ICQ9 Abstracts

Abstracts in Alphabetical Order by Presenting Author

A-C

D-F

G-I

"A Highly-Perfect Decagonal Quasicrystal in Al-Ni-O₃"
Eiji Abe¹, An-Pang Tsai²
¹National Institute for Materials Science, 305-0047 Tsukuba Japan
²IMRAM, Tohoku University

Presentation Type: Poster
Topic: Structure

Abstract:
Al-Ni-X alloys are well-known systems that form a various type of decagonal structures. However, there have been a few examples of a nearly-perfect basic-type decagonal structure; X=Co or Fe with quite limited composition ranges. We here report that a highly ordered decagonal phase is formed in an Al₇₅Ni_12.5Os_12.5 alloy annealed at 1373K. Electron diffraction studies have confirmed that a large number of sharp Bragg reflections appear in the patterns, whose features are the same as those of the basic structure in the Ni-rich Al-Ni-Co alloys. Therefore, the present Al-Ni-Os phase provides the additional example of a highly-perfect basic-type decagonal structure.

"The Structure of an Approximant Crystal in Al₇₁Ni₂₂Co₇"
Eiji Abe¹, Akiji Yamamoto¹
¹National Institute for Materials Science, 305-0047 Tsukuba Japan

Presentation Type: Poster
Topic: Structure

Abstract:
So-called Ni-rich Al-Ni-Co decagonal phases are of particular interests because they form a nearly-perfect quasiperiodic atomic structure at high temperature, which is often referred to as a basic Ni-rich type. At low temperatures, this basic structure transforms into a superlattice-ordered structure. During the study of this interesting transformation using transmission electron microscope (TEM), we have discovered a trace of an approximant crystalline structure in the Al₇₁Ni₂₂Co₇ alloy annealed at 973K, in which the decagonal superlattice structure coexists. We propose a plausible atomic model of this approximant crystal based on electron diffraction and atomic-resolution Z-contrast imaging. The structure can be interpreted as a periodic arrangement of fivefold symmetric decagonal clusters of 2nm across, with up-and-down pentagonal polarizations between the neighbors.

"Direct Imaging of Phason-Related Disorders in a Decagonal Al₇₂Ni₂₀Co₈ by Ultrahigh-Resolution Scanning Transmission Electron Microscopy"
Eiji Abe¹, Stephen Pennycook
¹National Institute for Materials Science, 305-0047 Tsukuba Japan
²Oak Ridge National Laboratory

Presentation Type: Contributed Talk
Topic: Structure

Abstract:
Phason is a unique elastic degree of freedom specific to quasicrystals, and may cause structural disorders at specific atomic sites. Using annular dark-field scanning transmission electron microscopy (ADF-STEM), we showed that some particular Al atoms in the decagonal Al₇₂Ni₂₀Co₈ reveal significantly large Debye-Waller (DW) factors at high-temperature. It was also demonstrated that these anomalous DW factors can be related to phason fluctuations within the context of hyperspace crystallography, in the sense that these anomalous Al sites are generated from the edge portions of the occupation domains (or atomic surfaces). Local DW anomalies may enhance short-range diffusional atomic jumps between their neighbor sites, and diffusional jumps would result in the "quenched phason disorders" that are detectable by experimental measurements on the quenched sample.

We describe the details of quenched phason disorders in the decagonal Al₇₂Ni₂₀Co₈ by recently developed ultrahigh-resolution STEM at Oak Ridge National Laboratory (aberration correction of the objective lens has successfully achieved the sub-Angstrom resolution). Because ADF-STEM provides incoherent images that can be well described by a convolution between the scattering object and the probe-intensity function, atomic structures can be directly addressed by a simple deconvolution procedure. After a deconvolution procedure using maximum entropy (ME) algorithm, the Al atomic sites have successfully emerged out even under the strongly enhanced amplitude contrast (Z-contrast) mode. We describe significant distributions of substitutional and occupational disorders across an entire quasiperiodic structure, for those not only the transition metal sites but also the Al sites. Local electronic structures will also be discussed at spatial resolution with almost atomic site-by-site, based on electron energy loss spectroscopy (EELS) combined with a sub-Å electron prone STEM.

"Electron Localization and Conduction Mechanisms of Al-Pd-Re Quasicrystals"
Tetsuji Akiyama¹, Yoshiki Takagiwa¹, Ikuzo Kanazawa¹
¹Tokyo Gakugei University, 184-0051 Tokyo Japan

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
The electric properties in quasicrystals are quite unusual. That is, the resistivity is anomalously large at low temperature, and decreases with increasing temperature. Especially the electronic transport properties of icosahedral Al-Pd-Re are highly anomalous and poorly understood [1]. Recently one of the present authors [2] has considered the transport properties in a randomly distributed system of correlated configurations (the aggregation such as an icosahedral cluster), in which the nearest distance between each configuration is ~ 2π/ 2k_F (the quasicrystal-like state). The quasicrystal-like state is regarded as the system composed of the Gaussian correlated distribution of the icosahedral cluster, which induces 2k_F-phase shift scattering. It is known that the mean-free path of electrons in quasicrystals is smaller than 15-20Å. Taking into account the short mean-free path, the transport property in the quasicrystal-like system is referential to one in the quasicrystals. In this study, we will analyze the conductivity data of Al-Pd-Re quasicrystal [1] in the framework of temperature-dependence of the conductivity derived from the theoretical formula in the quasicrystal-like system [3].

[1] R. Tamura, H. Sawada, K. Kimura, and H. Ino, in ICQ6, Proc, P.631, Eds. S. Takeuchi and T. Fujiwara, World Sci, (1998).
[2] I. Kanazawa, Physica B 328 (2003) 111
[3] I. Kanazawa, Modern Phys, Lett, B 17 (2003) 841

"Compressive Residual Stress Effects in Composites Consolidated from Al and Al₆₃Cu₂₅Fe₁₂ (Icosahedral) Gas Atomized Powders"
Iver Anderson¹, Fei Tang²
¹Ames Laboratory (USDOE), Iowa State University, Ames, Iowa 50011 USA
²Oak Ridge National Laboratory

Presentation Type: Poster
Topic: Mechanical Properties and Applications

Abstract:
Unlike the typical tensile residual stresses in the Al matrix of many particulate reinforced Al composites, e.g., Al/SiC, compressive residual stress (up to -215 MPa) was found by neutron diffraction in the Al matrix of Al/Al-Cu-Fe composites (15 to 30 vol.%) after powder consolidation. One benefit of the compressive stress in these composites, which also had excellent interparticle bonding, was a remarkable increase in the Al matrix tensile yield strength (YS) of up to 328% and ultimate tensile strength (UTS) of up to 220% after reinforcement by spherical Al₆₃Cu₂₅Fe₁₂ particles. The unusual compressive stress appears related to volume expansion of the reinforcement particles that transformed from quasicrystalline to crystalline omega-phase (Al₇Cu₂Fe), due to Al diffusion during hot forging or vacuum hot pressing (VHP) at 550°C. To explore this, a Al/30% Al-Cu-Fe composite sample (forged) with the highest residual compressive stress (-215MPa) and highest YS (227MPa) and UTS (304MPa) was relaxed by annealing at 550°C and converted to a small residual tensile stress (+ 55MPa). This is similar to the residual stress of VHP samples and consistent with CTE mismatch on cooling between the Al matrix and omega, isolated from transformation stresses. YS (178MPa) and UTS (235MPa) values of the forged/annealed composite also decreased to become similar to YS (191MPa) and UTS (238MPa) of VHP samples that experienced the same thermal cycle. This suggests that retention of high compressive stress is favored by the increased cooling rate of the forging process. To verify that compressive stresses were related to reinforcement phase transformation, through-thickness neutron diffraction was used to monitor the reinforcement transformation and matrix stress evolution during VHP (near 650C) of such composites. The results revealed that the transformation onset can preceed applied loading in the powder compact at high temperatures, but that transformation kinetics were accelerated by compressive loading which produced matrix particle deformation and enhanced contact area for interdiffusion.
Supported by USDOE-BES through contract no. W-7405-Eng-82 at Ames Laboratory.

"NMR Study of Hydrogen Mobility in Zr_69.5Cu₁₂Ni₁₁Al_7.5"
Tomaz Apih¹, Matej Bobnar¹, Janez Dolinsek¹, Lioba Jastrow¹, Daniela Zander², Uwe Koester²
¹J. Stefan Institute, SI-1000 Ljubljana Slovenia
²Department of Biochemical and Chemical Engineering, University of Dortmund, D-44221 Dortmund, Germany

Presentation Type: Poster
Topic: Thermal and Dynamical Properties

Abstract:
Hydrogen mobility was studied in hydrogenated Zr_69.5Cu₁₂Ni₁₁Al_7.5
melt-spun ribbons, both in partially quasicrystalline and in pure metallic glass form.
Using SGSE (Static Gradient Spin Echo) NMR technique in a stray field of a superconducting magnet, temperature dependence of hydrogen self-diffusion constant D was measured between room temperature and 430K. Several samples, differing in structure and covering hydrogen-to-metal (H/M) ratio up to 1.9 were used. The measured D was found in the range 10^-11 m²/s - 10^-13 m²/s, and obeys Arrhenious law D = D₀ exp(-E_a/k_bT). Concentration dependence of self diffusion constant D(H/M) and activation energy E_a(H/M) is discussed and compared with results of nuclear magnetic relaxation measurements.

"Comparison of Nucleation and Growth of Metallic Films on Different Quasicrystalline Surfaces"
Falko Baier¹, Jason Barrow², Pat Thiel²
¹Technical University Darmstadt, 64287 Darmstadt Germany
²Ames Laboratory, Ames, IA 50010, USA

Presentation Type: Poster
Topic: Surfaces

Abstract:
An atomistic understanding of the nucleation and growth of materials on top of quasi-crystalline surfaces is important for two reasons. First, it can provide a window on how quasicrystals form and propagate their unique structure. Second, this understanding may be a useful tool for growing "artificial quasicrystals" in the form of pseudomorphic thin films on quasicrystalline substrates. We performed experiments with different Al coverage and investigated the quasicrystal surface by STM and AUGER Spectroscopy. For the deposition of Al on the five-fold surface of icosahedral Al₇₂Pd_19.5Mn_8.5 different growth behaviors can be observed. Al atoms show a smooth growth behavior at low coverages (≤ 1 ML), and a rough growth at higher coverages. Also surface alloying can be observed, by which the step edge structures change drastically and terrace widths increase in a very narrow coverage regime around one monolayer. For a better understanding these results have been compared with Al deposition studies on icosahedral Al-Cu-Fe quasicrystals.

"6D Modelling of Bulk Icosahedral Quasicrystals as a Tool for Studying their Surface Properties"
Luc Barbier¹, Denis Gratias²
¹CEA, 91191 Gif-sur-Yvette France
²LEM/CNRS-ONERA, 92322 Chatillon France

Presentation Type: Poster
Topic: Surfaces

Abstract:
6D polyhedral models give a periodic description of icosahedral quasicrystals. They are powerful tools to describe their structural surface properties (chemical nature and structure). The bases of deriving the main surface geometrical properties out of the 6D description are given and exemplified with the interpretation of high resolution STM images of the 5f
surface of i-AlPdMn. It is found that both terrace structure and step-terrace height sequences observed on the STM images can be consistently interpreted within the framework of the 6D quasicrystalline bulk models. STM images show that the surface morphology exhibits extended 5f terraces that are, like for usual chemically ordered crystals, chemically selected (Al-rich) 5f planes over a high density underlying plane.

"Hydrogen Interactions with Quasicrystalline Al–Pd–Mn Surfaces"
Robert Bastasz¹, Josh Whaley¹, Thomas Lograsso², Cynthia Jenks²
¹Sandia National Laboratories, Livermore, CA 94551-0969 USA
²Ames Laboratory, Iowa State University, Ames, IA 50011 USA

Presentation Type: Poster
Topic: Surfaces

Abstract:
In this work we examine the interaction of molecular and atomic deuterium with the fivefold surface of icosahedral (i-) Al–Pd–Mn using angular-resolved low-energy ion scattering under ultrahigh vacuum conditions. i-Al–Pd–Mn has been studied extensively and is known to form a clean laterally-bulk-terminated surface after sputtering and annealing in excess of 800 K. Despite no two layers within a quasicrystal being identical, certain types of planes are favored, namely those that are Al-rich (> 77 atomic percent). The density of Al atoms on the clean surface of fivefold i-Al–Pd–Mn is about that of Al(111) and thus we compare our results to studies of molecular and atomic hydrogen on Al(111).

We are able to confirm, using ion scattering of 2 keV neon ions and direct recoil measurements, previous thermal desorption studies that indicated that molecular deuterium does not dissociate on this surface. Molecular deuterium, likewise, does not dissociate on Al(111). Atomic hydrogen, however, we find readily adsorbs on both surfaces. On fivefold i-Al–Pd–Mn we find that atomic deuterium adsorbs on top of the surface and that it strongly attenuates the signals for Al, Pd and Mn along particular azimuthal directions. Based on this information we will discuss the deuterium adsorption geometry on i-Al–Pd–Mn and compare the results to those for Al(111).

This work is supported by the Director, Office of Energy Research, Office of Basic Energy Sciences under Contracts no. W-405-Eng-82 and DE-AC04-94AL85000.

"Electronic Structure of Leached Al-Cu-Fe Quasicrystals used as Catalysts"
Esther Belin-Ferré¹, Marie Françoise Fontaine¹, Jean Thirion¹, Satoshi Kameoka², An-Pang Tsai²
¹Laboratoire de Chimie Physique Matière et Rayonnement UMR 7614 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France
²Institute for Multidisciplinary Research for Advanced Materials,2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
It is now well established that Al-Cu-Fe quasicrystals can be used as industrials catalysts for reforming of methanol provided they undergo appropriate chemical treatments. To try to understand the mechanisms involved in the catalytic behaviour, we have performed a study of the electronic structure of a series of Al-Cu-Fe samples after leaching and annealings in different conditions. We have used X-ray photoemission spectroscopy for the investigation of inner levels and X-ray emission spectroscopy for probing the electronic distributions of Al, Fe and Cu in the various samples. On the leached and annealed samples, Al and Fe appear oxidized over the whole probed sample thickness, Cu exhibits neither the structure of the pure metal nor that of the quasicrystalline compound. The data will be presented and discussed by comparison to the same measurements carried on the starting quasicrystalline compound.

"Fivefold and Tenfold in Three-Space"
Shelomo Ben-Abraham¹, Alexander Quandt²
¹Ben-Gurion University, IL-84105 Beer-Sheva Israel
²Institut für Physik, Ernst-Moritz-Arndt Universität, Domstrasse 10a, D-17489 Greifswald, Germany

Presentation Type: Poster
Topic: Mathematics

Abstract:
We have developed a two-stage variant of the cut-and-project method to study structures constructed by projection of a high-dimensional lattice into three-space so that a second projection yield a quasiperiodic tiling [1]. Here we apply the method to the Penrose tiling and its generalizations. In the original cut-and-project approach the five-dimensional simple cubic lattice, alias Z⁵ was projected into a suitable plane R² as parallel (physical) space while the window was in its complementary three-space R³ (cf. [2, 3]). Here we reverse the roles and focus on the projected structure in R³. The five-dimensional central unit cell projects into a rhombic icosahedron. Although the projection from five dimensions is not minimal [4] it appears to be the most convenient and transparent, particularly in the present context.

[1] S.I. Ben-Abraham, Y. Lerer, Y. Snapir, J. Non-Cryst. Solids, 334&335 (2004) 71-76.
[2] F. Gähler, J. Rhyner, J. Phys. A: Math. Gen. 19 (1986) 267-277.
[3] M. Senechal, Quasicrystals and Geometry, Cambridge UP 1995, pp. 195 ff.
[4] M. Baake, P. Kramer, M. Schlottmann, D. Zeidler, Int. J.
Mod. Phys. B, 4 (1990) 2217-2268.

"Dodecagonal Structure Constructed by Two-Stage Projection"
Shelomo Ben-Abraham¹, Alexander Quandt²
¹Ben-Gurion University, IL-845105 Beer-Sheva Israel
²Institut für Physik, Ernst-Moritz-Arndt Universität, Domstrasse 10a, D-17489 Greiswald, Germany

Presentation Type: Poster
Topic: Structure

Abstract:
Aperiodic crystalline structures quasiperiodic in a plane and periodic in its perpendicular direction or the other way round are by now quite commonplace. To understand them better it is interesting to study three-dimensional structures obtained by projection of periodic structures in some higher dimension D (>3) in such a way that a second projection be quasiperiodic in two dimensions. Here we present and visualize such an intermediate structure obtained by projecting onto three dimensions the root lattice D₄, alias the four-dimensional checkerboard lattice, but better known as the four-dimensional centered (both body and face centered) cubic lattice [1, 2]. It is interesting in itself, being the densest and most symmetric of all four-dimensional lattices. Its symmetries include the rotations 1, 2, 3, 4, 6, 8, 12. The unit cell is the 24-cell, alias polytope {3,4,3}; since it is self-dual it is also the Voronoi domain [3]. However, the respective 3D projections are different. The second projection onto two dimensions, based on the dodecagonal projection of the 24-cell, yields the desired final structure that is quasiperiodic in the basal plane and periodic in its perpendicular direction.

[1]M. Baake, D. Joseph and M. Schlottmann, Int. J. Mod. Phys. B5 (1991) 1927-1953.
[2]J.H. Conway and N.J.A. Sloane, Sphere Packings, Lattices and Groups,
3rd ed, Springer, New York 1999.
[3] H.S.M. Coxeter, Regular Polytopes, 3rd. ed., Dover, New York 1973.

"Characterization of the Growth Dynamics of Icosahedral AlPdMn Quasicrystals by In situ Synchrotron X-ray Imaging"
Guillaume Reinhart¹, Henri Nguyen Thi¹, Joseph Gastaldi², Thomas Schenk, Nathalie Mangelinck¹, Bernard Billia¹, Paola Pino³, Benjamin Grusko⁴
¹L2MP, CNRS UMR 6137, Universite Paul Cezanne - Aix-Marseille III, Marseille, France
²CRMC-N, CNRS UPR 7251, Campus Luminy, Marseille, France
³ESRF, Grenoble, France
⁴IFF, Forschungszentrum Juelich, Juelich, Germany

Presentation Type: Contributed Talk
Topic: Structural Evolution and Phase Stability

Abstract:
Quasicrystals display long-range orientation order with symmetries incompatible with periodicity. Whether the formation of the stable quasicrystal structure is constrained by local growth rules or by the establishment of long-range atomic correlation is still undetermined. Because translation symmetry is lacking, it seems unlikely that quasicrystals build up by the attachment of atoms, generating unit cells and the whole network. However, as icosahedric clusters have been identified in quasicrystal-forming liquids, the idea of growth by the attachment of these clusters at the liquid-solid interface is reviving. Besides, the post mortem observation that the solid grains often display facets perpendicular to the symmetry axes also suggests that quasicrystals may grow by the forward movement of facets resulting from terrace and step motion. It follows that it is critical and timely to deepen the understanding of the dynamical formation of quasicrystals during their growth from the alloy melt. In order to gain clear insight on both the shape of the growing grains and the morphology of the solid-liquid interface, using synchrotron X-ray imaging at ESRF we therefore carried out the first in situ and real time investigation of the growth dynamics of icosahedral AlPdMn quasicrystals during upward Bridgman solidification. The dynamical evolution of the quasicrystal grains observed in the AlPdMn samples was characterized. First, live observation unambiguously proved that the quasicrystals did grow with a facetted solid-melt interface. At low pulling velocity, facet edges were identified on the outline of the grains. Increasing the applied velocity, the solidification front globally receded to lower temperature, and the facets all together developed evidence of layer growth by the tangential motion of macrosteps at the solid-melt interface, namely striations parallel to the interface and notches on the grain sides. Second, the kinetic undercooling became large enough to allow the nucleation and free growth of new grains in the melt ahead of the columnar front. This easy nucleation can tentatively be explained by relating it to the local order in the molten alloy.

"Kinetics of the Crystallization of Icosahedral Phase in Ultra Thin Deposits of Al₆₂Cu_25.5 Fe_12.5 Alloys"
Nathalie Bonasso¹, Philippe Pigeat²
¹LERMPS, UTBM, 90010 BELFORT FRANCE
²LPMIA, UHP Nancy I, BP 239, 54046 Vandoeuvre, France

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
We have show elsewhere that the best film quality (smooth and homogeneous) of i-Al₆₂Cu_25.5Fe_12.5 alloys was obtained by simultaneous deposition of the three elements Al, Cu, Fe in the atomic proportion 62/25.5/12.5. Made at room temperature, the obtained deposit is amorphous. The mixture becomes quasicrystalline when subjected to heat treatment. To optimize this annealing, the crystallization paths of the deposit were studied. To this end several TEM grids were overlaid during the same deposition. Each grid was heated up to a different temperature threshold in the range of 350 °C to 600 °C. During these temperature treatments the crystalline structure of these 100 nm thick deposits was observed in real time by TEM. The quasicrystalline transformation of these amorphous alloys begins with a primary crystallization where the β-Al₅₀(Cu,Fe)₅₀ crystalline phase and α-Al precipitates appear in a residual amorphous matrix. Only during annealing above 450 °C are nucleation and growth of i-AlCuFe observed. This growth occurs around betagrains. The growth kinetics of i-AlCuFe particles observed during heat treatment performed at different temperatures > 450 °C, cannot be described by simple parabolic or cubic laws. In this communication a Mehl-Johnson-Avrami model (MJA) is proposed. In this model an instantaneous heterogeneous nucleation is assumed. Each particle is schematized as a cylinder with a radius R. Each isolated quasicrystallite growing on both sides of the beta-amorphous interface increases by bulk diffusion of different species, Al in the amorphous matrix, Cu and Fe in the β-phase (enriched with these elements) and of Al, Cu, Fe through the quasicrystallite. The differential equation is numerically integrated. Hence the result R=f(t) fits the experimental data very well and allows mean activation energies for volume diffusion to be calculated (in the range of 110-150 kJ/mol).When the theoretical evolution of the particles size increases monotonically, a discontinuity in the experimental growth kinetics is recorded. This modification is interpreted as due to the grains coalescence of neighboring icosahedral particles. Assuming that at this time the transformation factor of the sample in quasicrystalline phase is near 100%, a TTT curve is plotted. Nucleation densities (in the range of 2.1019-7.1019 m^-3 versus temperatures) and activation energy of nucleation of the i-Al ₆₂Cu_25.5Fe_12.5 phase (29.4 kJ/mol. K) are then calculated.

"Crystal Structure of the Novel Binary Compound Ir₂₃Ga₇₇ with Clusters Showing Local Icosahedral Symmetry and Correlated Positional Disorder"
Magnus Bostrom¹, Yurii Prots¹, Yuri Grin¹
¹MPI Chemische Physik fester Stoffe, 01187 Dresden, Germany

Presentation Type: Poster
Topic: Structure

Abstract:
The novel phase Ir23Ga77 was synthezised in equilibrium with a gallium-rich melt. Its crystal structure was established from single-crystal X-ray diffraction data. The compound crystallizes in the space group and represents a novel structure type (Pearson symbol cP662, a = 19.8471(8) ?). The structure is described as a packing of three different clusters, one at the center and one at the origin of the unit cell and one at 0.5, 0.19, 0. The two cluster at the center and the origin of the unit cell both have icosahedral symmetry. Two the clusters show several examples of positional disorder. In the central clusters the disorder is correlated, so that a change in one shell affects the occupancy in another shell.

"Non-Linearities in Undercooled Properties of Ti_39.5Zr_39.5Ni₂₁"
Rich Bradshaw¹, Aaron Arsenault¹, Robert Hyers¹, Jan Rogers², Tom Rathz³, Geun Lee⁴, Anup Gangopadhyay⁴, Ken Kelton⁴
¹University of Massachusetts, Amherst, MA 01003 USA
²NASA Marshall Space Flight Center, Huntsville, AL 35812 USA
³Center for Automation and Robotics, University of Alabama in Huntsville, Huntsville, AL 35899 USA
⁴Washington University, St. Louis, Missouri 63130 USA

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
Using electrostatic levitator (ESL) based containerless processing combined with non-contact measuring techniques; surface tension, viscosity and thermal expansion have been measured for Ti_39.5Zr_39.5Ni₂₁ in the superheated and metastable undercooled liquid states. A non-linearity in thermal expansion and sudden divergence in surface tension has been observed for this composition both of which correlate with a maximum in specific heat that has been measured by Kelton, et al. Despite the correlation of anomalous behavior between thermal expansion, surface tension and specific heat, viscosity does not exhibit the same trend. Data for thermal expansion, surface tension and viscosity will be presented.

"Local Order and Dynamics in Quasicrystalline and amorphous AlCuFe"
Richard Brand¹, Peter Häussler², Michael Krisch³
¹Department of Physics, 47048 Duisburg Germany
²Laboratory for the Physics of Thin Films, TU Chemnitz, Chemnitz Germany
³European Synchrotron Radiation Facility, Grenoble France

Presentation Type: Poster
Topic: Thermal and Dynamical Properties

Abstract:
We study the lattice dynamics of Al₆₂Cu_25.5⁵⁷Fe_12.5 in both the quasicrystalline and the amorphous states by Inelastic X-ray Scattering. Amorphous films were produced from the ternary alloy by flash evaporation. Identical compositions in the QC state were used as comparison. Electron diffraction studies of the amorphous samples have shown enlarged maxima at precisely the same values of the reciprocal vector Q where the QC shows the strongest diffraction lines, demonstrating that the short range order is very similar to that of the QC. Free-standing samples in both states were studied using Inelastic X-ray Scattering in the foreword direction. Data were collected at various values of scattering vector Q, and summed to build a vibrational density of states, as is commonly done in inelastic neutron scattering. The two states of AlCuFe show a similar vibrational DOS except for the presence of an excess density at low energy in the amorphous one. The resulting calculated thermal properties will be compared and discussed in terms of phase stability and entropy. Additional preliminary studies of the acoustic branch of QC Al₆₂Cu_25.5⁵⁷Fe_12.5 studied on a small single grain sample will be discussed.

"Iron Phason Hopping in Al-Cu-Fe Quasicrystals and Approximants: Mossbauer, Neutron and EXAFS Experiments"
Richard Brand¹, Juergen Voss¹, Francoise Hippert², Virginie Simonet², Bernard Frick⁴, Catherine Pappas⁵, Carlos Fehr⁵, Yvonne Calvaryac⁶
¹Department of Physics, 47048 Duisburg Germany
²LMGP, ENSPG, 38402 Saint Martin d'Hères Cedex France
³Laboratoire Louis Neel, 38042 Grenoble France
⁴Institute Laue Langevin, Grenoble France
⁵Hahn Meitner Institute, Berlin Germany
⁶C.E.C.M./C.N.R.S., 94407 Vitry France

Presentation Type: Poster
Topic: Thermal and Dynamical Properties

Abstract:
In an investigation of the phason dynamics in icosahedral quasicrystals, we have studied i-Al₆₂Cu_25.5⁵⁷Fe_12.5 using different methods. Phason dynamics is evidenced by anomalous changes in several scattering properties: The Mossbauer recoilless fraction f(T) decreases abruptly below the Debye result f_D(T) for temperatures above ca. 550 to 600 K. An additional broad quasielastic signal is found at an energy scale of a few μeV (time scale of h/(4πμeV) at T = 1050 K) corresponding to this loss of recoilless fraction. A break in the temperature dependence of the electric field gradient is seen at ca. 800 to 850 K, indicating a change from linear to rotational phason flips. Similar results were found for the two approximants α-Al₅₅Cu_25.5Fe_12.5Si₇ and α'-Al_76.7Cu_3.8Fe_17.5Si7, but not for the non-approximant cubic B2 phase β-Al₅₂Cu₃₅Fe₁₃. Quasielastic neutron scattering results on isotopic samples have confirmed the Mossbauer result and shown a quasielastic line whose area strongly increases with the iron scattering cross section. EXAFS experiments at the iron K-edge show for the first time high temperature anomalous changes in the Debye Waller factor, showing explicitly the local atomic jumps, while preliminary wide angle Neutron Spin Echo experiments show an anomalous variation of the SE Debye Waller consistent with the 800 K transition to rotational phason flips.

"Effective Potentials for Quasicrystals from Ab-Initio Data"
Peter Brommer¹, Franz Gähler¹
¹ITAP, Universität Stuttgart, 70550 Stuttgart, Germany

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
Classical effective potentials are indispensible for any large-scale atomistic simulations, and the relevance of all simulation results crucially depends on the quality of the potentials used. For complex alloys like quasicrystals, however, realistic effective potentials are practically nonexistent. We report here on our efforts to develop effective potentials especially for quasicrystalline alloy systems.

As experimental data to which potentials could be fitted are scarce, we employ the so-called force matching method. For a suitable selection of small reference configurations, the cohesion energies, stresses, and forces on individual atoms are computed by (quantum-mechanical) ab-initio methods, and the parametrized potentials are adapted to optimally reproduce these data. In this way, enough reference data can be obtained to determine even a large number of potential parameters. As reference configurations, we use several kinds of quasicrystal approximants, some of which are also distorted, or brought to higher tempertures, in order to have a sufficient
variety of local neighborhoods and interatomic distances.

The selection of reference configurations determines the type of situations the potentials will be able to handle: One may either develop specialized and highly accurate potentials for a particular task, or somewhat less accurate potentials with a broader range of applicability, but there is always a trade-off between accuracy and transferability to be made.

Although the method is suitable for any kind of effective potential, we concentrate on embedded-atom-method (EAM) potentials, which are especially suited for metals, and avoid many of the problems pure pair potentials have with metals. As a first application, several EAM potentials for decagonal Al-Ni-Co and icosahedral Ca-Cd quasicrystals are presented. The influence of their range and degree of specialization on the accuracy and other properties is discussed and compared.

"Structure Factor for Decorated Quasiscrystals and Other Related 1D Structures"
Pawel Buczek¹, Janusz Wolny²
¹University of Alberta, Edmonton, Canada
²University of Science and Technology, Krakow, Poland

Presentation Type: Poster
Topic: Mathematics

Abstract:
A systematic method of dealing with arbitrary decorations of quasicrystals is presented. The method is founded on a statistical description and utilizes the notion of average unit cell formalism. Therefore it operates in the physical space only, where each decorating atom manifests itself by a component of the displacement density function in the average unit cell. The cell is a straightforward extension of a unit cell in classical crystallography - decorating atoms are not represented as δ-like objects, but correspond to non-discrete "images". Such approach allows using almost all classical crystallography algorithms for structure refining based on experimental data and may meaningly decrease a number of parameters which have to be fit in order to resolve the crystal structure. Further help for such analysis may be use of proposed recently average Patterson function, here extended and applied to decorated sets. As an example we present a description of a class of decorated quasicrystals based on Sturmian sequence of two interatomic spacings. Structure factor is shown to have the following form:

F(k) = Σ_l f_l φ_l F_l,

where the sum runs over all types of decorating atoms, f_l and φ_l are respectively scattering power and relative concentration of an l-th type of atoms and F_l is a specific mode of a Fourier transform (corresponding to scattering vector k) of its displacement density function. Subsequently the shapes of average Patterson functions are calculated and used in a detailed and "ready to use" algorithm for pattern analysis. In the end we point the ways to include impact of structure imperfections: phonons, phasons and admixtured atoms.

"Can Motifs of Quasicrystals and Approximants also Pack to Form an Unusual Glass?"
Leonid Bendersky¹, Frederic Mompiou¹, Kil-Won Moon¹, Frank Biancaniello¹, John Cahn¹
¹Metallurgy Div., NIST, Gaithersburg, MD 20899 USA

Presentation Type: Contributed Talk
Topic: Structural Evolution and Phase Stability

Abstract:
We report glass formation by a first-order transition from Al-Fe-Si melts over a narrow concentrations range close to those of quasicrystals and approximants. This glass nucleates and grows from the melt as a primary phase as if it were an intermetallic crystal, with partitioning at the interface and Mullins-Sekerka instabilities when the melt is off composition. This is not a glass which forms when an undercooled melt becomes kinetically frozen. Instead it is highly ordered phase with its own stoichiometry, possibly composed of the same motifs as the quasicrystal, but packed in way which does not result in directional order. Its energy is found to be similar to that of the quasicrystal in the same system, and considerably less than that expected for a glass which forms from a kinetically frozen melt. We are engaged in structural studies.

"Influence of Gravity on Growth Behaviour of the Quasicrystalline Phase in Mg-Zn-Ga Aystem"
Amit Mondal¹, S. Sarma², M. Mittal², Kamanio Chattopadhyay³
¹Department of Metallurgy, Indian Institute of Science, Bangalore India
²Vikram Sarabhai Space Center, Indian Space research Organisation, Trivandrum India
³Indian Institute of Science, Bangalore India

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
Peritectic reaction is reported to be responsible for the formation of stable quasicrystal in Mg-Zn-Ga system. The density difference between Mg and Zn/Ga is large (density of Mg = 1.74 gm/cc while for Zn = 7.14 gm/cc and Ga = 5.91 gm/cc). Thus sedimentation is expected to influence the growth as well as perfection of the quasicrystals. In order to explore the influence of the gravity on the growth of quasicrystal, a microgravity experiment is planned using an isothermal furnace on a satellite platform. The presentation will report the results of extensive ground experiments carried out prelude to the actual microgravity experiments (scheduled for 2005) which have yielded important results. These include isothermal solidification experiments on prototype flight model of the isothermal furnace. Contrary to the ideal quasicrystal composition of Mg₃₆Zn₄₀Ga₂₄ reported earlier, we find that the alloy with Mg₄₀Zn₄₀Ga₂₀ composition yields the larger volume fraction of quasicrystal. It coexists with MgZn₂ and MgGa/Mg₂Ga eutectic. Detailed DSC experiments allow us to establish the sequence of phase evolution in this alloy. Increasing Zn contents beyond 40 at% keeping Mg content at 40 at% yield the ternary T-phase (Bergman phase) as the coexisting phase with quasicrystal. The influence of gravity leads to the appearance of phase distribution consistent with Zn rich alloys in the bottom of our candidate alloy Mg₄₀Zn₄₀Ga₂₀. The results will be analyzed in terms of a growth model incorporating gravity-induced sedimentation.

"On the Strip Projection Method and Symmetry Properties of the Window"
Nicolae Cotfas¹
¹University of Bucharest, 061513 Bucharest ROMANIA

Presentation Type: Poster
Topic: Mathematics

Abstract:
An algorithm for generating periodic/quasiperiodic patterns with predefined local structure and symmetry has been obtained [J. Phys.A:Math.Gen. 30(1997) 4283, 37(2004) 3125; Lett. Math. Phys. 47(1999) 111; http://fpcm5.fizica.unibuc.ro/~ncotfas ] by starting from the model proposed in 1986 by Katz and Duneau and independently by Elser for the icosahedral quasicrystals. For each finite group G and each G-cluster C one can define a representation of G in a higher-dimensional space Rⁿ, decompose this space into a sum of two orthogonal G-invariant subspaces E (physical space) and E' , and use the strip projection method in order to define a pattern Q representing a packing of interpenetrating copies of C. In the case of a two or three-shell icosahedral cluster n is rather large but, fortunately, the window W (projection of the cube [-1/2,1/2]ⁿ on E') is high symmetric. The hundreds of inequations describing the window can easily be generated from a small number of them by symmetry transformations. Each inequation corresponds to a pair of opposite (n-4)-faces of W, and is determined by the corresponding orthogonal vector. This vector can be obtained directly by using a generalization of the usual cross product, namely, by expending a determinant having the vectors of the canonical basis of Rⁿ on the first row, the coordinates in this basis of the vectors determining the considered pair of faces of W on the next (n-4) rows, and the coordinates of the three vectors of a basis of E on the last rows. Our pattern Q is exactly defined, has the desired symmetry and local structure, and all the remarkable properties of the patterns obtained by projection. The existing computer programs for generating periodic/quasiperiodic patterns proposed by Vogg and Ryder [J. Non-Cryst. Solids 194(1996) 135], Lord et al.[Bull. Mater. Sci. 23(2000) 119] allow one to use our algorithm in order to describe the atomic structure of real quasicrystals.

"Formation of an Unexpected, Ordered Surface Alloy upon Pb Deposition onto Ag(111):"
Julie Dalmas¹, Hamid Oughaddou¹, Jean Marc Gay¹, Bernard Aufray¹, Guy Tréglia¹, Guy Le Lay¹
¹CRMCN-CNRS, 13288 Marseille Cedex 9 France

Presentation Type: Poster
Topic: Surfaces

Abstract:
By depositing lead under ultra-high vacuum onto a silver single crystal (111) surface we have grown an ordered Ag₂Pb surface alloy in a two-dimensional √3x√3R30$deg; superstructure. This is particularly interesting because totally unexpected, since the bulk materials tend to phase separate. Surprisingly, the large Pb atoms are inserted into the top Ag layer as revealed in atomically resolved Scanning Tunneling Microscopy images and further confirmed by Grazing Incidence X-Ray Diffraction Experiments using synchrotron radiation. Indeed this surprising alloy may have peculiar electronic properties, and even, possibly, exhibit superconductivity as a result of proximity effects.

Further growth beyond one-third of a monolayer leads to the formation of a dense, slightly rotated, Pb monolayer displaying Moiré patterns in STM images.

We have followed by Auger Electron Spectroscopy the kinetics of dissolution of such a monolayer. The dissolution process follows a peculiar kinetic path, blocking on a plateau corresponding to the ordered surface alloy, which indicates its surprising stability.

The electronic properties are investigated through detailed synchrotron radiation photoelectron spectroscopy measurements (valence states and shallow core levels) as well as simulations based on a tight binding approach. These simulations confirm the embedment of the Pb atoms within the top layer for the √3x√3R30$deg;. Ag₂Pb surface alloy and, conversely, the de-alloying when the monolayer grows beyond one third of a monolayer.

"Quantum Dynamical Properties of Quasicrystals"
David Damanik¹
¹California Institute of Technology, Pasadena, CA 91125 USA

Presentation Type: Invited Talk
Topic: Mathematics

Abstract:
To analyze electronic transport properties of a quasicrystal model one has to study spectral and quantum dynamical properties of the associated Schrödinger operator. In this talk, we will survey methods and results dealing with this issue. The situation in one dimension is particularly well understood. The occurrence of extended states for isolated energies in certain models, observed in the early 1990's, was recently shown to have strong quantum dynamical implications. We will explain this connection and the resulting lower bounds on quantum transport. Higher-dimensional models and results, along with a number of open problems, will be discussed as well.

"Phonon Dynamics in the Icosahedral and 1/1 Periodic Approximant of the CdYb and ZnMgSc Phase"
Marc de Boissieu¹, Sonia Francoual¹, Kaoru Shibata², Roland Currat³
¹LTPCM, UMR CNRS 5614, ENSEEG BP 75 38402 St Martin d'Hères Cedex, France
²Neutron Science Research Center, Tokai Research Establishment, Japan Atomic Energy Research Institute, Japan
³Institut Laue-Langevin, BP 156, F-38042 Grenoble Cedex 9, France

Presentation Type: Invited Talk
Topic: Thermal and Dynamical Properties

Abstract:
The recent discovery of the i-Cd_5.7Yb and i-Zn₈₀Mg₅Sc₁₅ QC phases, for which both the icosahedral and the 1/1 cubic approximant phases can be easily grown as single grains, opened new possibilities for the understanding of the dynamics of quasicrystals. Quasicrystals and approximants can be described as a compact quasiperiodic or periodic packing of the same icosahedral atomic clusters. Comparing the phonon dynamics in both systems should allow a better understanding of what is the respective influence of the local order (atomic clusters) and of the long range quasiperiodic order on the dynamical response of the system. Moreover, the atomic clusters which constitute the CdYb and ZnMgSc phases are of a new kind, i.e., neither of Mackay type as in i-AlPdMn nor of Bergman type as in i-ZnMgY. We present a detailed comparative study of the phonon dynamics carried out on single-grains of the i- and approximant phases of the CdYb and ZnMgSc systems using Inelastic X-ray and Neutron Scattering. Those results will be compared with the one obtained in crystalline phases of moderated structural complexity for which dynamical calculations can be performed more easily.

"Scanning Tunneling Spectroscopy and High Resistivity in Quasicrystalline Alloys"
Julien Delahaye¹, Thomas Schaub¹, Claire Berger¹
¹LEPES - CNRS, 38042 Grenoble Cedex 9 France

Presentation Type: Invited Talk
Topic: Surfaces

Abstract:
Very low electrical conductivity σ is experimentally observed in a number of Al-based icosahedral alloys containing transition metals. Conductance scaling laws [1] and detailed analysis [2] of the temperature dependence of conductivity σ(T) all point to a metal-insulator transition in icosahedral Al-Pd-Re. The conductivity is essentially determined by the electronic mobility and the density of states (DOS) at the Fermi level E_F, which we will address here. There is a consensus for the presence of a broad pseudogap, about 1eV wide around E_F, but the structure of the DOS on a smaller energy scale (1-100meV) is still debated. We present our results obtained by low temperature Scanning Tunneling Spectroscopy (STS) [3] on three icosahedral phases (i-Al-Pd-Mn, i-Al-Cu-Fe and i-Al-Pd-Re). The measurements give evidence for the presence of a narrow square root energy-dependent pseudogap centered at E_F. Similar square root dependence was observed in disordered systems close to the Mott-Anderson metal-insulator transition. We propose a scenario to explain the similarity between disordered and quasicrystalline systems that give a coherent picture of the metal-insulator transition in these icosahedral phases. We also compare our results with other tunneling spectroscopy studies of quasicrystalline alloys.

[1] J. Delahaye et al. Phys. Rev. B 64, 094203 (2001).
[2] J. Delahaye et al. J. Phys.: Condens. Matter 15 8753 (2003).
[3] J. Delahaye et al. Phys. Rev. B 64, 214201 (2003).

"5-D Modelling of Decagonal Al-Co-Ni Quasicrystals, Starting from the W-Approximant"
Sofia Deloudi¹, Walter Steurer¹
¹Laboratory of Crystallography, ETH Zurich, 8093 Zurich Switzerland

Presentation Type: Poster
Topic: Structure

Abstract:

Even twenty years after the discovery of quasicrystals, not a single structure is known with the accuracy and reliability commonly expected for regular periodic structures [1]. One way leading towards understanding the structure of quasicrystals, is given by the examination of approximants. In this project, we plan to investigate the complete stability field of decagonal Al-Co-Ni by 5-D modeling, starting from the W-phase [2]. The latter is of particular interest, as it is the only known approximant in this system, containing the complete atomic cluster [3]. The focus will be upon order, disorder and phase transformations of decagonal Al-Co-Ni. In the present work, first 5-D modeling results of the Co-rich decagonal phase will be discussed. Via embedding the W-phase, models of the decagonal phase are rendered and phasonic modes are being applied.
[1] Steurer, W.: Z. Kristallogr. 219 (2004) 391-446.
[2] Hiraga, K., Ohsuna, T., Nishimura, S.: J. Alloy Comp. 325 (2001) 145-150.
[3] Sugiyama, K., Nishimura, S., Hiraga, K.: J. Alloy Comp. 342 (2002) 65-71.

"Microstructural Studies on Zr_69.5Cu₁₂Ni₁₁Al_7.5"
Raghav Savalia¹, Gautam Dey¹, Dinesh Srivastava¹, Srikumar Banerjee¹, Srinivas Ranganathan²
¹Bhabha Atomic Research Center, Mumbai, Maharasthra 400085 India
²Indian Institute of Science, Bangalore, India

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
In this study the microstructure of this alloy has been examined after different solidification and heat treatment conditions. The as solidified microstructure has been examined in the arc melted button as well as in the melt spun ribbon. The arc-melted button showed a very fine dendritic microstructure. In addition to the dendrites it has also been possible to see the presence of an amorphous phase in the arc melted buttons, an observation which suggests that glass formation does not require very high cooling rates in this alloy. Electron diffraction has indicated that the crystalline phases having the dentritic morphology were isostructural with Zr₂Ni and Zr₂Cu. The nature of the interfaces between the crystalline phases and the crystalline and the amorphous region has been examined in detail using high-resolution electron microscopy (HREM). Though electron diffraction indicated that the melt spun structure was fully amorphous HREM has indicated the presence of a few quenched in nuclei in the amorphous matrix.

The microstructure developed after crystallization of the arc-melted ingot as well as of the melt spun ribbons has been examined. Crystallization has been carried out at 633 K and 673 K. The nature of the phases forming after crystallization of the amorphous phase has been compared with those forming form the liquid during solidification. The interfaces present in the crystallized microstructure have also been examined and compared with those forming during solidification. Particular emphasis has been given to the study of the interface between the quasicrystalline and the crystalline phases. The melt spun ribbons have been deformed and the microstructure of the deformed specimens has been examined before and after crystallization in order to ascertain if the shear bands have acted as sites of nucleation of crystalline phases.

"The Growth of Xe on the 10-Fold Al-Ni-Co Quasicrystal Surface"
Nicola Ferralis¹, Renee Diehl¹, Raluca Trasca¹, Milton Cole¹, Wahyu Setyawan², Stefano Curtarolo²
¹Penn State University, University Park, PA 16802 United States
²Duke University, Durham, NC 27708 United States

Presentation Type: Poster
Topic: Surfaces

Abstract:
The growth of Xe films on ten-fold decagonal Al₇₃Ni₁₀Co₁₇ was studied using low-energy electron diffraction and He-atom scattering. Adsorption isobars indicate layer-by-layer growth of Xe in the temperature range of 60 K to 120 K. The first layer apparently possesses the symmetry of the substrate, but at the onset of the second layer, the film reorders into a 6-fold structure. This 6-fold structure has domains that are aligned along the 10-fold directions of the quasicrystal, leading to a diffraction pattern having 30-spot rings. The momentum transfer of the first-order diffraction beams from Xe coincides with that of the quasicrystal surface, indicating that the Xe inter-row spacing is related to a principal distance on the substrate. This coincidence probably facilitates the ordering of the Xe film. The Xe-Xe spacing was measured to be 4.4 Å, which is essentially identical to the bulk nearest-neighbor distance of Xe, and the domain size of the ordered Xe is at least 160 Å. The Xe bilayer is consistent with two layers of bulk fcc(111) Xe. Further adsorption produces further ordered growth of the film.

A Xe-substrate potential was computed using Lennard-Jones potentials. This potential was the basis for Grand Canonical Monte Carlo simulations that agree substantially with the experimental measurements. The simulated isotherms and layer density profiles provide more details of the layer-by-layer Xe growth in the temperature range 70-280K. By increasing the pressure at low temperatures (70-110K) the first layer, which conforms to the 5-fold ordering of the substrate, evolves continuously into a 6-fold hexagonal symmetry before the condensation of the second layer. Further layers retain this 6-fold symmetry. At low temperature (70K) the layer stacking is ABCABC, consistent with fcc(111), while at higher temperature (160K) the stacking is ABAB, indicating a possible fcc to hcp transition of the structure of the adsorbate film.

"Magnetic and Transport Properties of Complex Metallic Alloys"
Janez Dolinsek¹
¹J. Stefan Institute, University of Ljubljana, Ljubljana, Slovenia SI-1000 Ljubljana

Presentation Type: Contributed Talk
Topic: Electronic and Magnetic Properties

Abstract:
Complex Metallic Alloys (CMA) are intermetallic compounds with crystal structures based on giant unit cells with an edge length of several nanometers and containing many tens up to more than a thousand atoms per unit cell. The examples are cubic NaCd₂ with 1152 atoms/unit cell, β-Al₃Mg₂ (1168 atoms/u.c.), orthorhombic ξ'-Al₇₄Pd₂₂Mn₄ (320 atoms/u.c.) with its ψ-modification (about 1500 atoms/u.c), etc. Inside the giant unit cells the atoms are arranged in clusters with local quasiperiodic symmetries, where icosahedral atomic coordination plays a prominent role. Consequently, in CMA there exist two substantially different physical length scales, one defined by the cluster substructure and the other by the unit cell parameters, so that interesting physical properties may be expected from the competition between these different length scales. Although the giant-unit-cell CMA materials have attracted attention of crystallographers in determining their structure since long time ago (the pioneering work of L. Pauling on NaCd₂ dates 1923), the studies of their physical properties are still scarce due to the problem of preparing the materials in efficient quantities and in single-crystalline form. Recently, high-quality samples of orthorhombic Al-Cr-Fe, ξ'-Al-Pd-Mn and ψ-Al-Pd-Mn giant-unit-cell materials were prepared and their physical properties (magnetic susceptibility, electrical resistivity, thermal conductivity and thermoelectric power) were studied. The investigated physical properties are in many respects intermediate to those of regular metals and quasicrystals (QCs). Their electrical resistivities show very weak (or no) temperature dependences between room temperature and 4 K and the resistivity values are higher than for regular metals and lower than for Al-based QCs. The magnetic susceptibility results show the existence of a tiny fraction (of about 1 % for the Al-Cr-Fe and about 100 ppm for the ξ'-Al-Pd-Mn and ψ-Al-Pd-Mn) of localized magnetic moments with Curie-like temperature dependence. Thermal conductivity measurements show that the electronic and lattice contributions are of comparable size. While the electronic contribution can be described by the Wiedemann-Franz law, the lattice contribution can be reproduced by a sum of Debye (long-wavelength phonons) and hopping of localized vibrations terms. The thermoelectric power can be either positive or negative with a complicated temperature dependence, indicating the complexity of the electronic band structure. These results that are in many respects different from both regular periodic metals and alloys and QCs indicate that the magnetic and transport properties of the giant-unit-cell CMA materials are affected by both the quasiperiodic short-range atomic order and the long-range periodic order.

"Cluster-Based Composition Rules for Quasicrystals"
Chuang Dong¹, Jianbing Qiang¹, Yingmin Wang¹, Patricia Thiel²
¹State Key Lab. of Materials Modification, Dalian University of Technology, 116024 Dalian, China
²Ames Laboratory, Ames, IA 50011 US

Presentation Type: Invited Talk
Topic: Structural Evolution and Phase Stability

Abstract:
Although hundreds of quasicrystals have been found, little is known about their quantitative composition rules that can help design new materials. In this paper we propose cluster-based composition criteria that are related to quasicrystal structure and stabilization mechanisms. For a binary quasicrystal of the icosahedral glass type, the composition is mainly determined by topological close packing of the atoms in the basic 1st-shell icosahedron. A ternary quasicrystal is located at the crossing point of two kinds of specific lines in a ternary phase diagram, which we term e/a-constant and e/a-variant lines, respectively. The e/a-constant line reflects the common electronic structure shared by the quasicrystal and its e/a-constant approximants. The e/a-variant line, defined by linking the composition of a binary cluster composition, to the third element, implies the growth pathway of a basic cluster towards final quasicrystalline and crystalline phases along the line. These simple rules are general in quasicrystalline systems. Applications of these rules in two exemplary systems, Al-Ni-Fe and Zr-Ti-Ni, are presented. Similar rules are also extended to the bulk metallic glass forming systems.

"Bulk Ti-Zr-Ni Quasicrystals with Superior Mechanical Properties"
Jianbing Qiang¹, Yingmin Wang¹, Huogen Huang¹, Chuang Dong¹
¹State Key Lab of Materials Modification, Dalian University of Technology, Dalian 116024 China

Presentation Type: Poster
Topic: Mechanical Properties and Applications

Abstract:
Bulk and dense Ti-Zr-Ni quasicrystalline ingots were obtained by using copper mold suction casting method. At the composition of Ti₄₀Zr₄₀Ni₂₀, the ingot is a single-phase quasicrystal alloy, 3 mm in diameter and 15 mm in length. Its quasicrystal nature and structural homogeneity were confirmed by using X-ray diffraction and transmission electron microscopy. According to compression and indentation tests at room-temperature, the as-cast Ti₄₀Zr₄₀Ni₂₀ quasicrystalline rod has a hardness of 5.5 GPa and a fracture strength of 542 MPa. In particular, the Young's modulus (~43GPa) of the Ti₄₀Zr₄₀Ni₂₀ quasicrystals is considerably low while the Poisson's ratio (~0.48) is quite high. During monoaxial compression, this Ti₄₀Zr₄₀Ni₂₀ quasicrystalline alloy exhibits a highly elastic deformation capability of 1.25 %. The cleavage fracture morphologies point to a brittle fracture mechanism. At other compositions, the ingots contain a secondary phase of the a-Ti/Zr type. The strength indices of the icosahedral phase-based duplex phase alloy (a-Ti/Zr plus I phase) are 5.5GPa in hardness and 1000 MPa in fracture strength, respectively. The Young's modulus is in the range of 23 - 36 GPa. The as-cast Ti₆₀Zr₂₅Ni₁₅ QC-based composite alloy exhibits an elastic deformation capability of 4.2% during monoaxial compression, which is about 4 times those of Al-based quasicrystals. The quasi-cleavage fracture morphologies were observed, indicating a brittle fracture mechanism of the bulk quasicrystal-based composite alloys.

"Dodecagonal Quasicrystal in a Polymeric Alloy"
Tomonari Dotera¹, Tohru Gemma²
¹Kyoto University, Kyoto, Kyoto 615-8510 Japan
²University of Tokyo, Komaba, Tokyo 153-8902 Japan

Presentation Type: Poster
Topic: New Frontiers

Abstract:
We report the formation of a dodecagonal (DD) quasicrystal in a quasi-two-dimensional lattice Monte Carlo simulation of a star-shaped three component polymeric alloy. [1,2] It is associated with the recent striking experimental manifestation of the (3².4.3.4) Archimedean tiling consisting of triangles and squares, closely related to the σ phase of the Frank-Kasper phases, but whose edge length is about 80 nm [3] (The polymer is an ABC star-shaped polymer alloy molecule composed of polyisoprene, polystyrene and poly (2-vinylpyridine)). In simulations, with increasing C component, a series of phases (4.8²) → (3².4.3.4) → DD → (4.6.12) is observed. The simulation box with periodic boundary conditions (128*128*10) can be regarded as the Stampfli inflation of the (3².4.3.4) tiling, an approximant of the DDQC, and thus the result is at best we could achieve in a simulation box. The corresponding edge length of deflated squares and triangles is thought to be about 300 nm. Furthermore, the consecutive zipper move of the deflated square-triangle tiling is dramatically observed at an elevated temperature. Our simulation suggests that the realm of quasicrystals may extend to macromolecular systems.

[1] T. Gemma, A. Hatano, and T. Dotera, Macromolecule 35, 3225 (2002).
[2] A. Takano, S. Wada, S. Sato, T. Araki, K. Hirahara, T. Kazama, S. Kawahara, Y. Isono, A. Ohno, N. Tanaka, and Y. Matsushita, Macromolecules 37, 9941 (2004).
[3] A. Takano, W. Kawashima, A. Noro, Y. Isono, N. Tanaka, T. Dotera, and Y. Matsushita, submitted.

"Surface Energy of Complex — and Simple — Metallic Compounds"
Jean-Marie Dubois¹, Marie-Cécile de Weerd², Josef Brenner³, Andreas Merstallinger⁴
¹CNRS, 54042 Nancy France
²CNRS LSG2M-Ecole des Mines Nancy
³AC2T Research GmbH, Viktor-Kaplan Str. 2, A-2700 Wiener Neustadt
⁴ARC Seibersdorf Research GmbH, A- 2444 Seibersdorf

Presentation Type: Poster
Topic: Surfaces

Abstract:
Based on comparative tribological pin-on-disk experiments performed in vacuum, we have found a simple way to estimate with good accuracy the surface energy of metallic specimens. The comparison is made to pure metals and reference samples the surface energy of which is known either from experiment or from calculations.

As long as wear is negligible, it turns out that the friction coefficient measured in vacuum is dominated by the adhesion between the surface of interest and the steel pin used in the present study. Within restrictive conditions that will be discussed in the paper, the friction coefficient is shown to be proportional to the surface energy and inversely proportional to the hardness of the sample. This approximation allows us in turn to estimate the surface energy of unknown specimens as hardness is easy to measure.

We have found that the surface energy of quasicrystals is in the range 0.5-0.8 J/m² whereas it increases to 2 J/m² in metallic compounds like the ω-Al₇Cu₂Fe or λ-Al₁₃Fe₄ compounds. Binary compounds of the Hume-Rothery family show intermediate values of the surface energy, closer to that of pure aluminium (1.2 J/m²).

"The World of Complex Metallic Compounds and the CMA European Network of Excellence"
Jean-Marie Dubois¹, Louis Schlapbach², Knut Urban³
¹CNRS, 54042 Nancy France
²EMPA, Swiss federal Lab. for Materials Testing and Research, Ueberlandstrasse, 129, CH-8600 Zürich.
³Institut für Mikrostruktur Forchung, Forschungszentrum Jülich, D-52425 Jülich

Presentation Type: Contributed Talk
Topic: Past/Present/Future

Abstract:
The world of Complex Metallic Compounds encompasses a broad variety of compounds and phases that are characterized by i) large unit cells, comprising tens up to thousands of atoms, ii) the marked presence in the unit cell of well-defined atom clusters, most often of icosahedral point group symmetry, iii) the occurrence of disorder within the layers bridging the clusters. As a result, most physical and mechanical properties show distinct differences with respect to the behavior of normal metallic alloys. Such properties, which were explored in quasicrystals to a limited extent, may find applications in technology. It is clear however that many more compounds may be synthesized by combining transition metals and/or rare earths with s-p elements. Hence, the potential of as yet unknown compounds is by far larger than the few tens of compounds that could be investigated in detail in recent years thanks to the enormous progress generated in crystallography after Shechtman's discovery twenty years ago.

With the support of about sixty industrial companies, it is therefore proposed to contribute to a strengthening of the competitiveness of metallic materials for the benefit of European industries by the formation of a European Network of Excellence CMA. This Network will unite an appropriate number of specialised laboratories to form an integrated body dedicated to the intelligent search for new metallic materials and their development towards technological applicability. Collaborating on a first challenging topic, these laboratories are going to demonstrate that CMA is capable to embark upon the research and development of materials that could not be dealt with before. CMA will unite 20 high-reputation core-group members (representing a person-year critical mass of 252 p.y. and 84 Ph.D. students in 12 countries).

On this basis, the project is designed to strengthen the competitiveness of European industries wherever materials need to offer hybrid properties, being both structural and functional, or embody an extraordinary combination of properties that are mutually excluding in conventional materials. Innovative management procedures for knowledge handling and networking, grant administration, organisation of conferences, exhibitions, industrial open days and specific measures for personnel exchange, access to platforms and durable integration of women in science will be taken, together with an ambitious programme of summer schools and personnel training.

"Electron Microscopy Study of Approximants and γ-brass Phases in Al-Cr-Fe system"
Valérie Demange¹, Jaafar Ghanbaja², Jean-Marie Dubois³
¹LSGS, F-54042 Nancy France
²LCSM, F-54500 Nancy France
³LSG2M, F-54042 Nancy France

Presentation Type: Poster
Topic: Structure

Abstract:
We have characterized by transmission electron microscopy several new phases in bulk alloys of the Al-Cr-Fe system. These structures are an icosahedral phase, several approximants of the decagonal phase and two γ-brass phases. According to the indexation of the electron diffraction patterns (EDPs), some of the approximants are not Fibonacci phases. These EDPs correspond to nanodomains, which are due to disorder introduced by the quenching which follows the fusion of the pure elements. For these new types of approximants, we will propose a mathematical description related to the usual Fibonacci description. In addition, we will suggest construction of the corresponding tilings. These ones show orientation relationships between these structures. Concerning the γ-brass phases, we have identified a long-period superlattice, which is constituted of inversion domains separated by boundaries along specific structure planes. The last part of this study deals with a description of some structural links between γ-brass phases and approximants of decagonal and icosahedral quasicrystals.

"Structural Evolution of Nano-Scale Icosahedral Phase in Novel Multicomponent Amorphous Alloys"
Ki Buem Kim¹, Paul Warren², Brian Cantor³, Jurgen Eckert¹
¹Technische Universität Darmstadt, Petersenstrafle 23, D-64287 Darmstadt, Germany
²Department of Materials, Oxford University, Oxford, OX1 3PH, Oxford U.K.
³Vice-Chancellor's Office, University of York, Heslington, YO10 5DD, York U.K.

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
A series of novel multicomponent amorphous alloys has been developed using equiatomic substitution technique. Of those alloys, it was observed that (Ti₃₃Zr₃₃Hf₃₃)₇₀(Ni₅₀Cu₅₀)₂₀Al₁₀, (Ti₂₅Zr₂₅Hf₂₅Nb₂₅)₇₀(Ni₅₀Cu₅₀)₂₀Al₁₀ and (Ti₃₃Zr₃₃Hf₃₃)₇₀(Ni₃₃Cu₃₃Ag₃₃)₂₀Al₁₀ amorphous alloys transform into nano-scale icosahedral phase when heat treated through the first exothermic reaction. However, the decomposition sequence of the nano-scale icosaheral phase was significantly different in each alloy. The devitrified nano-scale icosahedral phase in the (Ti₃₃Zr₃₃Hf₃₃)₇₀(Ni₅₀Cu₅₀)₂₀Al₁₀ decomposed into a mixture of Zr₂Cu-type and icosahedral phase through the second exothermic reaction. This icosahedral phase still existed after heat treatment temperature up to 970 K, indicating high thermal stability. On the other hand, the devitrified nano-scale icosahedral phase in the (Ti₃₃Zr₃₃Hf₃₃)₇₀(Ni₃₃Cu₃₃Ag₃₃)₂₀Al₁₀ alloy transformed into a mixture of Zr₂Cu-type and icosahedral phase during the second exothermic reaction, and then decomposed into a mixture of Zr₂Cu-type and Ti₂Ni-type phases. The devitrified nano-scale icosahedral phase in the (Ti₂₅Zr₂₅Hf₂₅Nb₂₅)₇₀(Ni₅₀Cu₅₀)₂₀Al₁₀ alloy decomposed into a mixture of Ti₂Ni-type and MgZn₂-type phases during the second exothermic reaction. These different decomposition sequences of the nano-scale icosahedral phase will be described in terms of the effects of the equiatomically substituted Nb and Ag in (Ti₃₃Zr₃₃Hf₃₃)₇₀(Ni₅₀Cu₅₀)₂₀Al₁₀ alloy.

"Mechanical Properties of Dual-Phase Alloys of B2 and Icosahedral Phases in the Al-Cu-Fe System"
Takeshi Kitaura¹, Yeong Gi So¹, Yasushi Kamimura¹, Keiichi Edagawa¹
¹IIS, Univ. of Tokyo, 153-8505 Tokyo Japan

Presentation Type: Poster
Topic: Mechanical Properties and Applications

Abstract:
Though the quasicrystal generally shows high strength up to considerably high temperature, it cannot be used by itself as a structural material because of lack of ductility. However, if it is combined with a more ductile phase, the mixed phase material would possibly exhibit high strength, together with good ductility, as demonstrated by Kimura et al. [1] An intermetallic compound Al-Fe B2 phase offer advantages for engineering uses due to its lightness, high corrosion resistance, high melting temperature, high strength and relatively good ductility. However, it shows a dramatic decrease in strength above approximately 600K and therefore is not suitable for such high temperature uses. In the present study, we attempted to produce dual-phase alloys of B2 and icosahedral (I) phases in the Al-Cu-Fe system and investigated their mechanical properties, focusing mainly on the strength improvement at high temperatures.
Ternary alloys with various compositions between Al₆₂Cu₂₆Fe₁₂ and Al₄₇Cu₃₅Fe₁₈ were produced by arc-melting. The alloys were annealed at various temperatures. The phases formed were examined by powder X-ray diffraction measurements. Vickers hardness was measured at room temperature. Compression experiments were done in the temperature range between room temperature and 873K. Microstructures were examined by optical microscopy, scanning electron microscopy and transmission electron microscopy. A series of alloys consisting of the two phases with different formation ratio were successfully obtained. Vickers hardness at room temperature was found to increase monotonically as the amount of the I-phase increases. The yield stress at 723K for the single B2 phase sample was about 300MPa. For the samples containing a small amount of I-phase (less than 5% in volume fraction), the yield stress at 723K increased up to about 580MPa.

[1] H.M. Kimura et al.: J. Mater. Res. 15 (2000) 2737.

"The Phasonic Degree of Freedom in the Ξ-Approximants of Al-Pd-Mn-Quasicrystals: Metadislocations and Phasonic Phase Boundaries"
Michael Engel¹, Hans-Rainer Trebin¹
¹ITAP, Universität Stuttgart, 70550 Stuttgart, Germany

Presentation Type: Poster
Topic: Mechanical Properties and Applications

Abstract:
One of the properties which distinguishes quasicrystal approximants from ordinary complex metal alloys is the possibility of spatial rearrangements of the cluster substructure, resulting in new unit cell contents. Such rearrangements are discrete degrees of freedom well known from quasicrystals. By adopting quasicrystal terminology, these degrees of freedom are called phasonic degrees of freedom.

We demonstrate that two-dimensional tilings of the Ξ-(AlPdMn)-approximants ξ, ξ', and ξ'_n can be modelled by a simple cut and projection formalism in three-dimensional hyperspace, not the usual four- to six-dimensional hyperspace. It follows that in the ξ'_n-phases there is one phasonic degree of freedom, whose excitation results in a movement of structural defects, so-called phason-planes.

Furthermore we describe phasonic phase boundaries between different Ξ-phases and metadislocations, which are special textures of partial dislocations. Both are modelled geometrically in the hyperspace and compared to HREM images from the literature. We determine the burgers vectors of metadislocations with lowest energy and relate them uniquely to experimentally observed ones. Since moving metadislocations in the ξ'-phase create new phason-planes, we suggest a dislocation induced phase transition from ξ' to ξ'_n.

"Nondeterministic Substitutional Structures and Applications"
J. Escudero¹
¹Universidad Politecnica de Madrid, Departamento de Ciencia y Tecnologia Aplicadas, U.D.Matematicas. EUITA, Ciudad Universitaria s.n., 28040-Madrid, Spain

Presentation Type: Poster
Topic: Mathematics

Abstract:

Geometric constructions for the generation of several types of substitutional tilings have been introduced in [1]. Non deterministic structures can be obtained by composition of the inflation rules. The local rearrangements of tiles are included in the inflation rules and this property allows to compute the configurational entropy. Non deterministic derivations in formal grammars producing non periodic ordered structures, can be considered also for the analysis of the multimode pulsations of variable stars [2]. Artificial light curves for the semiregular UW Her and the Delta Scuti V784 Cas have been derived by concatenation of two sinusoidal fragments following certain word sequences . The two basic building blocks represent temporal segments in a golden ratio and the number of long and short segments in a word are also in a golden ratio. The analysis can be applied also to the Delta Scuti star V346 Ori.

References:
[1] Escudero J.G. Int. J. Mod. Phys B. Vol.17,n.15,.2925. (2003) ;ibid. Vol.18,n.10-11, 1602. (2004)
[2] Escudero J.G. Chin. J. Astron.Astrophys. Vol.3,n.3,235. (2003) ;ibid. Vol.4,n.4,343. (2004).

"Tunneling Spectroscopy of Tb-Mg-Zn Quasicrystals"
Roberto Escudero¹
¹Universidad Nacional Autönoma de México, Mexico, DF 04710 MEXICO

Presentation Type: Invited Talk
Topic: Electronic and Magnetic Properties

Abstract:
Theoretical studies on quasicrystals (QC) predict that the electronic density of states will have a rich and fine spiky structure, and a pseudo-gap. Experimentally the pseudo-gap feature has been found at the Fermi level and determined by different spectroscopic techniques and by specific heat measurements. However, the predicted rich and fine spiky structural characteristics on the density of states have not been observed and it has been very elusive. The problem with its absence has been related, from the experimental point of view, to poor structural characteristics of the studied QCs specimens. From the theoretical point of view recent calculation have shown that the fine structure indeed exist, but only for approximants phases. The understanding and improvement for fabrication of single grain QCs, actually have had great advances. Today single grain materials can be prepared with good quasicrytalline perfection and purity. In this report we are presenting tunnelling studies performed in samples of very high quality. We have observed for the first time, a spiky structure on the electronic density of states. Our results open again the debate about the theoretical predictions, and support the picture of the spiky nature on the density of states, and also confirm the existence of the pseudogap at the Fermi level. We found that in general the structural features and the pseudo-gap are distinguished only at low temperatures.

R. Escudero.
Email: escu@servidor.unam.mx

"Modeling of Island Formation During Deposition of Al on Al-rich Quasicrystal Surfaces"
Chandana Ghosh¹, Dia-Jang Liu¹, Cynthia Jenks¹, James Evans¹, Patricia Thiel¹
¹Iowa State University, Ames, IA 50011 USA

Presentation Type: Poster
Topic: Surfaces

Abstract:
STM experiments reveal the formation of 'starfish' islands during the initial stages of deposition of Al on AlFeCu. Plausibly this phenomenon is more general, and is expected to reflect diffusion-mediated heterogeneous nucleation at special "trap sites". To model this process, we identify the 'disordered network' of local adsorption sites for Al on AlFeCu using a Gratias model for the substrate, and Lennard-Jones (LJ )interactions to describe Al-substrate binding and diffusion barriers between neighboring sites. (The LJ parameters selected to match density functional theory or experimental values for Al diffusion barriers on Al, Cu, and Fe single crystal substrates.) One must also prescribe Al-Al adsorbate interactions as these could be significant in stabilizing islands. We find two types of deep (clusters of) adsorption sites having starfish and banana shapes. Within the framework of this model, we explore the deposition and diffusion of Al on AlFeCu, and the competition between these two types of deep sites with regard to the aggregation of Al into islands.

"Symmetry of Magnetically-Ordered Quasicrystals"
Shahar Even-Dar-Mandel¹, Ron Lifshitz¹
¹School of Physics & Astronomy, Tel Aviv University

Presentation Type: Poster
Topic: Mathematics

Abstract:
We have developed a theory of magnetic symmetry in quasicrystals [1]. This theory provides a practical formalism for the enumeration of spin point-groups and spin space-group types, as well as for the calculation of selection rules for neutron scattering. The formalism is based on an extension to multi-component fields of the Fourier space approach to crystallography, similar to the treatment in the case of color symmetry [2]. As an application of the theory we have enumerated all three-dimensional octagonal spin point-groups and spin space-group types, and calculated the resulting selection rules [3]. Previous calculations of magnetic symmetry for quasicrystals were partial and did not provide much detail regarding the enumeration process. This work provides, for the first time, a straightforward method for calculation, which can be applied to any quasicrystal.

[1] R. Lifshitz and S. Even-Dar Mandel, "Magnetically ordered quasicrystals: Enumeration of spin groups and calculation of magnetic selection rules", Acta Cryst. A60 (2004) 167.
[2] R. Lifshitz, "Theory of color symmetry for periodic and quasiperiodic crystals", Rev. Mod. Phys. 69 (1997) 1181.
[3] S. Even-Dar Mandel and R. Lifshitz, "Symmetry of magnetically ordered three-dimensional quasicrystals", Acta Cryst. A60 (2004) 179.

"Electronic Energy Spectra and Wave Functions on the Square Fibonacci Tiling"
Shahar Even-Dar-Mandel¹, Ron Lifshitz¹
¹School of Physics & Astronomy, Tel Aviv University

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
The electronic energy spectra and densities of states for an off-diagonal tight-binding hamiltonian on the square Fibonacci tiling [1] are studied by various methods. Hopping amplitudes are non-zero only for nearest neighbors and are taken to be 1 and t, arranged according to the Fibonacci sequence. With these definitions the model is separable, thus its spectrum is the sum of two 1-dimensional spectra.

For nearly periodic and intermediate cases (1<t<2), the exact 1d spectrum is calculated, and summed to obtain the 2d spectrum, for approximants up to the 20th Fibonacci number. For higher values of t, as well as for larger approximants, Monte-Carlo integration is used to find the total bandwidth of the 2d spectrum. We are performing these calculations in order to find the exact nature of the transitions between different spectral behaviors [2], as well as the scaling of the total bandwidth as it becomes finite.

The macroscopic degeneracy of some energy values in the spectrum is invoked as a possible explanation for the emergence of extended electronic wave functions. In particular, the zero energy wave functions of the 2d hamiltonian is F_n degenerate for an F_n×F_n approximant. Certain linear combinations of such degenerate wave functions are shown to be extended along particular directions of the approximant. For nearly periodic cases (t≈1) additional energy values become macroscopically degenerate, leading to the possibility of having extended wave functions along other directions of the approximant.

[1] R. Lifshitz, "The square Fibonacci tiling", J. of Alloys and Compounds 342 (2002) 186.
[2] R. Ilan, E. Liberty, S. Even-Dar Mandel, and R. Lifshitz, "Electrons and phonons on the square Fibonacci tilings," Ferroelectrics 305 (2004) 15.

"Complex Metallic Alloys: Structures, Defects and Plasticity"
Michael Feuerbacher¹
¹Forschungszentrum Juelich GmbH, 52425 Juelich Germany

Presentation Type: Invited Talk
Topic: Mechanical Properties and Applications

Abstract:
Complex metallic alloys (CMAs) represent a class of materials increasingly attracting interest. These materials show characteristic structural features substantially deviating from those of simple metals. They possess large lattice constants and correspondingly a high number of atomic positions per unit cell. As a result, a novel type of local order can form, which is frequently dominated by icosahedral-symmetric atom coordinations. These salient structural features open up the possibility for novel physical properties.

Representing a new field in materials science, the physical properties of CMAs have hardly been investigated to date. However, as a result of the particular structure of CMA phases, principle physical questions arise. In this talk, we will focus on the plasticity of these materials, and the following questions will be addressed: Is the macroscopic plastic behaviour comparable to that of ordinary crystals with simpler structures? What is the structure of defects, particularly of dislocations, if the translational invariant distances are much larger than energetically acceptable Burgers-vector lengths? What are the mechanisms of dislocation movement in such complex structures? Will planar faults be introduced upon dislocation movement?

In this contribution current work in the field of plasticity of complex intermetallic alloys will be reviewed. We will present experimental results on the plastic behaviour of a number of CMAs with up to 1500 atoms per unit cell. It will be demonstrated that completely novel mechanisms occur, involving new types of defect such as the recently discovered metadislocation [H. Klein, M. Feuerbacher, P. Schall and K. Urban, Phys. Rev. Lett. 82, 1999, 3468] and uncommon macroscopic plastic behaviour [M. Feuerbacher, H. Klein and K. Urban, Phil. Mag. Lett. 81, 2001, 639].

"Heterogeneous Nucleation of Icosahedral Phase from f.c.c Phase in Al-Mn-Be(-Si) Alloys"
Eric Fleury¹, Hye-Jung Chang², Won-Tae Kim³, Do-Hyang Kim²
¹Korea Institute of Science and Technology, 130-650 Seoul Korea
²Center for Non-Crystalline Materials, Yonsei University, 120-749 Seoul Korea
³Applied Science Division, Chongju University, 360-764 Chongju Korea

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
Since Shechtman's discovery, it is well known that the metastable icosahedral (i) quasicrystalline phase in binary Al-Mn alloy system forms in a wide range of composition upon rapid quenching of the liquid phase. Recent studies have shown that the addition of Be enhanced the quasicrystalline phase formation in Al-Mn-Be alloys. A few micrometer-sized i-phase were observed in 10 mm diameter rods prepared by conventional casting technique with compositions ranging from Al₉₅Mn₂Be₃ to Al₉₀Mn₆Be₄. In these ternary alloys, the i-phase formed as a primary phase together with a hexagonal approximant phase embedded in the Al solid solution, α_Al [1]. A two-phase microstructure comprised of α_Al + i could be obtained in cast Al-Mn-Be-Si for 2.5≤Mn≤8 at.% [2]. If the role of Si is to extend the composition range for the i-phase formation in these alloys, Be is detrimental for the quasicrystalline forming ability. A minimum of about 1.5 at.%Be is necessary for the i-phase formation under low cooling rate and we demonstrated the heterogeneous crystallization of the α-AlMnSi cubic approximant phase from the i-phase in quaternary Al_92.5Mn_2.5Be_1.5Si_1.5 alloy [3]. However, as the Be content increased to about 5 at.% in both ternary and quaternary alloys, a phase with cubic morphology was detected inside the i-phase particles. Microstructural observations indicated that, upon cooling from the liquid, the AlMn(Si)Be₄ f.c.c. compound (Pearson symbol cF24 with 24 atoms and lattice parameter 6.1 Å) formed as the primary phase and then the heterogeneous nucleation of i-phase took place from this cubic phase. The three-fold symmetry zone axis of i-phase was parallel to [111] zone axis of AlMn(Si)Be₄ phase with ∼2 degrees misorientation.
The aim of this paper is to describe the heterogeneous nucleation of the i-phase from the f.c.c. phase by means of TEM and Auger analyses, and to discuss the role of Be on the formation of the i-phase in these ternary and quaternary Al-rich alloys.

"Bi Nano-Structures Grown on the 5-Fold Surface of Al-Cu-Fe"
Vincent Fournée¹, Hem-Raj Sharma², Masahiko Shimoda², Amy Ross³, Tom Lograsso³, An-Pang Tsai²
¹CNRS- Nancy, 54 042 Nancy France
²NIMS, 1-2-1 Sengen, Tsukuba-shi, Ibaraki 305-0047 Japan
³Ames Laboratory, Iowa State University, Ames, Iowa 50011, USA

Presentation Type: Invited Talk
Topic: Surfaces

Abstract:
The growth of a Bi thin film deposited on the 5-fold surface of an Al₆₃Cu₂₄Fe₁₃ icosahedral quasicrystal is investigated by scanning tunnelling microscopy (STM) and reflection high energy electron diffraction (RHEED). The growth was investigated both at high temperature (250°C) and at room temperature. For deposition at high temperature, we basically observe the formation of the pseudomorphic layer expected based on the previous report by Franke et al. At submonolayer coverage, we found that the island density vary from terrace to terrace, suggesting that nucleation of Bi islands occurs at specific quasilattice sites having a density that is also terrace specific. For room temperature deposition, we observe the formation of crystalline Bi islands with flat tops and steep edges on top of a Bi wetting layer. The Bi islands exist in five different orientations (five-fold twinning) and they have uniform heights corresponding to two atomic layers (~ 6Å) or a multiple of this height. We will argue that the driving force for the island height selection is related to quantum size effects (QSE): the minimization of the energy of the electron confined within the Bi islands favors a preferred thickness. This shows that the unusual electronic structure of the substrate influences the film morphology, in producing a confinement barrier for the electron at the film/substrate interface.

"Dynamics of Phason Fluctuations in the i-AlPdMn Quasicrystal"
Sonia Francoual¹, Frédéric Livet², Marc De Boissieu², Flora Yakhou³, Françoise Bley², Antoine Létoublon⁴, René Caudron⁵, Joseph Gastaldi⁶
¹LTPCM, 38402 Saint Martin d'Hères, France and ILL, BP 156, F-38042 Grenoble, France
²LTPCM, UMR CNRS 5614, ENSEEG-INPG, BP 75, 38402 Saint Martin d'Hères, France
³ESRF, BP 220, 38043 Grenoble Cedex, France
⁴CEA-Grenoble, DRFMC, SP2M, NRS, 17 avenue des Martyrs, 38054 Grenoble Cedex 9, France
⁵LEM, UMR CNRS 104, ONERA, BP 72 92230 Chatillon, France
⁶CRMC2, CNRS, Campus de Luminy, case 913, 13288 Marseille Cedex 9, France

Presentation Type: Contributed Talk
Topic: Thermal and Dynamical Properties

Abstract:
We report on the study of the dynamics of long-wavelength phason fluctuations in the i-AlPdMn icosahedral phase using coherent x-ray scattering (1). When measured with a coherent x-ray beam, the diffuse scattering intensity due to phason modes exhibits a spiky structure or speckles pattern. From room temperature up to 500 °C, the speckle intensity distribution displays no time evolution which is consistent with frozen-in phason fluctuations at low temperature. At 650 °C and as expected for a diffusive process, the intensity correlation function decays exponentially with a characteristic time τ. In agreement with theoretical predictions, we find that τ is proportional to the square of the phason wavelength which demonstrates that phasons are collective diffusive modes in quasicrystals. The evolution of the characteristic time with the temperature indicates that phason modes are thermally activated with an activation energy of the order of 3 eV.

1. S. Francoual et al., Phys. Rev. Lett., 91, (2003), 225501

"X-ray study of the Diffuse Scattering in the i-Zn-(Mg,Ag,Co,Pd)-Sc Quasicrystalline Phases and in the Periodic 1/1 Approximant Zn-Sc"
Sonia Francoual¹, Shiro Kashimoto², Tsutomu Ishimasa², Marc de Boissieu³
¹LTPCM, 38402 St Martin d'Hères Cedex, France
²Division of Applied Physics, Graduate School of Engineering, Sapporo 060-8628, Japan
³LTPCM/ENSEEG, UMR CNRS 5614, INPG, BP 75, 38402 St Martin d'Hères, France

Presentation Type: Poster
Topic: Thermal and Dynamical Properties

Abstract:
We report on absolute scale measurements of the X-ray diffuse scattering in the Zn-Mg-Sc icosahedral quasicrystal (i-QC) and in its periodic 1/1 approximant. Whereas the diffuse scattering in the approximant is purely accounted for by thermal diffuse scattering, this is not the case in the QC where an additional contribution to the diffuse intensity is observed. This additional signal is related to phason fluctuations as indicated by its shape anisotropy and its dependence in the perpendicular component Qper of the Q wavevector. This difference in the diffuse scattering response of the QC and of its crystalline approximant demonstrates that indeed phason fluctuations are specific to the quasicrystalline state. Moreover, when compared with previous absolute scale measurements in the i-AlPdMn phase, the amount of diffuse scattering is found about 3 times smaller in the i-ZnMgSc phase what shows the very high structural quality of this i-phase. This is supported by the presence of a high density of weak Bragg peaks with a high Qper value in the diffraction pattern of i-ZnMgSc. We will report also on the diffuse scattering measured in samples where Mg has been substituted by Ag, Co and Pd, the observed differences pointing to a connection between phason disorder and chemical disorder.

"Superquasicrystalline Tilings with 8-, 10-, and 12-fold Point Symmetries"
Nobuhisa Fujita¹, Komajiro Niizeki²
¹Arrhenius Laboratory, Stockholm University, 10691 Stockholm, Swede
²Department of Physics, Tohoku University, Sendai 980-8578, Japan

Presentation Type: Poster
Topic: Structure

Abstract:
We present several limit-quasiperiodic tilings with 8-, 10-, and 12-fold point symmetries. These tilings are generated with inflation procedures as in the case of the Penrose tilings. Yet they are, to the best of our knowledge, the first tilings ever known with limit-quasiperiodic order and non-crystallographic point symmetry, and we categorize them as superquasicrystalline tilings.[1] The internal space structures of such tilings are not as simple as quasicrystalline tilings, because the atomic-surfaces depend on the lattice points on the relevant hyperlattices. We numerically investigate such atomic-surfaces by mapping the real-space structure into the internal space. It is strongly suggested that their perimeters have fractal shapes. The structure factors of these tilings are also calculated, in which successive generations of super-quasilattice reflections are clearly observed.

[1] K. Niizeki and N. Fujita, cond-mat/0411134.

"Molecular Dynamics Simulation of Aluminium Diffusion in Decagonal Quasicrystals"
Stephen Hocker¹, Franz Gähler¹, Peter Brommer¹
¹ITAP, Universität Stuttgart, 70550 Stuttgart, Germany

Presentation Type: Poster
Topic: Thermal and Dynamical Properties

Abstract:
Aluminium diffusion in decagonal Al-Ni-Co and Al-Cu-Co quasicrystals is investigated by molecular dynamics simulations with newly developed EAM potentials, and the results are compared to previous work with effective pair potentials. With both types of potential, strong aluminium diffusion is observed above two thirds of the melting temperature, and the general behaviour of the system is quite similar. Compared to pair potentials, however, the diffusivities in the decagonal plane are enhanced with EAM potentials, which can be attributed to additional diffusion processes. As a further difference, with EAM potentials the transition metal atoms are much more mobile, so that their diffusion is now also measurable. The diffusion constant is measured as a function of temperature and pressure, and the activation enthalpies and activation volumes are determined from the resulting Arrhenius plot. For a number of important diffusion processes, the energy barriers have been determined with molecular statics simulations. These barriers roughly agree with the activation enthalpies determined from the Arrhenius plot. The qualitative behaviour of the dynamics is also confirmed by ab-initio simulations.

"Cohomology and Mutual Local Derivability of Quasiperiodic Tilings"
Franz Gähler¹
¹ITAP, Universität Stuttgart, 70550 Stuttgart, Germany

Presentation Type: Poster
Topic: Mathematics

Abstract:
The local isomorphism class (LI class) of a tiling can be given a metric, which makes it a compact topological space, called the hull of the tiling. Many important properties of the tiling depend on the topology of its hull, which can be characterized to some extent by topological invariants like the Cech cohomology groups. Tilings wich are mutually locally derivable (MLD) have homeomorphic hulls, and thus isomorphic cohomology. For projection tilings with polygonal windows, it is relatively easy to compute the cohomology. We shall investigate here in how far the cohomology can actually distinguish different MLD classes. For this purpose, the MLD classes of tilings with dihedral or icosahedral symmetry are classified, and their cohomology is computed. In order to make the classification finite, the MLD classes considered had to be restricted in complexity. It turns out that with very few exceptions, different MLD classes also have different cohomology. Only very rarely, the cohomology of different MLD classes is isomorphic either by accident, or due to some non-local relationship between the hulls. For the MLD classes of two-dimensional tilings, also their frequency module is computed, which is the module generated by the frequencies of all finite subpatterns. If suitably normalized, the frequency module is an invariant of the MLD class, and plays an important role in the gap labelling theorem.

"In Situ Observation of Pore Evolution during Melting and Solidification of Al-Pd-Mn Quasicrystals by Synchrotron X-Ray rRadiography"
Thomas Schenk¹, Guillaume Reinhart², Joseph Gastaldi³, Holger Klein⁴, Juergen Haertwig¹, Nathalie Mangelinck², Benjamin Grushko⁵, Henri Nguyen Thi²
¹ESRF, Grenoble France
²CNRS-L2MP, Marseille France
³CNRS, 13288 Marseille cedex 9 France
⁴Laboratoire de Cristallographie, Grenoble France
⁵IFF, Juelich Germany

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
It is now generally admitted that pores are intriguing special features of quasicrystals. Two hypotheses have been put forward to explain their origin, namely the thermal vacancy condensation [1] and the hierarchical structure [2], but none of them is satisfactory [3]. Therefore, we have performed an “in situ” and real time observation of the pore evolution during directional solidification and melting cycles of an icosahedral Al-Pd-Mn bi-grained sample by synchrotron X-ray radiography. Rather surprisingly, no pore was observed to grow during the solidification stages. Nucleation and growth of pores were firstly seen during melting. These pores were subsequently shrinking during resumption of directional solidification. All these results will be related in detail, and tentative explanations will be proposed.

[1] C. Beeli, T. Godecke and R. L?ck, Phil. Mag. Letters 78, (1998), 339.
[2] L. Mancini, C. Janot, L. Loreto, R. Farinato, J. Gastaldi and J. Baruchel, Phil. Mag. Letters 78, (1998), 159.
[3] S. Agliozzo, J. Gastaldi, H. Klein, J. Hartwig, J. Baruchel and E. Brunello Phys. Rev. B 6914 (14) (2004) 144204.

"Study of Defects in a "Highly Perfect" Icosahedral Al-Pd-Mn Quasicrystal Grain by Synchrotron X-ray Topography"
Silvano Agliozzo¹, Jose Baruchel², Marc de Boissieu³, Joseph Gastaldi⁴, Jurgen Härtwig², Holger Klein⁵, Lucia Mancini⁶, Thomas Schenk²
¹Institute for Scientific Interchange Foundation, 10133 Torino, Italy
²European Synchrotron Radiation Facility, 38043 Grenoble, France
³LTPCM-INPG, 38402 St Martin d'Hères Cedex France
⁴CRMN-CNRS, F-13288 Marseille, France
⁵CNRS, Lab. Cristallographie, 38042 Grenoble, France
⁶Sincrotrone Trieste, 34012 Basovizza, Italy

Presentation Type: poster
Topic: Structure

Abstract:
Icosahedral Al-Pd-Mn quasicrystal grains, produced by slow solidification techniques, are probably those with the highest structural perfection obtained up to now. The best grains exhibit rocking curve widths comparable to those of the best metallic crystals and show diffraction effects, which have been explained in the framework of the dynamical diffraction theory, like anomalous transmission [1, 2]. However, they still contain a considerable amount of defects, which are the subject of intense studies [3, 4].
Defects in such kind of samples with a rather good structural quality are mostly: dislocations, pores and their strain fields, phason strain. Up to now the study of dislocations in quasicrystals was mainly carried out by electron microscopy (TEM) [5, 6]. This technique requires a minimum dislocation density of about 10⁵ to 10⁶ cm/cm³ in as-grown grains [5]. As it is known from crystals, this is the lower limit under which it is difficult to detect dislocations by TEM. Therefore, further investigations are needed using a technique appropriated to characterise defects at a low density level. X-ray diffraction topography is such a technique. It allows to image dislocations in relatively thick grains (0.2 –1 mm) in concentrations ranging from one single dislocation up to a density of roughly less than 10⁴ cm/cm³ [7].
We further investigated a rather perfect icosahedral Al-Pd-Mn grain with X-ray topography, in combination with phase-sensitive X-ray radiography and X-ray diffraction methods. We detected and characterized individual dislocations, obtained hints that ìhighly perfectî icosahedral Al-Pd-Mn quasicrystal grains contain practically no micrometer sized pores. In addition we confirmed the rather high perfection by local measurements of rocking curve widths and of anomalous X-ray transmission.

[1] S. W. Kycia et al., Phys. Rev. B 48, 3544 (1993)
[2] Y. Zhang et al., Acta Cryst. A 58, 385 (2002)
[3] J. Härtwig et al., J. Phys.D: Appl. Phys 34, A103 (2001)
[4] J. Gastaldi et al., Phil. Mag. A 83, 1 (2003)
[5] M. Wollgarten and K. Urban, Lectures on Quasicrystals, edited by F. Hippert and D. Gratias (Les Editions de Physique Publishing), p535 (1994)
[6] D. Caillard et al., Phil. Mag. A 80, 237 (2000)
[7] A. Authier, NATO ASI Series, Series B: Vol. 357, X-ray and Neutron Dynamical Diffraction: theory and applications, Erice, Italy, edited by A. Authier, S. Lagomarsino and B. K. Tanner (New York: Plenum Press), p. 1 (1996)

"Potential Energy Surfaces for Adsorbates on Al-rich Quasicrystal Substrates"
Chandana Ghosh¹, Dia-Jang Liu¹, Cynthia Jenks¹, James Evans¹, Patricia Thiel¹
¹Iowa State University, Ames, IA 50011 USA

Presentation Type: Poster
Topic: Surfaces

Abstract:
We examine the potential energy surface (PES) for the binding of an adsorbed Al atom on AlFeCu and AlPdMn quasicrystal surfaces. Appropriate geometric models are used to fix the positions of the substrate atoms. The interaction between the adsorbate and the substrate is described using simple pairwise Lennard-Jones (LJ) interactions. However, the LJ parameters chosen to fit 'exact' binding energies for Al on relevant single crystal substrates as obtained from density functional theory calculations. The form of the PES is used to identify and to characterize deep adsorption sites which likely act as traps for the heterogeneous nucleation of Al-islands during deposition. We also make qualitative comparisons between the PES for Al and Cu on AlFeCu (and for Al and Pd on AlPdMn).

"Intermediate-Range Order in Amorphous Metal Alloys: The Role of Icosahedral Order in Glass Formability and Stability"
Patrick Gibbons¹, Youtao Shen¹, Karyn Spence¹, Li-Qian Xing¹, Kenneth Kelton¹
¹Washington University, St Louis, MO 63130 United States

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
There are strong reasons to believe that some amorphous metal alloys have intermediate-range order beyond that expected in 'simple' glasses, in which there is no order beyond the second or third neighbor. Kelton and his collaborators have shown that a Ti-Zr-Ni alloy with equal numbers of Ti and Zr atoms, and 21 at% Ni, contains icosahedral clusters or fragments of icosahedral clusters in the undercooled liquid and even at temperatures above the liquidus (Kelton 2003). These clusters should likewise be present in a metallic glass obtained by rapid quenching. Direct detection of such small clusters is desirable, but detection by transmission electron microscopy or diffraction is difficult. Fluctuation electron microscopy (Dash 2003) produces information about higher-order correlations than pair correlations, and has been used successfully to replace the previously accepted structural model of amorphous Si with a better one (Voyles 2001). We are studying Ti-Zr-Ni, Zr-based metallic glasses with Ti added, and high-Al metallic glasses by FEM. Preliminary results will be described and compared with results from high-energy x-ray diffraction studies of undercooled liquids (Kelton 2003).

In addition, because the paracrystalline model now generally accepted for amorphous Si (Voyles 2001) seems applicable to glasses containing icosahedral clusters, we have attempted a simulation of FEM using a dense random packing (DRP) and a DRP modified by the addition of small (quasi)crystalline clusters. A cube-shaped DRP with 44 % packing fraction can be obtained in just an hour on a modern computer. At this packing fraction, however, density fluctuations are large enough to overwhelm the FEM signal expected to be produced by nanocrystals small and dilute enough to escape detection by diffraction. We are working to produce more tightly packed DRPs with lower density fluctuations.

R. K. Dash et al., J. Phys Conden. Matt Vol. 15 (2003) p. 2425.
K. F. Kelton et al., Phys. Rev. Lttrs. Vol 90 (2003) 195504.
P. M. Voyles et al., J. Appl. Phys. Vol. 90 (2001) p. 4437.

"Zn_1-xPd_x (x=0.14 to 0.32) Intergrowth Compounds: Towards 1D Quasicrystal Approximants"
Olivier Gourdon¹, Zunbeltz Izaola², Luis Elcoro², Vaclav Petricek³, Gordon Miller⁴
¹Los Alamos National Laboratory, Los Alamos, NM 87545 USA
²Univ. Pais Vasco, Fac Ciencias, Dept. Fis. Mat. Condensada, Apdo 644, E-48080 Bilbao, Spain
³Institute of Physics AVCR, Cukrovarnicka 10, 16t2 53 Praha, Czech Republic
⁴Department of Chemistry and Ames Laboratory, Iowa State University, Ames, Iowa 50011-3111

Presentation Type: Contributed Talk
Topic: Structure

Abstract:
Our investigations in the Zn rich portion of the Zn-Pd system are related to the idea that a prototype 1D quasicrystal, as defined by the Fibonacci sequence, can be regarded both as a quasicrystal and as an incommensurately modulated crystal. Consequently, there are two different ways to achieve the atomic structure. According to Hume-Rothery's supposition, which is verified in the Zn-Pd system, the crystal structures of various noble metal alloys are invariant with respect to specific valence electron concentrations (vec = e/a ratio) called 'electron compounds.' Since the formation of quasicrystalline compounds corresponds often to a specific valence electron concentration (vec) ≈1.6, we targeted such electron concentrations in the Zn-Pd system, i.e. Zn_1-xPd_x, x ~ 0.2, which leads to the γ-brass phase.

Six different, closely related, new structures in the Zn_1-xPd_x system have been synthesized. All data sets were consistent with orthorhombic symmetry and could be described either in an F or B (depending on the phase) centered orthorhombic Bravais lattice (a≈9.1 Å, b≈ n≈ 4.3 Å and c≈12.9 Å (where n is an integer). Misfit character of all the structures with two distinct main subsets and satellite reflections is clear from the diffraction patterns. Among these six structures, two compounds with formulations Zn₁₁Pd₂ (Zn_.846Pd_.154) and Zn₂₁₂Pd₆₄ (Zn_.768Pd_.232) have been refined by single crystal X-ray diffraction using classical refinement methods. Analogies with the Fibonacci sequence are possible if we define dimmer of icosahedron DI as short segments (S) and trimer of icosahedron TI as long segments (L). By defining these S and L segments, Zn₁₁Pd₂ and Zn₂₁₂Pd₆₄ correspond to the two first simplest approximants of a hypothetical 1D-quasicrystal. This binary code of S and L segments may also be presented by a so-called Farey tree. Finally, we propose a refinement using the (3+1)D formalism (superspace group Xmmm(0β0)s00) which allows us to refine all the structures of that family either the commensurate ones or the incommensurate ones.

"Shelling and Coordination Sequences for Aperiodic Tilings"
Michael Baake¹, Franz Gähler², Uwe Grimm³
¹Fakultät für Mathematik, Univ. Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
²Institut für Theoretische und Angewandte Physik, Pfaffenwaldring 57, 70550 Stuttgart, Germany
³Applied Mathematics Department, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK

Presentation Type: Contributed Talk
Topic: Structure

Abstract:
We consider averaged shelling and coordination numbers of aperiodic tilings. Shelling numbers count the mean number of vertices lying on radial shells of given radius around a vertex. Coordination numbers count the number of vertices in a coordination shells of a vertex, which means the number of vertices that can be reached by a given minimum number of steps along the edges of the tiling. We explain how these numbers can in principle be calculated, and discuss their behaviour with increasing distance. For the Ammann-Beenker tiling, we find that coordination shells consist of a number of complete shelling orbits, which allows us to calculate coordination numbers for rather large distances explicitly. The relation to topological invariants of tilings is discussed.

"Interplay between Magnetic and Spatial Order in Quasicrystals"
Elena Vedmedenko¹, Uwe Grimm²
¹Institut für Angewandte Physik, Jungiusstr. 11, 20355 Hamburg, Germany
²Applied Mathematics Department, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
The stable magnetization configurations of ferromagnets and antiferromagnets on quasiperiodic tilings are investigated theoretically. The exchange coupling is assumed to decrease exponentially with the distance between magnetic moments. It is demonstrated that the combination of geometric frustration and the quasiperiodic order of atoms leads to complicated noncollinear ground states. For both ferromagnets with a strong dipolar coupling and antiferromagnets, the structure can be divided into subtilings of different energies. The symmetry of the subtilings depends on the quasiperiodic order of magnetic moments. The subtilings are spatially ordered. However, the magnetic ordering of the subtilings in general does not correspond to their spatial arrangements. While subtilings of low energy are magnetically ordered, those of high energy can be completely disordered due to magnetic frustration.

"On Structural Complexity without Local Pentagonal Symmetry"
Yuri Grin¹, Magnus Botroem¹
¹Max-Planck-Institut fuer Chemische Physik fester Stoffe, 01187 Dresden Germany

Presentation Type: Invited Talk
Topic: Structure

Abstract:
Local pentagonal or pseudo pentagonal symmetry is one of the basic characteristics of the crystal structures of approximants (cf. Mackay or Bergman clusters). Several binary and ternary transition metal compounds with aluminium and gallium in the concentration region close to quasicrystalline phases and their approximants reveal this feature resulting in a strong structural complexity in different variations.

Detailed investigations were performed in binary systems of 4d and 5d transition metals (i.e., rhodium and iridium) with aluminium and gallium. Several new compounds with large crystal structures and different levels of complexity occur in the aluminium- or gallium-rich regions. Here, the complexity appears without local pentagonal or pseudo pentagonal symmetry, e.g., the binary compound Ir₁₃Al₄₅ is the first representative of a newstructure type (Pearson symbol oP232). The structure shows a partial local disorder in vicinity of some aluminium positions. This feature resembles many approximant structures. But, in contrary to the latter, covalent interactions between Ir and Al result in very irregular coordination of the iridium atoms. This causes low-symmetrical columnar packing of aluminium polyhedra centred by iridium atoms showing a pseudo pentagonal motif only in very rough approximation.

Chemical bonding in different structures belonging to this group is attempted to be interpreted applying the electron localization function. This kind of bonding analysis allows preliminary classifying of interatomic contacts in different groups and reveals a transition from two-centre to multi-centre bonds even in the neighbouring regions of the crystal structure.

"The Local Density of States on the 2-Fold Surface of the Icosahedral AlPdMn Quasicrystal"
Oliver Groening¹, Roland Widmer¹, Pascal Ruffieux¹, Pierangelo Groening¹
¹Empa Materials Science and Technology, CH-3602 Thun Switzerland

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
The advances in crystal growth and surface preparation techniques of quasicrystals (QC) have opened the possibility to investigate the local electronic structure by means of low temperature scanning tunneling spectroscopy (STS). We will present experimental data acquired on the 2-fold surface of Al₇₀Pd₂₀Mn₁₀ QC, where we succeeded in measuring atomically resolved scanning tunneling microscopy (STM) topography images and could combine the topography measurement with low temperature (4 K) STS measurements. We compare the atomically resolved STM images of the 2-fold surface to the 5-fold surface and to a model structure of the icosahedral AlPdMn QC based on the M-Model developed by Kasner and Papadopolos [1]. We find a good agreement with the 5-fold surface, where the position of the spots in the Fast Fourier Transform (FFT) of the 2-fold surface agree with projections of the FFT spots of the 5-fold surface. Also a good agreement with the model structure could be observed, indicating that the 2-fold surface prepared by sputter annealing cycles is close to the bulk termination, as observed for the 5-fold surface.

The STS measurements on this surface reveal a rich spectrum of states near the Fermi energy (E_F ± 0.5 eV) with strong local variations in peak positions and peak intensities. In STS spectra averaged over large surface areas (>10x10 nm²) a large (1 eV) parabolic pseudo gap is observed. As in tunneling experiments preformed on the 5-fold surface [2] a sharp dip (20-40 meV FWHM) in this large pseudo gap is observed, but not as an spatially extended but as a well localized feature. In order to understand qualitatively the observed spatial variability of the STS spectra we have performed tight binding calculations based on finite patches of the penrose structure and on the M-Model for the icosahedral AlPdMn QC. Special emphasis is put on the calculation of the LDOS, which is compared with the experimental results.

[1] Z. Papadopolos, G. Kasner, et al., PRB 66 (2002) 184207
[2] R. Escudero, J.C. Lasjaunias, et al.,J. Phys.: Condens. Matter 11 (1999) 383

"Derivation Rules for Decagon Coverings and HBS-Tilings"
Petra Gummelt¹
¹University of Greifswald, D-17487 Greifswald Germany

Presentation Type: Poster
Topic: Mathematics

Abstract:
For several decagonal quasicrystals and approximants, coverings of a single decagon cluster as well as HBS-tilings are used to describe the atomic structure. We study the relationship betweeen these two types of models in terms of geometrically defined transformation rules. In previous work, we discussed perfectly and randomly ordered decagon coverings and underlying larger HBS-supertilings. Here we consider HBS-tiles and decagon clusters at the same scale. In the perfect case, the correspondence is one-to-one and there are simple mutual local derivation (MLD) rules. If we describe a decagonal structure in terms of a 20 Å decagon cluster covering model satisfying the perfect decagon overlap rule, it can easily be replaced by a perfect HBS-tiling model with approximately 6.4 Å edge length. Vice versa, if we want to highlight all potential 20 Å decagon clusters in perfect HBS-tiling models with 6.4 Å edge length, we have to consider more than the observable decagon-shaped configurations. Most of the perfectly marked decagons are hidden, but they can be reestablished by using the MLD rules.
In the random case, we start from the maximal random HBS-tiling ensemble and study the density of decagon clusters. Vice versa, given a random decagon covering ensemble we get by suitably relaxing the overlap rule, we show that for every covering there is a corresponding HBS-tiling with equal edge length, but it is not unique. Moreover, we do not reach the complete HBS-random tiling ensemble. Since various HBS-tilings, which are used to model approximants, are not obtained this way, we are looking for alternative random decagon coverings.

"Low Temperature X-Ray Diffraction Studies of Decagonal Quasicrystals in the Ternary Al-Co-Ni System"
Kai Hassdenteufel¹, Walter Steurer¹
¹Laboratory for Crystallography, ETH Zurich, Zurich, Zurich 8093 Switzerland

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
In a project to clarify the low-temperature (LT) stability of decagonal Al-Co-Ni, several experiments have been carried out at temperatures between 10 K and 200 K. On one hand, this has been high-energy ball milling at liquid-nitrogen temperature and room temperature and subsequent annealing at temperature up to 900 °C. On the other hand a series of in-house and Synchrotron-based single-crystal LT X-ray diffraction experiments were performed on the decagonal phase with Type I superstructure (S1 and S2 satellite reflections) as well as on the basic Ni-rich phase. The use of area detectors also allowed in some cases to study the variation of diffuse scattering intensities. Beside in-situ LT experiments also ex-situ- long-time LT annealing experiments under high-pressure have been performed (diamond anvil cell in liquid nitrogen, 8 weeks, ≈ 4 GPa). The results of the experiments will be presented on the poster-presentation.

"Metadislocation Reactions and Networks in the Complex Metallic Alloy ξ'-Al-Pd-Mn"
Marc Heggen¹, Michael Feuerbacher¹
¹IFF, Forschungszentrum Juelich, 52425 Juelich Germany

Presentation Type: Poster
Topic: Mechanical Properties and Applications

Abstract:
The mechanisms of plastic deformation of complex metallic alloys (CMAs) and the defects mediating plastic deformation are largely unknown. Due to the large lattice parameters encountered in these materials, the concepts that are used to describe the plastic deformation of simple crystalline materials fail. For example, the introduction of a perfect dislocation into a CMA would lead to a high elastic line energy, which might exceed physically reasonable values. In the complex intermetallic phase ξ'-Al-Pd-Mn a novel mechanism of plastic deformation involving a new type of structural defect called metadislocation (H. Klein, M. Feuerbacher, P. Schall and K. Urban, 1999, Phys. Rev. Lett. 82, 3468) was found. As yet, the investigation of metadislocations in ξ'-Al-Pd-Mn in the literature was limited to the structure and properties of single metadislocations only.

We present a transmission electron microscopy study on metadislocation networks and reactions. This work shows that despite of the structurally complex and odd nature of metadislocations, they have the same basic attributes like dislocations in structurally simple materials. Metadislocations can, for instance, form dipoles or dissociate into partials with smaller Burgers vectors, leading to a decrease of the elastic line energy. Furthermore, connected groups of metadislocations can assume large and complex network structures. By this mechanism, effective large Burgers vectors, contributing massively to plastic strain, can be distributed over a large portion of the material.

"Predicting the Structure of Co-Rich Decagonal Al-Co-Ni"
Nan Gu¹, Christopher Henley¹, Marek Mihalkovic
¹Cornell University, Ithaca, NY 14853-2501 USA
²Slovak Academy of Sciences, Dubravska Cesta 9, Bratislava Slovakia 84228

Presentation Type: Poster
Topic: Structure

Abstract:
We apply the systematic, almost-ab-initio, decoration-based methods of Mihalkovic et al. [1] to predict the structure of the "basic Co-rich" subphase of the d(Al₇₀Co₂₀Ni₁₀) layered quasicrystal. Our inputs are interatomic pair potentials derived using Generalized Pseudopotential Theory and experimentally determined lattice constants. We assume initially that the quasicrystal can be described by a Penrose tiling of edge 2.45 Å with properly placed atoms on the Penrose tiles. Monte Carlo annealing is used to find low-energy configurations. Motifs abstracted from these results are converted into rules to decorate 4.0 Å edge tiles (larger, thus having fewer degrees of freedom). Simulations at this scale are used to develop a decoration model at an even larger scale. In contrast to the "basic Ni-rich" d(AlNiCo) subphase [1], the structure is dominated by decagon clusters with edge length 4 Å, having pentagonal cores. The final level of description is equivalent to a 10.5 Å edge binary tiling, with an even/odd alternation on the small sites. We come close to predicting the experimentally determined structure of W(AlCoNi), a crystalline approximant of "basic Co". [2] Relaxation and molecular dynamics are used to explore subtler aspects corresponding to small energy differences, such as (i) relative orientation of neighboring clusters (ii) puckering of atoms and doubling of lattice constant c from about 4 Å to about 8 Å.

[1] M. Mihalkovic et al., Phys. Rev. B 65, 104205 (2002).
[2] K. Sugiyama, S. Nishimura, and K. Hiraga, J. Alloys Comp. 342, 65 (2002).

"Chemical Decoration in Cubic Approximants and Quasicrystal in the Al-Cu-Fe System"
Virginie Simonet¹, Francoise Hippert², Richard Brand³, Yvonne Calvayrac⁴, Juan Rodriguez-Carvajal⁵, Anne Sadoc⁶
¹Laboratoire Louis Neel, CNRS, BP 166, 38042 Grenoble, France
²LMGP, ENSPG, Domaine Universitaire , BP46 , 38402 Saint Martin d'Hères France
³Institute of Physics, Universitat Duisburg-Essen, D-47048 Duisburg, Germany
⁴CECM, CNRS, 15 rue Georges Urbain, 94407 Vitry, France
⁵Laboratoire Leon Brillouin (CEA-CNRS) , CEA Saclay, 91191 Gif sur Yvette, France
⁶LPMS, Universite de Cergy-Pontoise, 95031 Neuville sur Oise, France

Presentation Type: Contributed Talk
Topic: Structure

Abstract:
In the Al-Cu-Fe quasicrystal (QC), an accurate description of the atomic positions is available but their chemical occupation remains uncertain. A way to tackle this problem is to study approximant phases. A cubic 1/1 approximant (the so-called α phase) is stabilized by a few percent Si for compositions around Al₅₅Si₇Cu_25.5Fe_12.5, close to those of the stable QC. Its crystallographic structure can be described by Mackay clusters and is nearly identical to that of the Al_71.1Si₇Cu_3.8Fe_17.5 cubic phase (M. Cooper, Acta. Cryst 23 (1967) 1106). Nevertheless these two cubic phases occupy distinct composition domains in the Al(Si)-Cu-Fe phase diagram and present different electronic properties (F. Hippert, V; Simonet, R.A. Brand and Y. Calvayrac, abstract submitted to this conference). It is therefore essential to analyse their structure in detail. Previously published results for the α phase were controversial. By performing a joint X-ray and neutron powder diffraction data refinement and X-ray absorption Fine Structure (EXAFS) analysis at both the Cu and Fe K-edges, we could determine the chemical decoration in several α samples. This allowed us to understand the chemical substitution law defining the composition domain of the α phase and the differences between the two cubic phases which mainly concern the location of Fe atoms. In addition, a successful analysis of EXAFS spectra at the Cu and Fe-K-edges in the quasicrystal Al₆₂ Cu_25.5Fe_12.5 could be performed by starting from the local chemical order in the α-Al₅₅Si₇Cu_25.5Fe_12.5. This lead us to propose a model for the chemical short-range order of the QC.

"Electronic Properties of Approximants and Quasicrystals in the Al(Si)-Cu-Fe System from Magnetic Susceptibility Measurements"
Francoise Hippert¹, Richard Brand², Virginie Simonet³, Yvonne Calvayrac⁴
¹LMGP, ENSPG, Domaine Universitaire, BP , 38402 Saint Martin d'Hères , France
²Institute of Physics, Universitat Duisburg-Essen, D 47048 Duisburg, Germany
³Laboratoire Louis Neel, CNRS, BP 166, 38042 Grenoble, France
⁴CECM, CNRS, 15 rue Georges Urbain, 94407 Vitry, France

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
In the Al-Cu-Fe system, several high order approximants (pentagonal, rhombohedral and orthorhombic) have been identified for compositions close to those of the stable quasicrystal (QC). A particularity of this system is the existence of two low order cubic 1/1 approximants, stabilized by a few percent Si, which occupy distinct composition domains. We have systematically investigated the magnetic properties of all these phases, as well as those of periodic non-approximant phases such as β and λ in the temperature range [4 K – 300 K]. Large variations of the susceptibility are observed, revealing an evolution of the conduction electron susceptibility. (The negative Larmor core contribution can be considered as identical for phases with similar compositions). No localized moments are present, if one excepts very small, sample-dependent, Curie terms in several cases. The conduction electron susceptibility is directly linked to the density of states (DOS) at the Fermi level. It varies from a large positive value in the non-approximant phases to a very small one in QCs and high order approximants. The behavior of the cubic Al₅₅Si₇Cu_25.5Fe_12.5 phase is very close to that of QCs and high order approximants, consistent with their similar transport properties (A. Quivy at al., J. Phys. Cond. Matter 8 (1996) 4223). In marked contrast the other cubic phase, Al_71.1Si₇Cu_3.8Fe_17.5, exhibits a strong electronic contribution, close to that found in non-approximant phases. The two cubic phases have the same atomic structure, described by Mackay clusters. This surprising difference in their DOS must be related to their different chemical decoration (V. Simonet, F. Hippert and R.A. Brand, abstract submitted to this conference). The DOS variations are confirmed by the analysis of the longitudinal relaxation rate of Al²⁷ nuclei in Nuclear Magnetic Resonance experiments.

"Phonon Spectra of Al-Ni-Co Decagonal Quasicrystals Modeled by Ab-Initio Interatomic Pair Potentials"
Seiji Hiramatsu¹
¹Chuo University, Bunkyou-ku, Tokyo 1128551 Japan

Presentation Type: Poster
Topic: Thermal and Dynamical Properties

Abstract:
Ternary Al-Ni-Co forms thermodynamically stable and highly perfect decagonal quasicrystals (QC) over a wide range of composition. One may predict QC structures by using interatomic pair potentials calculated from ab initio total-energy calculation. We have done such modeling for Al-Ni-Co decagonal QC in a wide range of composition. Our model structure consists of two atomic layers separated by 2 Å along the periodic axis (c-axis) and each layer is properly decorated two-dimensional Penrose tiling with a quasilattice constant 2.45 Å. We found two different typical tiling patterns for different compositions, that is, Ni-rich and Co-rich ones. In the present paper, we will report phonon spectra of these model QC. Atomic positions in the original model structures are at special positions in the two dimensional Penrose tiling. To calculate phonon spectra within a harmonic approximation, the atomic positions are relaxed to their equilibrium ones by using the interatomic forces calculated from the pair potentials. For the 2-layer model with a period of 4 Å, the dynamical matrix has a few negative eigenvalues and it turns out thatsome of the atoms are unstable against displacement along the c-axis. We try to construct the 4-layer model with a period of 8 Å by stacking the original 2-layer models and relaxing the atoms to preserve a mirror symmetry perpendicular to the c-axis. The phonon spectra for the 4-layer model are well-defined and acoustic modes and a bunch of optic modes are obtained. In particular, we find the optic mode with very low frequencies about a few meV. We shall present more detailed analysis of the low-frequency optic modes and discuss difference in the phonon spectra between the two different model structures for the Ni-rich and Co-rich phases.

"Short-Range Order in Undercooled and Stable Melts Forming Quasicrystals and Approximants"
Dirk Holland-Moritz¹
¹DLR, Institut fuer Raumsimulation, D-51147 Koeln Germany

Presentation Type: Invited Talk
Topic: Structural Evolution and Phase Stability

Abstract:
Following the pioneering work by Frank, an icosahedral short-range order should be energetically favored in undercooled metallic melts. This hypothesis was recently experimentally confirmed by diffraction experiments on stable and undercooled melts of pure metals.

In this work systematic in-situ studies on the short-range order of stable and undercooled liquids of alloys (Al₁₃(Co,Fe)₄, Al-Cu-Fe and Al-Cu-Co) forming quasicrystalline and polytetrahedral phases are presented. The liquids were containerlessly processed und undercooled by use of the electromagnetic levitation technique, which was combined with elastic neutron scattering and with energy dispersive diffraction of synchrotron radiation, in order to determine the structure factors of the liquids.
The diffraction studies indicate that an icosahedral topological short-range order prevails in all of the investigated stable and undercooled melts. For Al₁₃(Co,Fe)₄ melts partial structure factors were determined by variation of the Co/Fe ratio. This allows to analyze both the topological and the chemical short-range order as a function of the temperature. The partial structure factors indicate that the icosahedral topological short-range order is accompanied by a pronounced chemical short-range order such that the first coordination shell around the transition metal atoms consists preferentially of Al-atoms. An enhancement of the topological and the chemical short-range order is observed, if the temperature is decreased. The influence of the short-range order in the liquid phase on the nucleation behavior of solid phases in undercooled liquids is discussed.

This work was supported by Deutsche Forschungsgemeinschaft (DFG) under contract Nos. Ho1942/1, Ho1942/2 and Ho1942/4.

"New Methods for Solving Approximant and Quasicrystal Structures."
Sven Hovmöller¹, Xiaodong Zou¹, Markus Döblinger², Benjamin Grushko³
¹Stockholm University, SE-191 45 Stockholm Sweden
²Department of Materials, Oxford University, England

Presentation Type: Poster
Topic: Structure

Abstract:
Diffraction patterns of decagonal approximants and quasicrystals have rings of 10 very strong reflections at 2.0 Å resolution. In 3D, these reflections form a sphere. Crystallographic structure factor amplitudes and phases of those strong reflections are very similar, not only between a series of τ-related inflated approximants, but also between quite unrelated approximants, such as monoclinic and orthorhombic. These few reflections are so strong that they alone account for about half the total scattering power. Thus, an essentially complete crystal structure is obtained from just these very few reflections. Since the phases are similar from one approximant to another, we can solve the approximant structures from one structure that is well characterized, for example from X-ray crystallography. When this method was tested on structures solved by X-ray crystallography, typically over 90 % of all unique atoms are found at their correct positions, with just a few missing atoms or spurious peaks. The highest peaks correspond to heavy metal atoms Co, Ni etc. When applied to unknown structures where electron diffraction data are available, chemically reasonable structures were obtained. For τ-related inflated approximants, the strong reflections have hkl indices that are related as Fibonacci numbers. If 5 0 13 is strong in one approximant, then 8 0 21 is strong in the τ-inflated approximant. This method can be used for extrapolating to hypothetical τ-inflated structures that have not (yet) been found. In the Al-Co-Ni system, many different pseudo-decagonal (PD) approximants are found (Gruschko et al. 1998), called PD1, PD2... Many are related in series with one or two unit cell dimensions related by τ. For example PD2 with a = 23.2 Å, b = 32.0 Å and PD3 with a = 37.7 Å, b = 51.8 Å have both a and b axes related as τ. The next step in this τ-inflation is PD9 with a = 60.9 Å, b = 83.8 Å, the largest approximant in this system. The τ-inflated PD9 is related to PD2 such that an atom in PD2 becomes an icosahedral cluster in τ-PD9, an icosahedral cluster in PD2 becomes a ring of 10 icosahedral clusters in τ-PD9 etc ab infinito...

Reference Gruschko et al. J.Alloys Comp. 280 (1998) 215.

"What Good is Cohomology?"
Benji Fisher¹, John Huesman², David Rabson²
¹Boston College, Chestnut Hill, MA 02467 USA
²University of South Florida, Tampa, FL 33620 USA

Presentation Type: Poster
Topic: Mathematics

Abstract:
At previous meetings, we have demonstrated a homological formulation of Fourier-space crystallography. The formulation is mathematical, and while not difficult, it takes more time than available in a fifteen-minute talk to absorb. Before investing the time, the working crystallographer will want to know what good it is. Following are several things it has accomplished: (1) the complete classification of the two-dimensional reciprocal lattices; (2) the complete classification of plane groups (space groups in two dimensions); (3) the complete classification of the types of non-symmorphic elements in three dimensions (homological invariants); (4) prediction of an undiscovered type of symmetry and (possibly) of its experimental signature; and (5) development of an automated code for the calculation of all space groups derived from any given point
group and lattice.

Previous classifications of space groups have been limited in three ways. First, they have assumed that the point group of a lattice is no larger than necessary to accommodate the point group of the Fourier density. Physically, this restriction made sense for periodic crystals, but its application to quasiperiodic crystals is untested. Second, previous classifications have either dealt only with lattices of minimal rank compatible with the holohedry (thus excluding modulated crystals), or else they have proceeded one lattice at a time. Third, previous methods have had difficulty dealing with the "non-standard" lattices of 46-fold and higher symmetry. We lift these limitations, as well as the need in the pioneering work of Rokhsar, Wright, and Mermin on which our formulation is built to find a clever gauge: homology is explicitly independent of gauge (i.e., of crystal setting).

It is important to specify what the homological formulation does not accomplish. It does not supplant superspace crystallography in structural determinations, but it is possible to translate between the two languages (with the dictionary of Draeger and Mermin); the classification also assumes a finite indexing (rank) of the diffraction pattern.

"First Principles Study of Magnetism in AlPdMn Approximant"
Alexey Godoniuk¹, Eyvaz Isaev¹, Yuri Vekilov¹
¹Moscow State Institute of Steel and Alloys, 119049 Moscow Russia

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
Electron and magnetic properties of AlPdMn quasicrystalline approximant are studied by means of the density functional theory and all-electron PAW potentials. Magnetic moments of Mn atoms are analyzed in dependence on Mn concentration and its chemical environment. We have found that local magnetic moments of Mn atoms appeared at 8 at.% of Mn. Substituting of amount of Al atoms in AlPdMn by B atoms resulted in the enhancemnet of local magnetic moment of Mn atoms. The effect could be explained in terms of p-d hybridization of atomic orbitals. The total energy calculations were carried out for a number of magnetic configurations.

"Diamagnetism in Quasicrystals"
Yuri Vekilov¹, Eyvaz Isaev¹, Borje Johansson²
¹Moscow State Institute of Steel and Alloys, 119049 Moscow Russia
²Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
We have shown that the weak diamagnetism in perfect icosahedral quasicrystals is connected to atomic-like diamagnetic contribution of tightly bounded conduction electrons in electron pockets of multiconnected Fermi surface (FS). Landau-Peierls diamagnetic term is small due to large effective masses. At temperatures above the Debye temperature, electrons become "free" due to the intervalley electron - phonon scattering, and the temperature dependence of the Pauli paramagnetism, related to a pseudogap in the density of states (DOS) at the Fermi level, becomes important.

"Electron Structure and Diffusion of Hydrogen Atom in i-TiZrNi"
Albert Morozov¹, Eyvaz Isaev¹, Yuri Vekilov³, Yuri Kuzim¹
¹Moscow State Institute of Steel and Alloys, 119049 Moscow Russia

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
Quasicrystals are promising materials for hydrogen storage devices [1]. Among thermodynamically stable icosahedral quasicrystals, the i-TiZrNi is able to absorb a significant amount of hydrogen atoms (H/M=1.7). The efficiency of the devices is considerably dependent on the charge state of hydrogen atom in the quasicrystal and diffusion of hydrogen atom. In the present work, based on the density functional theory and all electron PAW potentials, we have studied charge state of hydrogen atoms in i-TiZrNi. We have found that hydrogen atoms are weakly charged. By means of ab-initio molecular dynamics, the diffusion coefficient of hydrogen atom in the quasicrystal is evaluated, and it is found to be in good agreement with available experimental data [2].

1. W.J. Kim, P.C. Gibbons, K.F. Kelton, W.B. Yelon, Phys. Rev. B, 1998, v. 58, p. 2578.
2. V. Azhazha, A. Grib, K. Khadzay, S. Malikhin, B. Merisov, A. Pugachov, J. Phys. Condensed Matter, 15, 5001 (2003).

"Non-Icosahedral Ordering of Transition Elements in Zn-TM-Sc Quasicrystals"
Yasushi Ishii¹, Kazuki Nozawa², Takeo Fujiwara³
¹Chuo University, Tokyo 112-8551 Japan
²SORST, Japan Science and Technology Agenct (SORST-JST), Kawaguchi, Saitama 332-0012, Japan
³University of Tokyo, Hongo, Tokyo 113-8654, Japan

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
Icosahedral quasicrystals (QC) have been obtained in ternary Zn-M-Sc alloys with various kinds of third element M such as simple metals (M=Mg), transition elements (M=Mn, Fe, Co, Ni, Pd, Pt) and noble metals (M=Ag, Au). It is naturally believed that an icosahedral cluster found in Zn-Sc cubic phase is a building block of the icosahedral Zn-M-Sc, in which some of Zn atoms are substituted with the third elements. Atomic radii of the third elements is scattered in a wider range (1.2-1.6 Å) around the one for Zn (1.38 Å) and it is of special interest to understand chemical origin stabilizing such atomic substitution and to predict possible ordering of the third elements in the icosahedral cluster. In this paper, we calculate electronic structures of the cubic approximant phases with various atomic ordering of the third elements and analyze their bonding behaviour. For the case of M=Fe, Co, Ni, reasonable stabilization is obtained if the transition elements are placed at sites on an innermost atomic shell with non-icosahedral symmetry and their neighboring sites on a second shell of the cluster. This is partly because the transition elements have small atomic radii and hence they can be placed at a pair of the sites in a very short interatomic distance. From the analysis of the electronic eigenstates, formation of strong bonding states between the transition atoms is found. Further analysis for the cases of M=Mg and noble metals will be presented.

"X-Ray Topographic Study of Dislocations in Icosahedral Quasicrystal of Zn₇₇Fe₇Sc₁₆"
Taro Hashizume¹, Ryo Maezawa¹, Tsutomu Ishimasa¹, Toshihiko Takama¹
¹Graduate School of Engineering, Hokkaido University, 060-8628 Sapporo Japan

Presentation Type: Poster
Topic: Structure

Abstract:
Recently, the present authors found a series of icosahedral quasicrystals with a new type of structure similar to the Cd-based quasicrystals. Burgers vector of dislocation was determined in the present study. The icosahedral quasicrystal of the Zn₇₇Fe₇Sc₁₆ was selected as specimen. Single quasicrystals were prepared by annealing at 969 K for 5 h, followed by a water quench. They were cut by a diamond saw into pieces of thin plate with the surface orientation parallel to the two fold axis. The surface was mechanically polished to remove strain induced by the cutting. The X-ray and the electron diffractions showed very sharp patterns. It suggests that the quasicrystal has a high quality in perfection. The Lang topographs were taken using Mo and Ag Kα radiations. It was extremely rare to find the isolated dislocation. Based upon the strong extinction condition of g_//·b_// = 0 in the topograph, it was determined that the Burgers vector in physical space is parallel to the two fold axis and normal to the dislocation line. The direction of the Burgers vector was the same as those observed in the quasicrystals with the Mackay and the Bergman types.

"Metallurgical Aspects of Tsai-Type Icosahedral Quasicrystals in the Zn-Sc Based Alloys"
Tsutomu Ishimasa¹, Ryo Maezawa¹, Yoko Kasano¹, Sonia Francoual², Shiro Kashimoto¹
¹Division of Applied Physics, Hokkaido University, Sapporo, 060-8628 Japan
²LTPCM, 38402 Saint Martin d'Hères, France and ILL, BP 156, F-38042 Grenoble, France

Presentation Type: Contributed Talk
Topic: Structural Evolution and Phase Stability

Abstract:
Formation of icosahedral quasicrystals has recently been reported in several Zn-Sc based alloys containing Mg, 3d-transition metals, or noble metals as the third element . These quasicrystals were discovered by the composition modification in the Zn₁₇Sc₃ crystal, which can be regarded as a 1/1 cubic approximant. They are members of new group of quasicrystals, named Tsai-type, which includes Cu-Ga-, Zn-, Ag-In- and Cd-based alloys and is characterized by the special icosahedral atomic cluster with triple shells. In this study, we have elucidated metallurgical aspects of some Zn-Sc based quasicrystals, namely Zn-(Fe, Co, or Ag)-Sc, in particular with respect to stoichiometric composition, phase stability, synthesis of single-quasicrystal and compositional relationship to ternary 1/1 cubic approximant.

The experimental results indicate that these three quasicrystals are stable at 700 °C, and exhibit very high quality. They are formed as single phases at very narrow composition regions, namely Zn₇₇Fe₇Sc₁₆, Zn₇₈Co₆Sc₁₆ and Zn_74.5Ag_9.5Sc₁₆, respectively. Small composition difference of Sc between the quasicrystal and the approximant has been recognized for the Fe and Ag alloys; the quasicrystal and the approximant contain 16 at% and 14.5 at% Sc, respectively. In the case of the Fe alloy, the single phase region of the approximant is limited at approximately 4 at% of Fe, while in the case of Ag that extends to more than 7 at% Ag. This difference corresponds to the composition of each quasicrystal, namely 7 at% Fe in the Zn-Fe-Sc and 9.5 at% Ag in the Zn-Ag-Sc quasicrystals. In both alloys, there is very small composition difference between the quasicrystal and the corresponding approximant. In these experiments, it was essential to prevent the evaporation of Zn during the alloying process. For this purpose, we introduced a sophisticated preparation method, which can suppress the weight loss within approximately 0.3 %. By applying this method, millimeter- sized single quasicrystals of these alloys were synthesized.

"Metastable Icosahedral Quasicrystal in the Zn-Mg-Yb Alloy System"
Tomokazu Mitani¹, Tsutomu Ishimasa²
¹Graduate School of Engine, Hokkaido University, 060 8628 Sapporo Japan
²Hokkaido University, 060 8628 Sapporo Japan

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
In the narrow composition region connecting Zn_76.2Mg_9.3Yb_14.5 and Zn_71.7Mg_16.7Yb_11.6, a primitive–type (P–type) icosahedral quasicrystal is commonly formed in the alloys quenched from approximately 800°C;. At the composition Zn_72.5Mg_15.0Yb_12.5, the quasicrystal is formed by the quenching from relatively low temperature, 649°C;, and exhibits rather nice quality. Therefore we have elucidated the stability of the icosahedral quasicrystal at this composition.

The alloys quenched from 580–830°C; after annealing for 1.5–72 hours were examined by means of powder X–ray diffraction and electron diffraction technique as well as electron probe microanalysis. The alloys quenched from the temperature higher than 746°C; include two phases; the P–type quasicrystal as a major phase and the hexagonal Z–phase. The latter is an isomorphic structure of Zn₆₅Mg₂₈Y₇. Between 649 and 746°C;, the 1/1 cubic Zn₁₇Yb₃–type phase is formed besides the above two phases. Below 620°C;, the 1/1 cubic phase and the Z–phase are formed, but no quasicrystal. The stability of these three phases were studied mainly by the following two methods; morphological observation and differential thermal analysis (DTA). It was noticed that the size of quasicrystal grains ranges approximately from 10 to 100 μm independently on quenching temperature, although the grain size of the 1/1 cubic phase becomes larger in the lower temperature. The DTA experiment of the ingot quenched from 580°C; indicated two endothermic reactions at 620 and 746°C;. These experimental results suggest that a liquid phase appears above 620°C and the quasicrystal is formed only by the quenching of the liquid phase.

"Formation Condition of the Icosahedral Phase in Rapidly-Quenched Ag-In-RE (RE=Rare Earth) Alloys"
Sumihisa Iwano¹, Ryuji Tamura¹, Shin Takeuchi¹, Kaoru Kimura²
¹Tokyo University of Science
²Department of Advanced Materials Science,University of Tokyo

Presentation Type: Poster
Topic: Structure

Abstract:
In order obtain insight into the origin of the stability of the Cd-Yb type icosahedral phases, formation condition of the i-phase has been investigated in rapidly-quenched Ag-In-RE (RE: Rare Earth metals) alloys. As a result, the i-phase occurs for Ag₅₀In₃₆RE₁₄ (RE=Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu) alloys while it does not for Ag₅₀In₃₆RE₁₄ (RE=La,Ce.Pr) alloys. For these i-phases, a strong correlation between the atomic radius of RE atoms and the quasilattice constant was obtained,suggesting that the atomic radius of RE atoms plays an important role in the stability of the i-phases. We then investigated the dependence of the quasilattice constant on the Ag/In ratio in Ag-In-RE alloys and have found that no appreciable change in the quasilattice constant occurs although the atomic radii are substantially different between Ag(0.144 nm) and In (0.163nm). Such a result cannot be explained by a simple replacement between Ag and In but it may be due to a systematic change in the number of atoms in a quasi unit cell with changing Ag/In ratio. Results of the density measurements on the Ag-In-RE alloys with changing Ag/In ratios will be also reported in the poster presentation.

"Insulator-Like Electrical Transport in Al_65-xSi_xCu₂₀Ru₁₅ (x= 0.5,1.0,1.5 & 2.0) Icosahedral Quasicrystals"
Archna Jaiswal¹, Niranjan Lalla¹
¹UGC-DAE Consortium for Scientific Research, Indore, M.P. 452017 India

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
Low-temperature resistivities and magnetoresistance have been measured down to 1.4K to 300K for stable icosahedral quasicrystals Al_65-xSi_xCu₂₀Ru₁₅ (x=0.0,0.5,1.0,1.5 & 2.0). The analysis of the magnetoresistance data shows an overwhelming presence of anti weak-localization effect (τ_so ~10^-12 sec). The low-temperature σ-T behavior, between 1.4K-15K, for the Si substituted samples is drastically different from that of the undoped one. The σ-T curves between 1.4K-15K, for 0.5,1.0,1.5 & 2.0 samples, have negative curvature and follow a power-law behavior with an exponent of 0.5 and at higher temperatures with an exponent of 1.0-1.2. Where as for x=0 the σ-T curve shows a positive curvature with clear upturn below 3K which changes to a 0.5 power-law on application of 8-tesla field. The in field σ-T for the sample with x= 0 has been found to follow lnσ-vs-T^-1/4below 5K. The observed σ-T behavior has been attributed to the occurrence of the proximity of metal-insulator (MI) transition. The power law behavior has been understood in terms of electron-electron and electron-phonon inelastic scattering dominated critical regimes in the proximity of the MI transition. The presence of variable range hopping, as indicated by lnσ-vs-T^-1/4 below 5K for x=0 sample and change in the σ-T behavior on Si substitution, indicate the occurrence of MI transition in Al-Cu-Ru quasicrystal on Si substitution.

"Distribution of Electric Field Gradients in Decagonal Quasicrystals"
Peter Jeglic¹, Tomaz Apih¹, Martin Klanjsek¹, Janez Dolinsek¹
¹Jozef Stefan Institute, 1000 Ljubljana Slovenia

Presentation Type: Poster
Topic: Structure

Abstract:
Nuclear magnetic resonance (NMR) spectroscopy is a very useful experimental tool for getting information about the local chemical environments. The dominant nuclear spin interaction that determines the ²⁷Al NMR lineshape of Al-based quasicrystals (QCs) is the electric quadrupolar interaction [1], which reflects the structure of the QC through the distribution of the electric field gradients (EFGs). In our search for a model of EFG distributions in the decagonal AlNiCo QCs, we first revisited the icosahedral AlPdMn QCs. The EFG distribution, which can satisfactorily reproduce the observed NMR spectrum, can be numerically simulated by a dense random packing of hard spheres using the point-charge model, known as spherically isotropic Czjzek model [2]. However, such model cannot yield the strong angular dependence of the NMR spectra of decagonal QCs [3]. The anisotropy was included by introducing an analogous stacked-planar model [4,5], where we assumed that the atoms are randomly packed in a layer, which is then equidistantly stacked along the orthogonal periodic direction. The obtained arrangement of atoms thus presents a layered structure, which is amorphous in a plane, and periodic in the third direction in accordance with the basic structural properties of the 2D QCs. We were surprised that a model distribution of EFGs, which basically holds for 3D or 2D amorphous solids, explained so well also the NMR spectra of the long-range ordered icosahedral and decagonal QCs. The answer most probably lies in the fact that the QCs exhibit a large amount of chemical and structural disorder, which is supported by X-ray measurements [5].

[1] J. Dolinsek, T. Apih, P. Jeglic, M. Feuerbacher, M. Calvo-Dahlborg, U. Dahlborg and J.-M. Dubois. Phys. Rev. B 65, 212203 (2002).
[2] G. Czjzek, J. Fink, F. Gôtz, H. Schmidt, J. M. D. Coey, J.-P. Rebouillat and A. Liénard. Phys. Rev. B 23, 2513 (1981).
[3] P. Jeglic, M. Klanjsek, T. Apih and J. Dolinsek. Appl. Magn. Reson. 27, 329 (2004).
[4] P. Jeglic and J. Dolinsek. Accepted in Phys. Rev. B.
[5] P. Jeglic. Ph. D. Thesis, University of Ljubljana, 2004.

"HRTEM studies on the Quasicrystal Nanoparticles"
Chihiro Kaito¹, Osamu Kido¹, Akihito Kumamoto¹, Yoshio Saito²
¹Ritsumeikan University, Kusatsu-shi, Shiga-ken 525-8577 Japan
²Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto-fu 606-8585 Japan

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
The preparation of quasicrystal nanoparticles by the gas evaporation method was reported in some binary system such as Al-Mn [1], Al-Cr and Al-V [2]. They were spherical with the size of 50-250 nm in diameter. We revealed that these quasicrystal nanoparticles have the characteristic properties which cannot be found in the ribbon or bulk quasicrystals, such as high thermal stability regardless of a metastable phase [3]. Therefore, the studies on the quasicrystal nanoparticles will be a subject of great importance to the quasicrystallography. In particular, the high resolution transmission electron microscopy (HRTEM) has been the most major method for the analysis of quasicrystal structure. The HRTEM images have been taken in Ni-Cr nanoparticles which have the twelve fold symmetry [4]. However, the quasicrystals having the eight or twelve fold symmetry have not been considered in quasicrystallography, i.e., they were considered as an exceptional quasicrystal phase. Therefore, the HRTEM study has been hardly used for the nanoparticles of general quasicrystal. In the present study, systematic HRTEM observation on the quasicrystal nanoparticles has been done. We have recently developed the preparation method for ternary quasicrystal nanoparticles by using the advanced gas evaporation method. So far, we succeeded in preparing some quasicrystal nanoparticles and taking HRTEM images of Al-Pd-Mn by using a special 2-axsis inclination stage. We will show the HRTEM images of stable and metastable quasicrystal nanoparticles and characterization of the phason strain in the nanoparticles due to thermal annealing.

[1] Y. Saito, K. Mihama, H.S. Chen, Phys. Rev. B35 (1987) 4085
[2] T. Okazaki, T. Maki, Y. Saito, K. Mihama, Jpn. J. Appl. Phys. 28 (1986) 195
[3] O. Kido, H. Suzuki, Y. Yuki, T. Sato, Y. Saito, C. Kaito, Physica E, 25 (2004) 619
[4] T. Ishimasa, H. U. Nissen, Y. Hukano, Phys. Rev. Lett, 55 (1985) 511

"Systematic Study of Magnetic Property in Zn-Based Tsai-Type Icosahedral Quasicrystals and their Approximants"
Shiro Kashimoto¹, Chihiro Masuda¹, Ryo Maezawa¹, Sonia Francoual², Senni Motomura³, Susumu Matsuo⁴, Tsutomu Ishimasa¹
¹Division of Applied Physics, Graduate School of Engineering, Hokkaido University, Kita-ku, Sapporo 060-8628 Japan
²LTPCM/ENSEEG, UMR CNRS 5614, INPG, BP 75, 38402 St Martin d'Heres France
³Division of informatics for Natural Science, Graduate School of Human Informatics, Nagoya University, Chikusa-ku, Nagoya 464-8601 Japan
⁴Department of Complex Systems Science, Graduate School of Informatic Science,Nagoya University, Chikusa-ku, Nagoya 464-8601 Japan

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
We have investigated the magnetic property of Zn-based Tsai-type icosahedral quasicrystals (i-QCs) and their 1/1 cubic approximants. These quasicrystals are formed by an addition of the third element M to the Zn₁₇Sc₃ 1/1 cubic approximant. The remarkable feature of the present i-QCs is a variety of the additional M elements ranging from light metal Mg and 3d transition metals to noble metals. It has been reported that one can use Mg, Mn, Fe, Co, Ni, Cu, Pd, Ag, Pt and Au as M for the formation of the i-QCs with high structural perfection up to now. This is an advantage in the systematic study of the behavior of local magnetic moments of 3d elements in such Zn-based i-QCs. The magnetic susceptibility of Zn₈₀Mg₅Sc₁₅ and Zn_74.5Ag_9.5Sc₁₆ i-QCs shows diamagnetism and an increase of magnetic susceptibility with a rise in temperature above 90 K. This behavior is accounted for by a temperature dependence of the Pauli paramagnetism. On the other hands i-QCs containing 3d transition metals clearly shows the local magnetic moment formation unlike well-studied stable Al-based i-QCs. The magnetization curves show Curie-like paramagnetism, where the magnetization are 4.2 cgsemu/g at 5 K for Zn₇₇Fe₇Sc₁₆ and1.6×10^-3 cgsemu/g at 2 K for Zn₇₅Ni₁₀Sc₁₅ under 70 kOe. The temperature dependence of magnetic susceptibility obeys Curie-Weiss law in the wide temperature range for M = Mn, Fe, Co and Ni. Moreover, Zn₇₇Fe₇Sc₁₆ and Zn₇₅Mn₁₀Sc₁₅ show the spin glasslike cusp of magnetic susceptibility with a freezing temperature 7.2 K and 14 K, respectively. We also confirmed appearance of Curie-like paramagnetism for Zn_85.5-xFe_xSc_14.5 (x = 1, 2, 3, 3,5) 1/1 cubic approximants. These experimental results indicate that the divalent Zn of major component in the i-QCs may be favorable for the moment formation of 3d transition metals in contrast with the case of the trivalent Al-based i-QCs.

"Local Configurations on Surfaces of Icosahedral Quasicrystals"
Gerald Kasner¹, Zorka Papadopolos²
¹Inst. fuer Theoretische Physik, Univ. Madeburg, 39016 Magdeburg, Germany
²Inst. fuer Theoretische Physik, Univ. Tuebingen, 72076 Tuebingen Germany

Presentation Type: Poster
Topic: Surfaces

Abstract:
The surfaces of icosahedral quasicrystals of an F-phase, orthogonal to a 5fold symmetry axis have a terrace-like appearance. On highly resolved images of these terraces one observes different local configurations. We search for possible corresponding configurations in the bulk model M [1,2], based on an icosahedral tiling [3].

[1] Z. Papadopolos et al., Atomic Positions for the Icosahedral
F-Phase Tiling, in: "Proc. of the Int. Conf. on Aperiodic Crystals,
Aperiodic 1997", eds Marc de Boissieu et al., World Scientific,
Singapore (1998), p.173
[2] G. Kasner et al. Phys.Rev.B60 (1999) 3899
[3] Z. Papadopolos et al., Discrete Mathematics 221, No 1-3, (2000) p.101

"Hydrogen Storage in Ti-Zr- and Ti-Hf- Based Quasicrystals"
Kenneth Kelton¹
¹Washington University, St. Louis, MO 63130 USA

Presentation Type: Invited Talk
Topic: Mechanical Properties and Applications

Abstract:
The depletion of the world's petroleum reserves and the increased environmental impact of conventional combustion-engine-powered automobiles are leading to renewed interest in hydrogen storage materials. The numerous tetrahedral interstitial sites in icosahedral quasicrystals, structurally favorable sites for hydrogen adsorption, give these novel phases potential technological importance. The most promising are the Ti-Zr-Ni and Ti-Hf-Ni i-phases, due to their favorable alloy chemistry and the low-cost of the constituent materials. While several years ago, these were demonstrated to store large amounts of hydrogen (to a weight percent of 2.5%, exceeding that of LaNi₅) with relatively rapid absorption, the hydrogen equilibrium vapor pressures of the hydrogenated quasicrystals were too low at reasonable desorption temperatures to be practically important. Recent advances, however, have improved their prospects. We have recently discovered a relatively flat plateau at high pressures (100 – 200 psi) for hydrogen concentrations up to 2.8 hydrogen atoms per metal atom (H/M = 2.8) in a Ti_41.5Zr_41.5Ni₁₇ quasicrystal, exceeding a weight percent of 4%. Hydrogen desorption from H/M = 2.8 to 1.6 was demonstrated at temperatures between 100 °C and 200 °C. Absorption and cycling are improved in mechanically milled powders that contain nanometer i-phase grains. The formation of a crystal hydride in Ti_41.5Zr_41.5Ni₁₇ during absorption from the gas phase is dramatically decreased in Ti₄₀Hf₄₀Ni₂₀ quasicrystals, suggesting an improved cycling ability for those quasicrystals. An overview of research on hydrogen in quasicrystals will be presented, focusing both on their potential for technological applications and the fundamental insight that can be gained by using hydrogen as a probe of quasicrystal structure and dynamics.

*Supported by the National Science Foundation under grant DMR 03-07410.

"Influence of Oxidation of i-AlCuFeB Particles on the Formation of Al-Based Composites Prepared by Solid State Sintering"
Samuel Kenzari¹, Patrick Weisbecker¹, Guillaume Geandier², Vincent Fournée³, Jean-Marie Dubois¹
¹LSG2M, CNRS-UMR 7584, Parc de Saurupt, ENSMN, 54042 NANCY Cedex, France
²ESRF, 6 rue Jules Horowitz, BP 220, 38043 GRENOBLE Cedex 9, France

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
We present new results concerning the formation of Al-based composites reinforced by icosahedral i-AlCuFeB quasicrystalline particles. The AlCuFeB and Al powders (<25μm) are blended under argon in a planetary ball mill and sintered at elevated temperature (500°C). As already observed, the diffusion of Al from the matrix to the quasicrystalline particles induces the transformation of the i phase into the ω tetragonal phase. Here, we study the influence of an oxidation pre-treatment of the quasicrystalline particles onto the kinetics of this phase transformation.

We have prepared two composites by cold compaction, both containing 15 vol.% of AlCuFeB particles. The AlCuFeB particles were previously annealed at 600°C for 45 minutes, either in vacuum to prevent oxidation or in air to promote the formation of a thicker oxide layer. In situ measurements were performed at ESRF (European Synchrotron Radiation Facility) on ID15B beamline (λ=0.014 nm) in order to quantify the kinetics of the phase transformation. The two composites were heated up to 500°C and maintained at this temperature until the phase transformation was completed. For non-oxidized AlCuFeB particles, the formation of the ω phase starts at 450°C and the phase ratio evolves very quickly associated with the rapid disappearance of the i phase. The i → ω is almost completed after three hours at 500°C, which corresponds to the minimum time needed to obtain a sintered composite. On the contrary, the phase transformation is largely inhibited in the case of pre-oxidized AlCuFeB particles, and the onset temperature is increased to 490°C. Consequently, the kinetics of Al diffusion is reduced by the oxide barrier and becomes compatible with the sintering process. We could produce sintered composites with no ω phase. We have investigated the mechanical and tribological properties, especially the influence of the oxidation on the interface strength.

"Preparation of Ternary Quasicrystal Nanoparticles by Gas Evaporation Method and their Phase Transition Mechanism"
Osamu Kido¹, Akihito Kumamoto¹, Yoshio Saito¹, Chihiro Kaito¹
¹Ritsumeikan University, Kusatsu-shi, Shiga-ken 525-8577 Japan

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
Nanoparticles of the binary quasicrystal such as Al-Mn, Al-Cr and Al-V were prepared by advanced gas evaporation method (AGEM)[1,2]. These quasicrystal nanoparticles with the size of 50-250 nm in diameter are spherical. However, the conventional gas evaporation method was not the efficient preparation for some quasicrystal nanoparticles due to its low-quenching rate. So, we developed the gas flow evaporation method (GFEM), in which the quenching rate is one order higher than that of previous method [3]. By using GFEM, the Al-Cr quasicrystal nanoparticles were formed efficiently. Since the binary quasicrystal nanoparticles were metastable phase, they transforms into more stable crystal phases upon heat treatment. We developed the direct observation method of the phase transition of a nanoparticle by a transmission electron microscope (TEM). It was found that the phase transition temperatures of Al-Mn and Al-Cr quasicrystal nanoparticles were higher than that of the ribbon specimens[3,4]. Therefore, there is some possibility that nanoparticles have more characteristic properties which cannot be found in the ribbon or bulk quasicrystals.

In this study, we also developed the tow preparation method for ternary quasicrystal nanoparticles. One is the AGEM, in which three metals are simultaneously evaporated from the independent heat sources. The other is the flash evaporation method, in which mixture powders of three metals were evaporated by dropping onto a tungsten heater. So far, we succeeded in preparing Al-Pd-Mn and Al-Cu-Fe quasicrystal nanoparticles. These quasicrystal nanoparticles are spherical and composed of a single grain. We will show the results of the preparation of ternary quasicrystal nanoparticles, and their thermal stability by in situ observation using TEM heating stage.

[1] Y. Saito, K. Mihama, H.S. Chen, Phys. Rev. B35 (1987) 4085
[2] T. Okazaki, T. Maki, Y. Saito, K. Mihama, Jpn. J. Appl. Phys. 28 (1986) 195
[3] O. Kido, M. Kurumada, K. Kamitsuji, T. Tanigaki, T. Sato, Y. Kimura, H. Suzuki, Y. Saito and C. Kaito, Physica E, in submitting
[4] O. Kido, H. Suzuki, Y. Yuki, T. Sato, Y. Saito, C. Kaito, Physica E, 25 (2004) 619

"Effect of precipitation of quasicrystals on the mechanical properties of Zr-Ti-Be-Cu-Ni metallic glasses"
Hye-Jung Chang¹, Jin-Man Park¹, Do-Hyang Kim¹
¹Center for Non-crystalline Materials, Dept. of Metallurgical Eng., Yonsei Univ.

Presentation Type: Poster
Topic: Mechanical Properties and Applications

Abstract:
Recently, formation of quasicrystalline phase in Zr-, Ti- and Al-based bulk metallic glasses has attracted significant interest, because strength and ductility of the metallic glasses can be significantly enhanced by precipitation of nano-scale quasicrystalline phase in the amorphous matrix. Our study on the Ti-rich Ti-Zr-Be-Cu-Ni alloys shows that they have high glass forming ability in the wide composition range. Moreover, stable icosahedral phase precipitates in the amorphous matrix upon annealing treatment within a limited alloy composition.[1] Recently, it has also been reported that quasicrystal precipitates in the Zr-rich Ti-Zr-Be-Cu-Ni alloys during annealing treatment [2]. Therefore, in the present study, we investigated the mechanical property of bulk glass forming Zr-rich and Ti-rich Ti-Zr-Be-Cu-Ni alloy. Special emphasis has been placed on the effect of precipitation of nano-scale quasicrystalline phases on the enhancement of plasticity. The result shows that the plastic strain before failure in as-cast Ti-Zr-Be-Cu-Ni metallic glasses varies in a large degree depending on the alloy composition, i.e. when the first crystallization product is quasicrystalline phase, higher plastic strain is observed, while when the first crystallization product is crystalline phases, much lower plastic strain is observed. Possible reasons for the enhancement of plasticity are existence of quenched-in icosahedral nuclei embedded in the amorphous matrix. The plastic strain also increases when a few nanometer size quasicrystalline phase particles form in the glassy matrix by the partial crystallization treatment.

[1] Y.C. Kim, J.M. Park, J.H. Na, J.K. Lee, W.T. Kim, and D.H. Kim, Appl. Phys. Lett., Vol. 83, 15 (2003) p.3093
[2] I. Martin, T. Ohkubo, M. Ohnuma, B. Deconihout and K. Hono, Acta Mater., Vol.52, 15 (2004) p. 4427

"Improvement of Thermoelectric Performance for AlPdRe Icosahedral Quasicrystals by Chemical Bond Control with Other Element Substitution"
Kaoru Kimura¹, Junpei Okada², Takehito Hamamatsu¹, Tomohiro Nagata², Kazuhiro Kirihara³
¹The University of Tokyo, Kashiwa-shi, Chiba 277-8561 Japan
²The University of Tokyo, Tokyo 113-8656 Japan
³National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8565 Japan

Presentation Type: Contributed Talk
Topic: Mechanical Properties and Applications

Abstract:
Using the MEM/Rietvelt method, we successfully obtained the clear images of inter-atomic covalent bonds between Al and Re and those in Al (or Si) icosahedron in the Mackay icosahedral cluster of α-AlReSi approximant crystal. The bond strength distributes widely from weak metallic to strong covalent bond, and the intra-cluster bonds are stronger than the inter-cluster ones. This means that α-AlReSi is located at the intermediate state of molecular, metallic- and covalent-bonded solids. Composition dependences of atomic density and quasi-lattice constant for AlPdRe icosahedral quasicrystals show the existence of covalent bonding nature and the above situation is considered to be the same in the quasicrystals. The above situation provides the possibility of large electrical conductivity, σ, like metals, large Seebeck coefficient, S, like semiconductors, low thermal conductivity, κ, like molecular solids, and as a result high thermoelectric performance, ZT. We report composition dependences of thermoelectric properties of ternary AlPdRe and quaternary AlPdReRu icosahedral quasicrystals, and relate them to that of effective mass determined by the balance of bond strength of intra- and inter-Mackay Icosahedral clusters. By the Ru substitution for Re, electrical conductivity, σ, increases greatly, S slightly and κ dose not change almost, in consequence ZT enhances by a factor of 1.5. The above discussion has been developed, and substitution of Fe for Re or Ga for Al has been predicted and will be carried out.

"Non-Isotropic Distribution of Atomic Environments in Icosahedral Quasicrystals"
Martin Klanjsek¹, Eeuwe Zijlstra², Janez Dolinsek¹
¹"Jozef Stefan" Institute, Jamova 39, 1000 Ljubljana, Slovenia
²Department of Physics, Brock University, St. Catharines, Ontario, Canada L2S 3A1

Presentation Type: Contributed Talk
Topic: Structure

Abstract:
The distribution of atomic environments in icosahedral (i) quasicrystals (QCs), i-AlPdMn single-grain sample in our case, can be experimentally studied via the local physical quantity, which is a fingerprint of an arrangement of atoms around lattice sites. Such a quantity is an electric field gradient (EFG) at the sites of a particular nuclear species (²⁷Al in our case) in a sample under study. The information about its distribution can be revealed by nuclear magnetic resonance (NMR). Among the interactions that perturb the main Zeeman interaction, the interaction between nuclear quadrupole moments and local EFG is by far the strongest in the case of QCs. The satellite part of the NMR spectrum is thus determined entirely by the first order perturbation of this interaction. In contrast to some previous NMR studies of single grain i-QCs [1], where NMR spectra were found to be independent of the orientation of the sample in an extrenal magnetic field, in our case the satellite part of the NMR spectrum exhibited weak orientation-dependence [2]. This proved directly that the distribution of the EFG principal axes system (PAS) orientations is not isotropic in space, in contrast to the previous belief. We developed a simple qualitative model in order to demonstrate, how global i-symmetry can lead to the orientation-dependence. Combining the experimental data with the outcomes of the model, we obtained the features of the most probable distribution of the EFG PAS orientations. We found that the main EFG principal axes, i.e. the one associated with the largest EFG eigenvalue, point preferentially along the directions of the three-fold symmetry axes of the sample [3]. In addition, we compared this outcome to the theoretical one, which we obtained by calculating the EFGs in a periodic approximant phase corresponding to the sample under study. We made use of two relaxed structural models and calculated the EFGs with the full-potential linearized augmented plane-wave method.

[1] A. Shastri et al. Phys. Rev. B 50, 15651 (1994).
[2] T. Apih, M. Klanjsek et al. Phys. Rev. B 61, 11213 (2000).
[3] M. Klanjsek. PhD thesis (2004).

"Defected Quasicrystals and Their Diffraction Patterns"
Bartlomiej Kozakowski¹, Janusz Wolny¹
¹AGH University of Science and Technology, 30-059 Krakow Poland

Presentation Type: Poster
Topic: Structure

Abstract:
Recently, the structure factor for arbitrary decorated rhombic Penrose tiling was successfully derived [1] in average unit cell approach. For any decorating atom the triangular distribution of atomic positions was found in physical and perp- spaces. These distributions are related by an oblique projection of the atomic surface onto physical space. Fourier transforms of such triangular distributions added with appropriate phases for all decorating atoms leads to the structure factor of the decorated quasicrystal. Analytical expression for the structure factor allows extending the obtained formulas to defected structures, including phasons and phonons or some other defects. Randomness of decorations can be observed in diffraction pattern by disappearance of some super structure reflections. Intensities of such forbidden reflections have been derived from general formula for the structure factor of decorated 2D quasicrystals and tested for some model structures. Vibration of atoms along certain directions produced essential reduction of diffraction peaks' intensities described, in a first approximation, by a well-known Debye-Waller factor. Such anisotropic factors have been found for different types of vibrations. Possibility of extension of derived formulas to the diffuse scattering is also discussed. Average Patterson function is very powerful tool in the analysis of diffraction pattern for aperiodic structures. This function is the Fourier transform of diffraction pattern, afterwards reduced to an average unit cell. It can be also calculated as a self-correlation function for the average unit cell distributions. Influence of defects on an average Patterson function is also discussed.

[1] Kozakowski B. and Wolny J., Structure factor for decorated Penrose tiling, preprint 2004

"Ab-Initio Study of Quasiperiodic Monolayers Adsorbed on Five-Fold Surface of Icosahedral AlPdMn Quasicrystal"
Marian Krajci¹, Juergen Hafner²
¹Institute of Physics, Slovak Academy of Sciences, SK-84511, Bratislava, Slovak Republic
²Institute for Physics of Materials and CCMS, University of Vienna, A-1090, Vienna, Austria

Presentation Type: Contributed Talk
Topic: Surfaces

Abstract:
The structure and the stability of quasiperiodic monolayers formed on a five-fold surface of an icosahedral AlPdMn quasicrystal have been investigated using ab-initio density-functional methods. Structural models for a series of rational approximants to the structure of bulk i-AlPdMn have been constructed using the cut-and-projection technique in the 6-dimensional hyperspace according to the Katz-Gratias-Boudard model. A five-fold surface has been prepared by cleaving the bulk structure along a high atomic density plane perpendicular to a five-fold axis. The atomic structure of the clean surface can be described by a P1 tiling consisting of pentagons, thin rhombi, pentagonal stars and a boat. The vertices of the P1 tiling are located in the centers of B (Bergman) clusters and occupied by Pd atoms. For the ab-initio calculations we have prepared slab-models of the surface based on the 2/1 approximant. The skeleton of the P1 tiling fixed by the transition-metal atoms represents a stable surface-termination, but considerable rearrangement of the Al-atoms and large relaxations of the interlayer distances are observed. The structural model of the adsorbed monolayer has been constructed on the basis of a mapping of the potential-energy landscape of an isolated adatom on the fivefold surface of i-AlPdMn. The vertex positions and the mid-edge positions of the P1-tiling and Mn atoms exposed at the surface are identified as favorable adsorption sites. This leads to a structural model of the quasiperiodic addlayer with atoms at the vertices and mid-edge positions of the P1-tiling, centered pentagonal motifs decorating the pentagonal tiles, and one additional atom in the center of the pentagonal star and of the boat tile. For this structure we calculate a surface coverage of 0.09 atoms/Å² in perfect agreement with experiment. The diffraction pattern of the monolayer exhibits (pseudo)decagonal symmetry. The stability of the quasiperiodic monolayer has been tested for Sn, Bi, and Sb monolayers via relaxation based on the forces from ab-initio DFT calculations.

"Influence of the Processing Route on the Development of Icosahedral-Short-Range Order in Zr-Pd Metallic Glasses"
Matthew Kramer¹, D. Sordelet¹
¹Ames Laboratory and Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011 USA

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
Many of the Zr-TM (transition metal, Nb, Ni, Cu, Pd, Pt) alloys can form metallic glasses when rapidly quenched. For many of these alloys, the first devitrification product is a metastable quaiscrystalline (QC) phase. While there have been many studies on the role trace elements, e.g., oxygen, have on promoting the formation of the QC phase, little has been done to try to understand the role the short-range order (SRO) may have in the phase selection process. We investigated the origin of the SRO of Zr-Pd based metallic glasses by different processing routes: magnetron sputtering (MG), melt spinning (MS) and mechanical milling (MM). The atomic pair distribution function (PDF) was observed using high energy synchrotron x-rays. Reverse Monte Carlo (RMC) simulations of the experimentally measured atomic pair distributions were performed to identify contrasting aspects of the partial pair distributions and differences in local chemical order between isochemical amorphous alloys prepared by these very different synthesis techniques. The MS alloy is shown to have a SRO dominated by a random polyhedral configuration close to icosahedral symmetry. The MM samples have SRO to medium-range order which is topologically similar to the MoSi₂-type structure (I4/mmm), but chemically closer to the Al₂Cu-type structure (I4/mcm). Determination of the SRO of the MG films is on-going. The role of chemical versus topological ordering on the phase selection process will be discussed.

"Correlations between the Solid and Liquid Structure of the Cd₈₄Yb₁₆ Quasicrystalline Compound"
Matthew Kramer¹, D. Sordelet¹, Dongmei Wu², T. Lograsso², U. Dahlborg³, G. Cuello⁴
¹Ames Laboratory and Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011 USA
²Ames Laboratory, Iowa State University, Ames, IA 50011 USA
³LSG2M, CNRS UMR7584, Ecole des Mines, F-54042 Nancy Cedex, France
⁴Institut Max von Laue - Paul Langevin (ILL), Grenoble, France

Presentation Type: Contributed Talk
Topic: Structural Evolution and Phase Stability

Abstract:
Do bulk metallic liquids prefer configurations that favor a local coordination with five-fold symmetry i.e., icosahedral clusters? The first clue comes from the fact that simple liquids can be considerably supercooled. If the short-range order (SRO) of the liquid is already icosahdedral, what implications does this have on the nucleation barrier and the ability to undercool the liquid? The Cd₈₄Yb₁₆ (QC) system is an intriguing piece to this puzzle since it is a stable solid with long range aperiodic order. Secondly, the atomic structure of a two component system is easier to solve. Thirdly, there is a very close cubic approximate, Cd₆Yb, which has been demonstrated to have polyhedral clusters of nearly the same size and composition. Understanding of the SRO of the liquid state of the QC system would be the first step to understanding this puzzle. The down-side of this compound is the highly reactive nature of the molten Cd, Cd's high vapor pressure and the poor stability of the compound in the presence of oxygen. Special care in materials preparation and containment have allowed us to perform neutron and high energy X-ray diffraction studies to determine the partial pair correlations in the liquid state of QC.

The neutron experiments were performed at ILL, D4 using ¹¹²Cd at 0.349 Å and ^natCd at 0.500 Å at a nominal temperature of 900 K. The high energy synchrotron experiments were performed at the Advance Photon Source on single grain QC aligned along the 2 and 5-fold axes. The single grain samples melted congruently, < 1 K. The liquid is 7% less dense than the solid. Upon cooling from the melt, the QC phase is readily reformed. The amount of undercooling, ~ 5 to 25 K, was dependent on the cooling rate. The total scattering function of the liquid is consistent with a dilute liquid Cd structure. These results are consistent with Frank's hypothesis that the structure of the liquid must undergo reordering in order to form the solid phases but there is no compelling evidence for icosahedral short-range order in the liquid prior to the formation of the quasicrystalline structure.

"Icosahedral Cd-Yb at Low Temperature"
Guenter Krauss¹, Walter Steurer¹, Amy Ross², Thomas Lograsso²
¹Swiss Federal Institute of Technology, 8093 Zurich Switzerland
²Ames Laboratory, Ames, IA, USA

Presentation Type: Poster
Topic: Structure

Abstract:
Icosahedral Cd-Yb is of special interest as a model compound for the understanding of quasicrystal formation and stabilization. As a line phase in the system Cd-Yb, its phase width is very small and chemical disorder can be excluded as essential entropic contribution. Its cubic 1/1 approximant Cd₆Yb undergoes an order-disorder transition at 110 K. Assuming the same clusters as building units in Cd₆Yb and icosahedral Cd-Yb, a structural transition may also be expected for the quasicrystalline phase. Icosahedral Cd-Yb was studied in the temperature range between 90 K and RT using single-crystal X-ray diffraction, area detectors, and reciprocal-space reconstruction techniques. Peak profiles and the shape of the diffuse scattering as a function of temperature and possible structural relations between icosahedral Cd-Yb and its cubic 1/1 approximant Cd₆Yb will be discussed.

"A Case Study of Complex Metallic Alloys: Structure and Disorder Phenomena of Mg-Pd Compounds"
Julien Makongo¹, Yurii Prots¹, Ulrich Burkhardt¹, Rainer Niewa¹, Guido Kreiner¹
¹MPI für Chemische Physik fester Stoffe, 01187 Dresden Germany

Presentation Type: Contributed Talk
Topic: Structure

Abstract:

Complexity in the sense of giant unit cells and inherent structural disorder is the most pronounced feature of complex metallic alloys (CMA). Prominent examples of CMA's are approximant phases of icosahedral quasicrystals with the Mackay cluster as fundamental structural unit. Recently, we reported on the synthesis and crystal structure of Mg-rich complex alloys in the binary Mg-Pd system [1]: Mg₆Pd, Mg₅₇Pd₁₃, Mg_56.4Pd_13.5, Mg₃₀₆Pd₇₇, Mg_78.5Pd_21.5, Mg₃Pd, Mg₅Pd₂, Mg₂Pd. The first five are CMA's of Mackay cluster type with giant unit cells containing several hundred atoms. In order to achieve a deeper insight into the relation between the homogeneity ranges, crystal structure and disorder phenomena in the CMA's we have determined the extent of the single-phase fields by chemical, metallographic, thermal and X-ray powder analyses. Single crystal X-ray structure analyses of specimens of varying composition reveal intricate patterns of disorder in the CMA's. Mg₆Pd, e.g., crystallizes with a homogeneity range of 2.3(1) at-% Mg. At the Mg-rich border the truncated tetrahedron between the Mackay clusters is randomly occupied with a MgPd₄ unit or a Mg₄ tetrahedron. With increasing Pd concentration the Mg4 unit vanishes and constitutional vacancies at the vertices of the truncated tetrahedron are formed instead. Moreover, the study serves as a demonstration project for the specifics of phase diagrams containing a great number of CMA's.
[1] Makongo, J.P.A., Moguilnikov, Yu., Kudla, C., Grüner, D., Schäpers, M., Kreiner, G.: Mat. Res. Soc. Symp. Proc., Vol. 805 (2004) LL2.1.1--11.

"Local Atomic 3D Real Space Structure Analysis of Icosahedral Mg-Zn-RE (RE=Y and HO) Alloys: Strategy, Method and Models"
Stefan Brühne¹, Eckhard Uhrig¹, Wolf Assmus¹, Guido Kreiner²
¹Johann Wolfgang Goethe-Universität, D-60054 Frankfurt am Main Germany
²MPI für Chemische Physik fester Stoffe, 01187 Dresden Germany

Presentation Type: Poster
Topic: Structure

Abstract:
A complementary strategy to circumvent the problems posed to X-ray structure analysis of quasicrystals by the absence of 3D periodicity is discussed. Approximant models can be assumed to represent the quasicrystalline structure locally. The local structure of any condensed matter, however, is accessible via the reduced atomic pair distribution function (G(r) or PDF) from powder diffraction [1]. PDFs for well-ordered fci-Mg₂₅Y₁₁Zn₆₄, fci-Ho₉Mg₂₆Zn₆₅ and si-Ho₁₁Mg₁₅Zn₇₄ alloys have been obtained from in-house X-ray and synchrotron data.
Least squares refinements of suitable 1/1- and 2/1-approximant local structure models fit the experimental PDFs in spheres of diameter up to 54 Å [2-4] and reveal the following results:
Coordination shells of the single atoms are predominant of Frank-Kasper-type. RE atoms always have CN16. The atoms combine to units of 104 or 105 atoms, the 3-shell Bergman cluster of diameter ~15 Å. Its core may be void or filled, depending on Zn-content and fci or si 6D Bravais lattice type. Concerning the RE partial structure, RE₈ cubes of edge length 5.4 Å are inscribed in the 2nd cluster shell. No shorter RE-RE contacts are present. The cubes are tilted with respect to each other in adjacent clusters to allow for overall symmetry m-3-5. The clusters share common edges and hexagonal faces following the Canonical Cell Tiling scheme. Few glue atoms fill the cluster interstices to achieve nearly tcp packing of all atoms. But presumably there exists covering of clusters in the long range quasicrystal structure, pointing to an interdependence of the tiling and its decoration. From the structural findings a hypothesis is given for the absence of real rational approximants Mg-Zn-RE systems. The prospective use of the PDF for structural investigation of periodic complex metallic alloys (CMAs) with huge unit cells but comprising severe intrinsic atomic disorder is discussed.

[1] Egami & Billinge, Underneath the Bragg peaks, Pergamon (2003)
[2] Brühne et al., submitted to J. Phys.: Cond. Mat. (2004); arXiv:cond-mat/0410534
[3] Brühne et al., Z. Kristallogr. 219 (2004) 245
[4] Brühne et al., Cryst. Res. Technol. 38(12) (2003) 1023

"Effect of Cooling Rate on Phase Formation in Zr₆₀Ti₂Nb₆Cu₁₄Ni₉Al₉ Cast Rods"
Uta Kuehn¹, Juergen Eckert², Sergio Scudino¹, Ludwig Schultz¹
¹IFW-Dresden, 01069 Dresden Germany
²TU Darmstadt FB11, Material- und Geowissenschaften FG Physikalische Metallkunde, 64287 Darmstadt

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
The crystallization behavior of copper mold cast Zr₆₀Ti₂Nb₆Cu₁₄Ni₉Al₉ has been investigated by thermal analysis, X-ray diffraction and transmission electron microscopy investigations. In the as-cast state, the alloy forms predominantly an i phase with a grain size of more than 1 μm and presents additionally two glassy phases which different compositions. Glassy phase I forms near the mold with a composition close to the nominal one. After precipitation of the i phase a glassy phase II formed from the residual melt at the interfaces of the quasicrystals in the center of the sample. This suggests that the solidification of the melt occurs in three steps (1. glassy phase I, 2. i phase and 3. glassy phase II) because of the change in the cooling rate within the sample. The influence of the compositional differences of the two glassy phases on the crystallization behavior at elevated temperatures was studied. It was found that the glassy phase near the mold shows the lowest thermal stability of the three existent phases. The second exothermic peak of the DSC curve corresponds to the devitrification of the glassy phase in the center and the third exothermic event is linked with the transformation of the quasicrystals into several crystalline phases.

"Kondo-Like Magnetic Scattering in Electrical Transport of Al_70-xB_xPd₂₀Mn₁₀ (x= 0,0.5,1,2, & 4) Quasicrystals"
Archna Jaiswal¹, Rajeev Rawat¹, Niranjan Lalla¹
¹UGC-DAE Consortium for Scientific Research, Indore, M.P. 452017 India

Presentation Type: Contributed Talk
Topic: Electronic and Magnetic Properties

Abstract:
The zero-field and in-field (at 8-Tesla) electrical-conductivity variation with temperature (σ-T) and magnetization behavior of single-phase Al_70-xB_xPd₂₀Mn₁₀ (x=0,0.5,1,2 & 4) quasicrystals has been studied. The magnetoresistance and magnetic property measurements (χ-T & M-H) have also been carried out for some the selected samples. The observed σ-T behavior for these boron-containing samples has been found to be drastically different from the one-without boron substitution. Even a 0.5 at.% replacement of Al by B causes drastic change in the σ-T behavior. The relative depth of the σ-T minima although looks shallow but the nature of pre and post minima slope is identical to the boron undoped sample. The ratio R=(σ₄/σ₃₀₀) and the σ-T minima temperature T_min, both increases with boron concentration. T_min goes even above room temperature for x=4 sample. χ-T & M-H carried out for x=2 sample exhibit the occurrence of anti-ferromagnetic interaction in these samples. It has been observed that boron substitution brings drastic changes in σ-T behavior of Al-Pd-Mn quasicrystals. The σ-(ln^-1T) plots show linear behavior and get scaled in a {σ(T)-σ_min)/(σ₄-σ_min)}-vs- (ln^-1T/ln^-1T_min) plot. The magnetoresistance shows a +H² behavior, which is expected in the case of spin-fluctuating systems. The observed σ(T) behavior has been fully accounted to the occurrence of Kondo-type spin-flip scattering by the interacting moments in Al-B-Pd-Mn and Al-Pd-Mn quasicrystals in the presence of weak-localization. With decreasing temperature the spin-flip rate increases where as the electron-phonon inelastic scattering rate decreases. It has been very first time shown that the observed conductivity minima is basically an out come of the above competing interaction and due to the spin-orbit scattering.

"An STM Study of Si Adsorption on the Five-Fold Al-Pd-Mn and Ten-Fold Al-Ni-Co Quasicrystal Surfaces"
Julian Ledieu¹, Paul Unsworth¹, David Martin¹, Amy Ross², Thomas Lograsso², Ronan McGrath¹
¹The University of Liverpool, L69 3BX Liverpool UK
²Ames Laboratory, Ames, IA 50011 USA

Presentation Type: Poster
Topic: Surfaces

Abstract:
The growth of Si on the five-fold surface of the icosahedral Al-Pd-Mn and on the ten-fold surface of the decagonal Al-Ni-Co quasicrystals has been studied by low energy electron diffraction, Auger electron spectroscopy and scanning tunnelling microscopy. For both samples, Si adsorption follows a 3-d growth mode. Above 1 monolayer coverage, LEED patterns vanish and STM images reveal rough surfaces. Independently of the coverage, both quasicrystalline structures are restored after flashing the Al-Pd-Mn and Al-Ni-Co to 600 K and 570 K respectively and each display very sharp LEED patterns. However at submonolayer coverage, a unique site of adsorption has been recognised on the Al-Pd-Mn surface. This is supported by a Fast Fourier Transform and an autocorrelation function obtained when one selects only the Si atoms deposited on the surface. The radial distribution function calculated has been compared with those obtained from different theoretical adsorption sites. It appears that Si atoms are located in the centre of the so-called i`flowersi^ present across the entire five-fold Al-Pd-Mn surface. Finally, self-assembled pentagonal clusters have been recorded on the ten-fold Al-Ni-Co surface at submonolayer coverage. The clusters resembling starfish point in two distinct orientations. Their adsorption site is still under investigation.

"Equivalence Between Pure Point Diffraction and Being Model Sets on Substitution Point Sets"
Jeong-Yup Lee¹
¹University of Washington, Seattle, WA 98195 USA

Presentation Type: Poster
Topic: Mathematics

Abstract:
It has been known that any regular model sets are pure point diffractive.
But the converse has been a conjecture. We show here that the converse is also true
in substitution point sets dropping the condition of regularity, that is to say,
substitution point sets are to be model sets
if and only if they are pure point diffractive. We introduce a new coincidence concept
which connects the model sets and the pure point diffractive point sets in substitutions.

"Glass Formation and Crystallization in Binary Zr-Pt Systems"
M. Lee¹, X. Yang¹, M. Besser¹, M. Kramer¹, D. Sordelet¹
¹Materials and Engineering Physics Program, Ames Laboratory (USDOE), Ames, IA 50011 USA

Presentation Type: poster
Topic: Structural Evolution and Phase Stability

Abstract:
Numerous studies by many researchers have proposed that deep eutectics are preferable for glass formation upon cooling a liquid. However, in many systems improved glass formation is found at off-eutectic compositions. It may be proposed that the stabilization of liquid state at low temperature the formation of clusters or short-range order in the liquid are different concepts; the nature the latter is still quite speculative. It is known that the oxygen may exhibit an important role during glass formation of binary Zr₈₀Pt₂₀ metallic glasses. The eutectic Zr₈₀Pt₂₀ alloys may form meta-stable quasicrystals either during devitrification of an amorphous phase or directly upon cooling from a liquid. This structural change depends on the critical amount of oxygen because the structures of several metastable phases including quasicrystals, β-Zr(Pt) superstructure and Zr-Pt-O large-unit cell structure all have similar structural components.

In the binary Zr-Pt system, when the oxygen concentration was fixed at a very low level (<500 ppm mass), an amorphous structure can not be obtained even in the Zr₈₀Pt₂₀eutectic composition alloy by high speed melt spinning. However, when the Zr content is decreased from the eutectic composition, a fully amorphous structure is obtained by the same melt spinning technique. At low oxygen level the Zr₈₀Pt₂₀ system is mainly composed of metastable β-Zr(Pt) phase with a small portion of a quasicrystal phase. We propose that the formation of an amorphous random structure was prevented by the crystal or quasicrystal short-range order in the liquid state. Therefore, the primary crystalline phase was can be easily formed during the solidification process.

In the hypo-eutectic Zr-Pt systems, an amorphous structure was the dominant phase, presumably by precluding the formation of crystalline or quasicrystalline short-range order in the liquid phase. It appears that decreasing the Zr content shows similar effects to the increasing of relative oxygen concentration. A decreased degree of short- range order at lower Zr contents in the binary Zr-Pt system was examined by comparing high-energy x-ray and electron diffraction data including pair distribution functions between eutectic and off-eutectic alloys.

"Icosahedral to Decagonal Transition in [AlCuFe]-[AlCoNi] Pseudo-Binary Alloy System"
Yi Lei¹, Jean-Marie Dubois², Monique Calvo-Dahlborg³
¹Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, P. R. China
²Institut Jean Lamour (FR 2797 CNRS-INPL-UHP), LSG2M-Ecole des Mines, Parc de Saurupt, F-54042 Nancy
³LSG2M, CNRS-UMR7584, Ecole des Mines, Parc de Saurupt, 54042 Nancy Cedex, France

Presentation Type: Poster
Topic: Structure

Abstract:
Pseudo-binary IQC_100-xDQC_x alloys made from mixing Al₆₂Cu_25.5Fe_12.5 icosahedral quasicrystal (IQC) and Al₇₀Co₁₅Ni₁₅ decagonal quasicrystal (DQC) together in progressively increased proportions were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDAX). Through this way the gradual transition from IQC to DQC was manifested by the evolution of their constituent phases in each alloy. Generally three approximants were found as common phases in all these pseudo-binary alloys: λ-Al₁₃Fe₄, β-AlFe and τ3-Al₃Ni₂. It is found that, with the increment of the DQC content in the alloy, the λ phase changes from Al₁₃Fe₄ to Al₁₃Co₄ and the τ3 phase changes from Al₃Cu₂ to Al₃Ni₂. The formation of these phases were found to follow the evolution of their corresponding valence electron concentration per atom (e/a) lines in the Al-(Cu,Ni)-(Fe,Co) pseudo-ternary phase diagram. In addition to the formation of these approximant phases, an AlCuCo-type decagonal phase was found in the annealed IQC₈₀DQC₂₀ alloy. The study of this phase by XRD, transmission electron microscopy (TEM) proved that this phase is a superstructure of the AlCuCo-type decagonal quasicrystal. Further study by high resolution electron microscopy (HREM) proved this deduction by the manifestation of its distinct tiling of the basic icosahedral clusters in the high resolution images taken with the incident beam parallel to the ten-fold symmetry axis. The formation of this QC phase is discussed within the framework of the interaction of the two related quasicrystal systems and its formation is proved to play crucial important role in the evolution process of the two QCs.

"Dislocation Dynamics in a Dodecagonal Quasiperiodic System"
Gilad Barak¹, Ron Lifshitz¹
¹School of Physics & Astronomy, Tel Aviv University

Presentation Type: Poster
Topic: Thermal and Dynamical Properties

Abstract:
We have developed a set of numerical tools for the quantitative analysis of defect dynamics in quasiperiodic structures, with the intention of addressing some of the open questions regarding the dynamics of dislocations in quasicrystals. We are applying these tools to study dislocation motion in the dynamical equation of Lifshitz and Petrich [1] whose steady state solutions are quasiperiodic, exhibiting dodecagonal symmetry.

Here we demonstrate—by showing real-time computer simulations—our ability to inject an arbitrary set of dislocations, parameterized by the homotopy group of the D-torus, and quantitatively follow the positions of these dislocations as the equation evolves in real time. We measure and analyze the dislocation velocity as a function of applied stress and shear, as well as the phonon and phason strains that accompany this motion as the system evolves in time. These results display intriguing differences with respect to the behavior of dislocations in periodic solutions of the dynamical equation.

[1] R. Lifshitz and D.M. Petrich, "Theoretical model for Faraday waves with multiple-frequency forcing," Phys. Rev. Lett. 79 (1997) 1261.

"Discovery of Icosahedral Quasicrystalline Phase by Electronic Tuning"
Qisheng Lin¹, John Corbett¹
¹Iowa State University, Ames, IA 50011 USA

Presentation Type: Poster
Topic: Structure

Abstract:
Discoveries of quasicrystals (QC) are still mostly by chance or by experience. We here report our recent progress on the discovery of stable icosahedral QC (i–QC) as well as their approximants (AC) by tuning according to the band structures of selected normal crystals. The point stems from our years of experience understanding on the crystal and electronic structures of intermetallic compounds containing the heavy triel elements Ga, In, Tl [1].

Extended Hückel calculations for Mg₂Cu₆Ga₅ reveal that the densities-of-states (DOS) exhibit a pseudogap and unfilled bonding levels just above the Fermi level [2]. Four more electrons per f.u. appear necessary to tune the Fermi level to the pseudogap and to optimize the bonding states; therefore, two strategies are feasible, variation of the composition or introduction of an active element richer in valence electrons. Tuning by optimization of the Cu/Ga ratio results in a novel Mg₃₅Cu₂₄Ga₅₃ phase with interpenetrating Bergmann clusters [3]. Although the fcc symmetry is not close to icosahedral, the homogenous bonding and the remarkably tight cation and anion frameworks without any spacer atoms herein are impressive. A breakthrough wasmade by replacing part of the Mg with Sc, which results in both the 1/1 AC Sc₃Mg_0.16(4)Cu_10.5Ga_7.34(4) and the i–QC Sc_15.1(2)Mg_2.4(2)Cu_48.4(4)Ga_34.1(4) [4]. The same strategy applied to the Mg₂Zn₁₁ and Na₂Au₆In₅ compounds also works. Parallel tuings based on the DOS analyses leqad to the discoveries of both the 1/1 ACs, the 2/1 ACs and the i–QCs in respective systems [4].

Despite the fact that a rigid band with a change in structure has been assumed, electron tuning to QC from the Mg₂Zn₁₁ prototype structures works well. The method may open a new avenue of rational design of QC, parallel to that for normal crystals. Extended investigations of other systems such as Tl compounds, Laves phases, and further band structure interpretations are ongoing.

[1] Corbett, J. D. Angew. Chem. Int. Ed. 2000, 39, 670.
[2] Lin, Q.; Corbett, J. D. Inorg. Chem. 2003, 42, 8762.
[3] Lin, Q.; Corbett, J. D. Inorg. Chem. 2004, accepted.
[4] Lin, Q.; Corbett, J. D., unpublished results.

"Real-Space Long-Range Properties of Al-Pd-Mn"
Jean-Nicolas Longchamp¹, Mehmet Erbudak¹, Rouven Luescher¹, Yves Weisskopf¹
¹Swiss Federal Institute of Technology Zurich, 8093 Zurich Switzerland

Presentation Type: Poster
Topic: Structure

Abstract:
For ordinary crystals, secondary-electron imaging (SEI) patterns show high electron intensities within well-defined bands, the so-called Kikuchi bands, testifying to quasi-elastic scattering of electrons in the keV range at periodically placed atomic planes. In contrast to crystals, quasicrystals lack periodicity, yet possess a high-degree of orientational order. Experimental data available to date suggest both layer- and cluster-based bulk structures. Owing to the aperiodic atomic arrangement, these indications are not necessarily contradictory. The SEI pattern from the icosahedral Al-Pd-Mn quasicrystal also displays distinct Kikuchi bands connecting high-symmetry directions. Further, the energy dependence of their widths is similar to that found for crystalline samples, implying the presence of atomic layers with well-defined interplanar distances in the quasicrystal. We observe such Kikuchi bands perpendicular to fivefold- and twofold-symmetry directions, but not to threefold. Using quantum mechanical computations based on single-site scattering, we have simulated scattering of electrons at differently stacked planar systems as a function of their areal atomic densities. One major result of our study is that a stacking based on the Fibonacci sequence also leads to Kikuchi bands. The results are compared to the experimental data as well as to existing bulk models.

"Nine Colours in a Single-Class Six-Fold Quasiperiodic Pattern"
Reinhard Lueck¹, Max Scheffer²
¹Max-Planck-Institut für Metallforschung, D-70569 Stuttgart Germany
²Chemnitz

Presentation Type: Poster
Topic: Mathematics

Abstract:
Nine colours in a single-class six-fold quasiperiodic pattern

Reinhard Lueck, Stuttgart, Max Scheffer, Chemnitz (Germany)

The colouring of vertices of a six-fold tau-related pattern with nine colours is studied. The investigated pattern has a single translation class. It is found that essentially different colourings are possible.

"A Phenomenological Study of Quasicrystals as Thermoelectric Materials"
Enrique Macia¹
¹Universidad Complutense de MAdrid, 28040 Madrid Spain

Presentation Type: Contributed Talk
Topic: Mechanical Properties and Applications

Abstract:
The potential of quasicrystals (QCs) in the quest for novel thermoelectric materials TEMs is discussed on the basis of a recently introduced phenomenological approach[1]. To this end, we obtain a closed analytical expression for the thermoelectric figure of merit (ZT) in terms of a set of phenomenological coefficients relating the electronic structure main features to some characteristic features of the experimental electrical conductivity and S(T) transport curves. In this way, composition-dependent effects in the ZT function are addressed, concluding that those samples whose Fermi level is located close to the pseudogap’s minimum are not useful as TEM, while relatively large ZT values may be expected for samples exhibiting narrow spectral features close to the Fermi level [2]. These general trends agree with experimental results reported for both QCs, [3] and approximants. [4] From the knowledge of ZT, we present a prospective study on the possible use of certain QCs in order to improve the efficiency of segmented thermoelectric generators on the basis of their related compatibility factor. [5] By comparing the obtained analytical results with available experimental data we conclude that a promising high temperature TEM can be found among AlPdMn based icosahedral QCs. Accordingly, a systematic research of the icosahedral AlPd(Mn)Re system, aimed to improve the figure of merit and compatibility factor values observed to date, would be appealing.

[1] E. Macia, Appl. Phys. Lett. 81, 88 (2002); Phys. Rev. B 66,174203 (2002); J. Appl. Phys. 93, 1014 (2003); Phys. Rev. B 69,132201(2004); C. V. Landauro, E. Macia, and H. Solbrig, Phys. Rev. B 67, 184206 (2003).
[2] E. Macia, Phys. Rev. B 69, 184202 (2004).
[3] K. Kirihara and K. Kimura, J. Appl. Phys. 92, 979 (2002)
[4] T. Takeuchi, T. Otagiri, H. Sakagami, T. Kondo, U. Mizutani, and H. Sato, Phys. Rev. B 70, 144202 (2004).
[5] G. J. Snyder and T. S. Ursell, Phys. Rev. Lett. 91, 148301 (2003).

"Simulation study on Antiferromagnetic Order of Ising Spins in a Zn-Mg-Ho Model Structure"
Susumu Matsuo¹, Hiroshi Nakano², Senni Motomura³, Tsutomu Tsutomu Ishimasa⁴
¹Graduate School of Information Science, Nagoya University, Nagoya, Aichi 464-8601 Japan
²Kumamoto University, Kumamoto 860-8555 Japan
³Graduate School of Human Informatis, Nagoya University, Aichi-ken 464-8601 Japan
⁴Division of Applied Physics, Graduate School of Engineering, Hokkaido University, Sapporo, 060-8628

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
A long-range antiferromagnetic order was studied on Ising spins in a Zn-Mg-Ho model structure for Ruderman-Kittel-Kasuya-Yosida-like alternating magnetic interaction in simulated annealing calculations. The order was analyzed in the magnetic diffraction, the phason space, and magnetic interaction connections. The order showed sharp magnetic diffraction spots with the same peak widths as the ordinary lattice diffraction with a diminished intensity of the central peak, which indicated a long-range antiferromagnetic order. More number of strong magnetic diffraction spots were found in the present case of a Fermi wave vector, 0.8 times as short as a former study, in the 5-fold diffraction plane. The order was most adequately interpreted as an order in the separation of the occupation domain in the phason space. The global feature of the separation was similar to the former study, but the separation was clearer than the the former one. Magnetic interaction connections were analyzed in the phason space for the clear separation of the occupation domain.

"Numerical Studies of Electronic Transport in Quasicrystals"
Guy Trambly de Laissardiere¹, F. Triozon², Jean-Pierre Julien³, Didier Mayou⁴
¹Laboratoire de Physique Théorique et Modélisation Université de Cergy Pontoise 95302 Cergy Pontoise
²CEA/DSM/DRFMC/SPSMS 17 Avenue des Martyrs 38054 Grenoble France
³LEPES/CNRS, BP 166, 38042 Grenoble Cedex 9 France
⁴CNRS, BP166 38042 Cedex 9 Grenoble France

Presentation Type: Contributed Talk
Topic: Electronic and Magnetic Properties

Abstract:
Electronic transport under the action of an applied electric field is one of the fundamental properties of the condensed state of matter. In the limit of low frequency field, the electronic transport is intimately related to the electronic diffusion in the equilibrium state. In many materials the electronic diffusion is well described by a semi-classical picture such as the Drude model or the Bloch-Boltzmann approach, yet there are also circumstances where a semi-classical approach is unsufficient and a fully quantum description is necessary to properly treat electronic transport. This is indeed the case for quasicrystals and related materials such as approximant phases.

In view of the complexity of the electronic structure of these systems, a purely numerical approach allows to treat the problem without uncontrolled approximations, giving insight in the physics of transport. A newly developed, and extremely efficient, mathematical algorithm is applied to tight-binding models of quasicrystalline phases such as the Penrose lattice. It allows to analyse for the first time the diffusion of electrons in these structures as a function of their energy. As we show this provides us with a deep insight in conduction properties of these systems in relation with anomalously slow electronic diffusion. The method can also be applied to realistic tight-binding models wich are derived from ab-initio calculations. This gives, to our knowledge, the first ab-initio description of transport in realistic electronic structure models of quasicrystals and approximants. The results are discussed in relation with experimental measurements on high structural quality samples.

"Molecular Adsorption on a Quasicrystal Surfaces: Benzene on i-Al₇₀Pd₂₁Mn₉"
Jon Hoeft¹, Julian Ledieu¹, Sam Haq¹, Thomas Lograsso², Amy Ross², Ronan McGrath¹
¹The University of Liverpool, Liverpool, Merseyside L69 3BX United Kingdom
²Ames Laboratory, Iowa State University, Ames IA 50011, USA

Presentation Type: Poster
Topic: Surfaces

Abstract:
Molecular adsorption on quasicrystals is of interest in testing the reactivity of the surfaces in comparison to adsorption on single crystal metal surfaces. There is also the possibility of molecular templating, i.e. the formation of an ordered overlayer of molecules on the surface through molecule-surface and inter-molecular interactions.

For these reasons, the adsorption behaviour of molecular benzene on the icosahedral (five fold) planes of the AlPdMn quasicrystal has been investigated. Reflection absorption infrared (RAIR) spectra taken while dosing at different rates indicate molecular adsorption at temperatures around 21 K. Depending on the total dose the adsorption geometry of the molecules changes from parallel to the surface to oblique geometries.

The molecular layer does not affect the low-energy electron diffraction (LEED) pattern which remains sharp and unchanged. Scanning tunnelling microscopy (STM) shows molecules adsorbing in bridge-like sites on the clean surface; however overall the adsorbed monolayer is dsordered. Under certain conditions the adlayer becomes transparent which is attrbuted to tunnelling resonances as discussed by Pascual et al. [Phys. Rev. B 62 (2000) 12632] for benzene adsorption on Ag(110). The adsorbed layer completely desorbs at 153 K leaving the substrate unaffected.

"Czochralski Growth of Decagonal AlCoNi-Quasicrystals - State of the Art"
Birgitta Bauer¹, Götz Meisterernst¹, Jürgen Härtwig², Thomas Schenk³, Peter Gille¹
¹Crystallography Section, Dept. of Earth and Environmental Sciences, LMU München, Germany
²E.S.R.F., BP 220, F-38043 Grenoble, France

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
In the system Al-Co-Ni large single crystals of high perfection can be reproducibly grown by the Czochralski method [1]. By using native seeds from former experiments, it is possible to grow crystals with various orientations. In order to investigate the kinetics of anisotropic growth, we carried out growth experiments along the periodic direction [00001], the two quasicrystalline directions [10000] and [10100], and the directions [01101] and [10102], which are inclined towards the periodic axis. The morphology of the grown quasicrystals is strongly affected by the growth anisotropy. The as-grown crystals show the forms {00001}, {10000}, {10100}, as well as {01101} and {10102}, which can be observed as well-developed facets. To investigate the influence of the growth direction on the structural perfection of quasicrystals, slices perpendicular and parallel to the growth direction were examined by transmission X-ray diffraction topography at the synchrotron (ESRF) and in the laboratory with the Lang method. All the samples were single crystals of the same composition Al₇₂Co₉Ni₁₉, which made it possible to image large areas. Because of the high density of defects, orientation contrast dominated. Variation in composition or spatial segregation effects did not contribute to the contrast of the topographs. Areas tilted by low-angle grain boundaries can mainly be observed parallel to the periodic direction, independent of the chosen growth direction. A column-like growth parallel to this direction can be assumed. If a growth direction perpendicular to the periodic axis is chosen, misoriented regions are limited by the diameter of the crystal and do not pass through the whole crystal.

[1] P. Gille, R.-U. Barz, L.M. Zhang
In: H.-R. Trebin (Ed.) Quasicrystals. Structure and Physical Properties, Wiley-VCH, Weinheim 2003, p. 73-87.

"Cold Welding and Fretting Tests"
Muriel Sales¹, Andreas Merstallinger¹, Pierre Brunet², Marie-Cécile de Weerd², Jean-Marie Dubois²
¹ARC Seibersdorf Research GmbH, 2444 Seibersdorf Austria
²Laboratoire de Science et Génie des Matériaux et de Métallurgie (UMR 7584 CNRS-INPL-UHP)

Presentation Type: Poster
Topic: Mechanical Properties and Applications

Abstract:
Cold-welding and fretting are especially relevant regarding opening and closing of engineering mechanisms, either placed in vacuum or embarked on spacecrafts because they condition the life of the mechanism and its proper functioning. Two specially designed devices — called "impact facility" and "fretting facility" — have been developed at the Austrian Research Centre Seibersdorf (ARCS) and were used to investigate several combinations of bulk quasicrystals and approximants for their tendency to prevent cold-welding. Reference bulk materials, namely two stainless steels and an aluminium alloy were tested in contact with themselves for comparison. The following general conclusions have been found: adhesion forces increase with static load, i.e. with contact pressure. The adhesion forces, on conventional alloys, found for the three different contact types, static, impact and fretting, increase by orders of magnitude: ≤ 0.5N (static), ≤ 2N (impact) and up to 13 N (fretting). This means, that vibrations in the pure elastic contact cause much more severe cold welding than impacts, which cause plastic deformation. Stainless steel 17-7 PH shows adhesion forces up to 1.5 N in impact, but more than 13 N in fretting contact. Similar tests performed on different quasicrystals in contact with various steels show that there is no adhesion (≤ 0.096 N) for the quasicrystal i-Al_62.5Cu_25.3Fe_12.2 versus the standard bearing steel AISI 52100, whereas an approximant phase, γ-Al_67.6Cr_23.3Fe_9.1 shows adhesion (1.5 N). In general, icosahedral quasicrystals exhibit no adhesion against AISI 52100 steel (0.15 N for i-Al_59.5Cu_25.3Fe_12.2B₃ and 0.26 N for i-Al₇₀Pd_22.1Mn_7.9).

"Tile-Decoration Model of Co-rich d-AlCoNi Derived from W-AlCoNi Approximant"
Marek Mihalkovic¹, Christopher Henley², Nan Gu³, Michael Widom⁴
¹Institute of Physics, Slovak Academy of Sciences, 845 11 Bratislava Slovakia
⁴Carnegie Mellon University, Pittsburgh, PA 15213 USA

Presentation Type: Invited Talk
Topic: Structure

Abstract:
We use ab-initio total energy calculations to refine chemical ordering of the W-AlCoNi approximant structure (Sugiyama et al., 2002), and calculate its stability relative to other ternary and binary competing compounds. The approximant structure has 8Å stacking periodicity along pseudo-5-fold axis, but due to its monoclinic centering the 8Å periodicity results from stacking two identical 4Å thick bilayers, with stacking vector not parallel to the pseudo-5-fold axis. We show how this ordering generalizes into a tile-decoration model of a quasicrystal phase, in which 4Å bilayers are bound to a "binary" (3-level) version of Hexagon-Boat-Star decagonal tiling with 6.5Å edge length, while tilings adjacent along the stacking direction may exhibit mismatches bounded within octagonal supertiles, analogical with octagonal clusters facilitating tile-reshuffling update move for binary Penrose tiling. In such description, the approximant structure is a dense packing of the octagon clusters, suggesting a negative energy coefficient for the mismatched regions, and 8Å superorder at low temperatures.

"New Insight into Amorphous Structures linked to Amorphous-Quasicrystal Transformation"
Z. Liu¹, D. Miller¹, J. Tao², J. Zuo², S. Marchesini³, D. Sordelet⁴, Matthew Kramer⁴, Cynthia Jenks⁴
¹Argonne National Laboratory, Argonne, IL 60439 USA
²University of Illinois at Urbana-Champaign, 1304 W Green Street, Urbana, IL 61801, USA
³Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA
⁴Ames Laboratory, Iowa State University, Ames, IA 50011 USA

Presentation Type: Contributed Talk
Topic: Structural Evolution and Phase Stability

Abstract:
Amorphous structures are of significant relevance as a precursor to quasicrystal fomation. However, our knowledge about amorphous structures is astoundingly limited due to the limitations of current structural characterization techniques. The advent of two transmission electron microscopy (TEM) techniques- fluctuation electron microscopy (FEM) and diffractive imaging- has enabled us to study such fundamental issues as the existence of a local order and its role on crystallization behaviors. We applied FEM and diffractive imaging to different Zr-based amorphous structures that look alike when examined by many characterization approaches yet have distinctly different crystallization behaviors. The amorphous Zr-based alloys were prepared by one of two fundamentally different processes: rapid solidification or mechanical milling. While mechanically milled powders crystallize directly into equilibrium phases, rapidly solidified ribbon first crystallizes into the metastable icosahedral (quasicrystalline) phase. Our FEM results showed that the rapidly solidified ribbon has a very high degree of medium-range order compared to the mechanically milled powders. This is the first time such a clear structural difference was observed and quantified in amorphous alloys prepared by different approaches. We speculate that the high degree of the MRO in the rapidly solidified ribbon arises from frozen-in local correlations that exist in the liquid. Diffractive imaging allows us to visualize the ordered clusters, we found that the length scales of the homogeneously distributed medium-range order in the RSR range from 0.5 nm to 2 nm. Our work suggests that medium-range order is the key to understand the amorphous structures, with such order necessary to lead to crystallization of quasicrystals in the Zr-based alloys.

This work was supported by the U.S. Department of Energy, Offices of Science. Electron microscopy was carried out in the Electron Microscopy Center at Argonne National Laboratory and at the Center of Microanalysis of Materials, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign.

"Pseudogaps and Bonding in Complex Intermetallics and Approximants to Quasicrystals: A Cluster Viewpoint"
Gordon Miller¹
¹Iowa State University, Ames, Iowa 50011 USA

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
The icosahedron and its symmetry-related polyhedra receive much attention as a fundamental structural and electronic unit for quasicrystals. From such a cluster viewpoint, numerous crystalline approximants and quasicrystalline structures can be constructed. However, since flat three-dimensional space cannot be filled exclusively with icosahedra, such a construction necessarily yields new clusters between icosahedra. With respect to the resulting electronic structures, the 'lone pair'orbitals associated with the icosahedral units will become bonding/antibonding orbitals within the new clusters upon construction of the complete crystalline or quasicrystalline structure. This presentation will examine the structures and electronic structures for a variety of intermetallic systems, e.g. aluminides, Bergman and Mackay systems, and Hume-Rothery type Zn-Pd binaries, to identify chemical bonding rationales for pseudogaps and electron counting rules in complex intermetallics. This cluster-based approach will also be applied to the Zn-Pd system for the possibility of creating one-dimensional quasiperiodic structures.

"The Hume-Rothery Electron Concentration Rule for a Series of γ-Brasses Studied by FLAPW Band Calculations"
Uichiro Mizutani¹, Ryoji Asahi², Hirokazu Sato³, Tsunehiro Takeuchi⁴
¹Nagoya University, Dept.Cryst.Mat.Sci., 464-8603 Nagoya Japan
²Toyota Central R&D Laboratories, Inc., Nagakute, Aichi, 480-1192, Japan
³Department of Physics, Aichi University of Education, Kariya-shi, Aichi, 448-8542, Japan
⁴Ecotopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

Presentation Type: Invited Talk
Topic: Electronic and Magnetic Properties

Abstract:
The theoretical interpretation of the Hume-Rothery electron concentration rule has been attempted by performing the FLAPW (Full-Potential Linearized Augmented Plane Wave) band calculations for a series of γ-brasses, which include Cu₅Zn₈, Cu₉Al₄, Pd₂Zn₁₁, Ni₂Zn₁₁, Co₂Zn₁₁ and Fe₂Zn₁₁. In the past, the LMTO band calculations, in which the wave function is composed of the atomic orbitals or muffin-tin orbitals, had been exclusively employed for structurally complex alloy phases like 1/1-cubic approximants and consistently pointed to the importance of orbital hybridization effects on the formation of the pseudogap. This is, however, not always best suited to explore the Hume-Rothery electron concentration rule. We believe the FLAPW method to extract most transparently the Fermi surface-Brillouin zone interaction responsible for the formation of the pseudogap, regardless of the degree of orbital hybridizations, and to allow us to discuss the Hume-Rothery rule in a more straightforward manner. We evaluated the Fourier spectrum of the FLAPW wave function for states at the point N on the {110} zone plane at energy eigenvalues sustaining the pseudogap. It directly proved that the {330} and {411} zones do play a crucial role in the formation of the pseudogap below the Fermi level for Cu₅Zn₈, Cu₉Al₄, Pd₂Zn₁₁ and Ni₂Zn₁₁ γ-brasses. This is taken as a straightforward demonstration of the empirical Hume-Rothery matching rule for γ-brasses beyond the oversimplified free electron model by Mott and Jones in 1936. There exist, however, exceptions to this universal behavior. In the case of the Co₂Zn₁₁ and Fe₂Zn₁₁, the pseudogap is still formed by resonating with {330} and {411} zones but is positioned above the Fermi level because of the presence of Co- and Fe-3d states across the Fermi level. The discussion will be focused on what is meant on the Hume-Rothery electron concentration rule for all γ-brasses studied by taking into account both the FLAPW band calculations and equilibrium phase diagrams.

"The Plasticity of Icosahedral Al-Pd-Mn Quasicrystal"
Frederic Mompiou¹, Daniel Caillard²
¹NIST, Gaithersburg, MD 20899 USA
²CEMES/CNRS, Toulouse, France

Presentation Type: Contributed Talk
Topic: Mechanical Properties and Applications

Abstract:
Quasicrystals are known to be extremely brittle at room temperature and to become ductile only above 0.7Tm. Their mechanical properties are thus closer to those of semi-conductors than to those of ordinary intermetallic alloys. It is now well known that dislocations with well-defined 6D Burgers vectors are responsible for the plasticity of Al-Pd-Mn icosahedral quasicrystal. In this talk we report results of conventional and in-situ transmission electron microscopy (TEM) experiments showing that dislocations move by climb in a large range of temperature and that glide is at least 1000 a slower process under a comparable stress. As climb proceeds by absorption or emission of 'vacancies,' dislocation motions are impeded respectively by a sub- or super- saturation of vacancies in the system. Thus plastic deformation can be understood only if dislocations acting as sources and sinks of 'vacancies' are introduced. We show that the corresponding back stress could be as large as half the applied stress. At high temperature, dislocations move viscously and exhibit straight segments along dense directions. This additional analogy with semi-conductors suggests that dislocations experience a high Peierls potential and that dislocation motion can be treated in terms of jog-pair nucleation and propagation by climb. The corresponding activation parameters are discussed in the frame of a model derived from the Hirth and Lothe model of gliding dislocations in covalent crystals. This leads to an activation energy larger than the vacancy self-diffusion energy and a stress exponent larger than unity. Their dependences with stress are also in good agreement with experimental data.

"New Results on Diffraction and Dynamical Systems"
Robert Moody¹
¹University of Alberta, Edmonton, AB T6G 2G1 Canada

Presentation Type: Poster
Topic: Mathematics

Abstract:
Diffraction is a central issue in quasicrystal theory, and the mathematics of it has proven remarkably tricky. One of the key tools that has emerged is the use of dynamical systems. Rather than studying a single distribution of points or atoms, one collects together the family X of all possible distributions, along with an associated probability measure. This becomes a dynamical system with translation, rather than time, providing the transformation group.

The idea of studying a dynamical system by use of spectral theory goes back to Koopman in the early 1930's. Here the situation is goes further. We study the spectralproperties of the diffraction by relating it to the spectrum of the associated dynamical system, which in turn is connected to the geometrical and statistical nature of the family X in question.

The poster will outline this connection and how it works. A number of interesting developments have taken place in the area of the past few years, and these will be emphasized.

"Nanomechanical Characterization of Al-Co-Ni Decagonal Quasicrystal"
N. Mukhopadhyay¹, A. Belger², P. Paufler², P. Gille³
¹Department of Metallurgical Engr, IT, Banaras Hindu University, Varanasi , UP 221 005 INDIA
²Institut für Strukturphysik, Fachrichtung Physik, TU Dresden, D-01062, Germany
³Institut für Kristallographie und Angewandte Mineralogie, Universität München, D-80333, Germany

Presentation Type: Poster
Topic: Mechanical Properties and Applications

Abstract:
The aim of the present work is to address some of the issues related to the nanomechanical responses of decagonal quasicrystals employing nanoindentation techniques. Hysitron Triboscope with a Berkovich indenter and conical indenter (up to16mN load) were used to carry out the nanoindentation tests on the single crystals of Al-Co-Ni decagonal quasicrystal. The impression of indent after complete unloading was examined under the AFM (Digital Nanoscope-III) attached with the Hysitron triboscope. The reduced modulus and nanohardness were calculated from the standard procedure developed by Oliver and Pharr (1992). The decrease of nanohardness with increase in load was noticed. This effect, which is known as indentation size effect (ISE), is attributed to the strain gradient plasticity theory. The anisotropy in terms of nanohardness at higher load could be observed, whereas the Young's modulus was found to be indistinguishable in all these cases. The discontinuity in the load-displacement plot, which is known as pop-in-effect, was observed. This discontinuity could not be related neither to any cracking nor phase transformation effect and therefore it can be attributed to plastic yielding phenomena related to nanodeformation of this material. The load displacement curve shows a stepwise characteristic with alteration of elastic and plastic sections causing a formation of shear bands. It is interesting to point out the present plastic flow is similar to that observed in Al-Fe-Cu QC, Ti-Zr-Ni QC and Zr-based bulk metallic glasses. In all these cases the plastic deformation is found to proceed by stepwise formation of shearbands around the indent. Attempts will be made to reconcile the experimental data with the structural models of quasicrystals.

"Curious Phase Transformations in Al₇₀Ni₂₄Fe₆ Decagonal System During High Energy Ball Milling"
T. Yadav¹, N. Mukhopadhyay², R. Tiwari¹, O. Srivastava¹
¹Department of Physics, Banaras Hindu University, Varanasi-221005 India
²Department of Metallurgical Engr., Banaras Hindu University, Varanasi-221005 India

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:

Al-Ni-Fe system exhibits the existence of a stable decagonal phase with the various degree of order in periodic and quasiperiodic planes. In the present investigation the decagonal quasicrystal phase has been synthesized in Al₇₀Ni₂₄Fe₆ system by melting the constituent pure metals and cooling it slowly in RF induction furnace. In order to study the structural and microstructural stability during ball milling, this material was employed for mechanical milling in an attritor ball mill for 1, 2, 4, 8, 12 and 40 h. The milling was carried out in a hexane medium with a powder to ball ratio of 1:100. The milled powder was annealed in argon atmosphere for durations ranging from 1 to 40h at 500°C. All the samples have been characterized by X-ray diffraction, scanning and transmission electron microscopic techniques. The as-cast alloy was found to consist of micron size decagonal phase along with minor amount of Al₃Ni and Al₁₃(Fe,Ni)₄ crystalline phases. During the course of milling the disordering of decagonal phase has been observed from the broadening of the X-ray peaks. The phase transformation from decagonal phase to a B2 crystalline phase has been observed. Powders milled for more than 4h contain predominantly the B2 phase. The Voigt function analysis of the x-ray data has been used to calculate grain size and residual strain. The crystallite size of the milled powder has been estimated to be around 14 nm after 40 h of milling. The crystallite size is found to decrease with increase in milling duration. A lamellar microstructure consisting nano grains of B2 phase evolves after 8 h of ball milling, however at a milling time of 40 h, the lamelar structure disappeared. Isothermal annealing treatment at 500°C for 10, 20 and 40 h of 12 h mechanically milled powder leads to the transformation B2 phase to τ 3 type vacancy ordered phase. The implication of these transformations and the evolution of lamellar microstructure will be discussed with reference to the relative stability of decagonal and other crystalline phases of various grain size during milling and subsequent annealing treatments.

"Phason Hierarchy and Electronic Stability of Quasicrystals"
Gerardo Naumis¹
¹Instituto de Fisica, UNAM, Mexico, DF 01000 Mexico

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
We show that there is a hierarchy in the probability of making a phason in real space. This effect divides the quasicrystal in a stable backbone plus an unstable part. The stable backbone and the unstable sites are obtained by the deflation rules of the corresponding quasicrystalline lattice.
Then we discuss how the electronic stabilization of a quasicrystal is affected by phason jumps in unstable sites. As a result, it is shown that phasons in unstable sites do not compromise the electronic stabilization of the structure since the density of electronic states is not affected; they only affect low amplitude diffraction spots of the quasicrystal. In that sense, the Hume-Rothery mechanism is stable against phason jumps in certain sites of the lattice.

"Efficient Generation of Harmonics by using Anharmonic Quasiperiodic Systems"
Gerardo Naumis¹, I. Limas¹, F. Salazar², C. Wang²
¹Instituto de Fisica, UNAM, México, DF 01000 Mexico
²Instituto de Investigaciones en Materiales, UNAM, México, DF 01000 Mexico

Presentation Type: Poster
Topic: Thermal and Dynamical Properties

Abstract:
The effects of anharmonic interactions on the phonon modes in quasiperiodic systems are studied by looking at the time-evolution Fourier spectrum. The results reveal an efficient generation of high-amplitude harmonics in Fibonacci chains. We suggest that this effect is due to the dense reciprocal space structure, which allows momentum conservation to be satisfied for almost any wave-vector in a phonon collision. An energy-level-spacing analysis of the spectrum is also performed. Anharmonic contributions do clearly manifest by changing the level clustering behavior observed in quasiperiodic chains, and contrary to the periodic case, where the distribution is insensitive to weak anharmonic interactions. This result suggests a structural instability of the self-similar vibrational spectrum in quasiperiodic systems.

"Limit-Quasiperiodic Ammann Bars and Two-Dimensional Limit-Quasiperiodic Structures"
Komajiro Niizeki¹
¹Tohoku University, 980-8578 Sendai Japan

Presentation Type: Poster
Topic: Mathematics

Abstract:
A one-dimensional (1D) binary tiling produced by the substitution rule, S → SLS and L → LSSL, is limit-quasiperiodic (LQP) if the lengths of the two intervals, S and L, are chosen so that |L|/|S| = √2, where the Frobenious eigenvalue, τ, of the relevant substitution matrix is a non-unit Pisot number, 2 + √2. The 1D tiling defines an LQP array of Ammann bars in 2D. A superposition of four sets of LQP Ammann bars is an octagonal LQP structure in 2D if the angles between different Ammann bars are multiples of π/4. The set of all the points at which four Ammann bars intersect is a ``superquasicrystal", i.e., an LQP Delone set with a non-crystallographic point symmetry. The superquasicrystal has a selfsimilarity whose selfsimilarity ratio is equal to τ. Alternatively, the dual to the superposed Ammann bar is another superquasicrystal, which is locally derivable from the former and vice versa.

These methods of constructing superquasicrystals are readily extended to the case of the decagonal or dodecagonal point symmetry; the requisite to a relevant 1D LQP tiling is that τ belongs to an appropriate algebraic field. A simplest example is the case of the substitution rule, S → LL and L → LSL for which τ = 1 + √3 and |L|/|S| = τ. It yields a dodecagonal superquasicrystal.

"Low-Temperature TEM Studies of the Cd-Based 2/1 Approximants and Quasicrystals"
Kazue Nishimoto¹, Ryuji Tamura¹, Shin Takeuchi¹, Keiichi Edagawa², Masaki Ichihara³
¹Tokyo University of Science
²Institute of Industrial Science, University of Tokyo
³Institute for Solid State Physics, University of Tokyo

Presentation Type: Poster
Topic: Structure

Abstract:

Low-temperature phase transitions were reported for 1/1-Cd₆Yb and 1/1-Cd₆Ca at 110K and 100K, respectively, and their origin has been attributed to an orientational ordering of a Cd-tetrahedron residing in the center of the icosahedral clusters located at the vertex and the body-center of the unit cell. In nearby compositions, there exist 2/1 cubic approximants, Cd₇₆Yb₁₃, Cd₇₆Ca₁₃, and icosahedral quasicrystals, Cd_5.7Yb, Cd_5.7Ca, which are also believed to possess a disordered Cd-tetrahedron in the center of icosahedral clusters at room temperature. Therefore, in order to clarify a possible occurrence of a similar type of phase transition, the 2/1 approximants and the quasicrystals have been investigated by TEM at low temperatures using liquid-N₂ and liquid-He holders. For the 2/1 approximants, the appearance of superlattice spots were observed at low-temperatures suggesting an occurrence of a phase transition. For the quasicrystals, no clear change has been seen at low temperatures down to 16 K up to now. A further investigation on the quasicrystals is now in progress and the results will be discussed in details in the poster presentation.

"First-Principles Studies on Orientational Ordering of Atomic Clusters in Cd₆Ca"
Kazuki Nozawa¹, Yasushi Ishii¹
¹Chuo University, SORST-JST, 112-8551 Tokyo Japan

Presentation Type: Poster
Topic: Structure

Abstract:
Cd₆Ca, which is a cubic approximant crystal of binary quasicrystals Cd_5.7M (M=Yb, Ca), exhibits an order-disorder transition at about 100 K. The crystal structure of Cd₆Ca is described as a bcc packing of a four-layered atomic cluster. Although structures of outer three shells are almost determined, that of the innermost first shell is still unclear. The order-disorder transition seems to be originated in a change of orientations of first shells. A candidate of the first shell is a regular tetrahedron. The tetrahedral first shell, however, seems to be unreasonable in terms both of the electronic and elastic aspects. Recently, Gomez and Lidin proposed another structure of the first shell, which is constructed by replacing each vertex of a regular tetrahedron with three splitting site. We perform first-principles calculations based on the density functional methods using plane wave basis sets and pseudo-potentials technique. It is found that the newly proposed structure is more stable than the one with the regular tetrahedral core. The stable ordering of two first shells is studied with atomic positions determined by experiments and it is found that an anti-parallel-like ordering is stable than a parallel-like ordering as the case of the regular tetrahedral core. Moreover it turns out that the most and second stable states are almost degenerate. We will examine the effect of the structural relaxation by first-principles calculations. Furthermore, we plan to investigate the ordering with larger unit cell.

"Direct Evidence of Fine Gap Near Fermi Level in Density of States of Quasicrystals"
Junpei Okada¹, Toshikazu Ekino², Yoshihiko Yokoyama³, Tomoaki Takasaki², Yasuhiro Watanabe⁴, Susumu Nanao⁴
¹Department of Advanced Materials Science,The University of Tokyo, Kashiwa-shi, Chiba 277-8561, JAPAN
²Faculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima, 739-8521, JAPAN
³Materials Science and Engineering, University of Hyogo, Himeji 671-2201, JAPAN
⁴Institute of Industrial Science, The University of Tokyo, Meguro 153-8505, Japan

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
Theoretical works have predicted that quasicrystals(QCs) have conspicuous fine electronic structure near Fermi level(E_F). This fine structure has been believed to be an origin of the extremely high resistivity of QCs. However, though the vigorous efforts for more than ten years, no experiment has succeeded in observing the fine electronic structure. We have investigated the density of states around E_F on icosahedral (i-)AlCuFe, i-AlPdMn, and decagonal-AlCuCo quasicrystal with break-junction tunneling spectroscopy and found clear pseudogaps ranging from 3.3meV to 23meV at 4.2K. This is the first experimental result that confirms the above predicted fine electronic structure directly.

"Structure of the Pseudodecagonal Al-Co-Ni Approximant PD4"
Petr Oleinikov¹, Jeppe Christensen¹, Xiaodong Zou¹, Sven Hovmöller¹, Markus Döblinger², Benjamin Grushko³
¹Stockholm University, 10691 Stockholm Sweden
²Department of Materials, University of Oxford, Oxford, OX1 3PH, UK
³IFF, Forschungszentrum, Jülich GmbH, D-52425, Jülich, Germany

Presentation Type: Poster
Topic: Structure

Abstract:
Crystalline approximants are key materials for determining the structures of quasicrystals, because of their similarity to the quasicrystals in terms of their compositions, densities and diffraction patterns. Several crystal structures of these so-called approximants were determined [1-2]. Several periodic pseudodecagonal (PD) structures were found in the Al-Ni-Co system. They are named PD1, PD2 etc. and exhibit diffraction patterns with almost perfect tenfold symmetry and quasiperiodic reflection arrangements [3]. We collected X-ray diffraction data from a single crystal of PD4 phase using Oxford Xcalibur™ 3 diffractometer. The unit cell parameters for this structure are a = 101.302 Å, b = 32.102 Å, c = 4.1803 Å. There were difficulties with intensities extraction from the obtained data. However we were able to overcome them using several different approaches, involving local peak fitting procedures for strong reflections. The data processing resulted in the extraction of intensities of over 3800 crystallographically unique reflections to 0.85 Å resolution.

The model for the psedodecagonal approximant PD4 was deduced from the orthorhombic phase o-Al₁₃Co₄ [1]. First, the crystallographic phases of strongest reflections [4-5] were determined using direct methods in the centro-symmetric space group Bbmm. The comparison of the phases showed that they are matching exactly, which allows us to assume that the crystallographic phases of other strong reflections for the PD4 approximant should be the same. The electron density map was calculated using the phases of reflections from o- Al₁₃Co₄ (reindexed accordingly to the PD4 approximant diffraction pattern). In a peak search procedure 136 peaks were found in the asymmetric unit for the given space group. The refinement of the structure using the deduced model is in progress.

[1] C. Freiburg, B. Grushko, R. Wittenberg, W. Reichert. Mater. Sci. Forum 228-231 (1996) 583-586.
[2] X. L. Ma and K. H. Kuo. Metall. Trans. 23A (1992) 1121.
[3] B. Grushko, D. Holland-Moritz, R. Wittmann, G. Wild. J. Alloy. Comp. 280 (1998) 215n230.
[4] J. Christensen, P. Oleynikov, S. Hovmöller and X.D. Zou, Ferroelectrics, 305, 273, 2004.
[5] X.D. Zou, J. Christensen, H. Zhang, P. Oleynikov, S. Hovmöller, 22^nd European crystallographic meeting, 26-31 August, 2004, Budapest, Hungary.

"Transition from Quasicrystals to Amorphous Structures"
Dariusz Orzechowski¹, Janusz Wolny¹
¹AGH University of Science and Technology, 30-059 Krakow Poland

Presentation Type: Poster
Topic: Structure

Abstract:
The diffraction pattern of Fibonacci chain consists of main peaks and series of satellite peaks. We can construct the average unit cell of the length equal to the average distance between nodes and obtain probability distribution in such average unit cell. Fourier transformation of such probability distribution P(u) gives an envelope function which shows correct intensities of all main reflections. By creating separate average unit cell of the length τ-times bigger we are able to describe correctly also intensities of all satellite peaks. In the (u,v) parameter space probability distribution P(u,v) proves to be only one line with the constant value on it and zero elsewhere.[1]

Exactly the same reasoning can be used to describe disordered structures. For example, the perfect Fibonacci chain nodes positions can be perturbed by adding to them a random value ε of the Gaussian distribution with the standard deviation σ.

The scope of the work was to observe influence of the ε value on the diffraction pattern - as well main and satellite reflections. We have investigated also changes in the P(u,v) distributions as a function of the sigma parameter. There are two well-defined extreme cases: for σ = 0 we have just pure Fibonacci chain (i.e. perfect one-dimensional quasicrystal) and for s going to infinity the structure becomes completely random. Between these two extremes, however, large variety of partially disordered structures exist and we can observe transition from amorphous to the quasicrystalline state.

[1] J. Wolny, Average unit cell of the Fibonacci chain, Acta Cryst. A54 (1998) 1014-1018.

"Quasicrystal Formation and Nanocrystallization in Zr-Cu-Ni-Al-Ta Metallic Glasses and Metallic-Glass-Matrix Composites"
Ryan Ott¹, Matthew Kramer¹, Todd Hufnagel², Daniel Sordelet¹
¹Materials and Engineering Physics, Ames Lab, Ames, IA 50011 U.S.A.
²Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD 21209 U.S.A

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
Small additions of early transition metals such as Nb, Ta, and V have been found to promote the formation of quasicrystals in Zr-based metallic glass alloys during devitrification. We have recently found that additions of Ta as low as 2% have a significant effect on the crystallization behavior of (Zr₇₀Cu₂₀Ni₁₀)_90-xAl₁₀Ta_x alloys. We find that isochronal annealing of non-Ta containing alloys results in two distinct exotherms while alloys containing greater than 2% Ta exhibit four exotherms when annealed. The first exotherm for the non-Ta containing alloys corresponds to the nucleation of several different intermetallic phases, while the first exotherm for the Ta-containing alloys corresponds to the formation of an icosohedral quasicrystalline phase. Additionally, we find that for Ta-containing alloys isothermally annealed through their second exotherm, the (ex situ) XRD pattern does not show any obvious difference from the XRD pattern corresponding to annealing through the first exotherm. TEM analysis of samples annealed through the first and second exotherm shows that for both annealing conditions, the microstructures consist of quasicrystals dispersed in residual amorphous matrix. To further examine the phenomenon associated with the second exotherm, we have used time-resolved in situ high-energy x-ray scattering to monitor the phase evolution during isothermal and isochronal annealing. The differences between the in situ XRD results and the ex situ XRD and TEM results are discussed

"Thin or thick Atomic Layers as Terminations of Quasicrystals"
Zorka Papadopolos¹, Gerald Kasner²
¹Inst. fuer Theoretische Physik, Univ. Tuebingen, 72076 Tuebingen Germany
²Inst. fuer Theoretische Physik, Univ. Madeburg, 39016 Magdeburg, Germany

Presentation Type: Poster
Topic: Surfaces

Abstract:
The clean surfaces of icosahedral quasicrystals, orthogonal to the directions of the main symmetry axes (two, three and fivefold) have a terrace-like appearance. We extend the Bravais' rule for crystals (the surfaces are the maximally dense atomic planes in a crystal) to the quasicrystals, allowing that instead of a single atomic plane, a layer of atomic planes may form a bulk termination. In [1] we introduced a concept of the "thin" layers (relative to the shortest interatomic distances) that may replace the atomic planes. There arise some open questions in the frame of the "thin" layer concept, broadly explained in Ref [1]. In the same paper [1] we anticipated a concept of the "thick" layer terminations as well. We present our results now and show the advantages of the new concept.

[1] Z. Papadopolos et al., A maximum density rule for bulk terminations of quasicrystals, in Phys. Rev. B 69, 224201 (2004)

"Novel Tribological Properties of Decagonal Quasicrystals"
Jeong Park¹, D. Ogletree¹, Miquel Salmeron¹, R. Ribeiro², P. Canfield², C. Jenks², P. Thiel²
¹Lawrence Berkeley National Laboratory, Berkeley, CA 94706 USA
²Ames Laboratory, Iowa State University, Ames, IA 50011 USA

Presentation Type: Contributed Talk
Topic: Mechanical Properties and Applications

Abstract:
The structural and tribological properties of the interface between decagonal Al-Ni-Co quasicrystals and conductive cantilevers have been investigated. This was accomplished using a combined atomic force and scanning tunneling microscopy apparatus in ultrahigh vacuum. Atomically resolved STM images of the 2-fold Al-Ni-Co surface were obtained showing a periodicity along 10-fold direction, and a quasi-periodicity (following Fibonacci sequences, and inflation symmetry with Golden Mean) along the 2-fold direction. To decrease the high adhesion between the atomically clean quasicrystal surface and the metallic AFM tip (with high adhesion force as large as 1000 nN), the tip or the surface were passivated by hydrocarbon molecules (ethylene and alkylthiol), which made possible the formation of stable contacts. This allowed us to observe an elastic to plastic transition occurring at a threshold load. With alkylthiol passivated tips the friction properties of the highly anisotropic 2-fold surface could be studied. We found a strong dependence of the friction force on the scanning direction, with low friction occurring along the aperiodic direction and high friction in the periodic direction. This remarkable result will be discussed in light of friction models of the interface based on (i) existence or not of commensurability between the two contacting surfaces, (ii) energy dissipation by electrons, and (iii) molecule-substrate interaction.

"3D-Structures of Quasicrystals and Higher Approximants in the M-Cd Systems (M = Ca, Yb)"
Cesar Pay-Gómez¹, Hiroyuki Takakura², Akiji Yamamoto³, Marc de Boissieu⁴, An Pang Tsai¹
¹IMRAM, Tohoku University, 980-8577 Sendai, Japan
²Research Center for Molecular Thermodynamics, Osaka University, 560-0043 Osaka Japan
³Advanced Materials Laboratory, NIMS, 305-0044 Tsukuba, Japan
⁴LTPCM/ENSEEG, UMR CNRS 5614, INPG, UJF, PB75, 38402, St.Martin d'Hères Cedex, France

Presentation Type: Contributed Talk
Topic: Structure

Abstract:
The binary quasicrystals found in the Yb-Cd and Ca-Cd systems have attracted great interest because of their unique combination of features.[1, 2] The structures of several 1/1 and 2/1 approximants have recently been studied by single crystal X-ray diffraction, [3-5] these results combined with diffraction data collected on the quasicrystals, [6] have enabled the structural modeling of the quasicrystals and higher approximants using only two types of building units without introducing further 'glue atoms'. The two identified building units consist of a triacontahedral cluster and a double Friauf polyhedron. By observing the different linkages between clusters in the approximants a set of construction rules have been postulated. These rules are in accordance with the cluster arrangement obtained by high-dimensional projection for a 3/2 approximant derived from an ideal quasicrystal.[7] All the approximants, including the 3/2 approximant could be decomposed into canonical cells with identical atomic decorations.
Single crystal data collected on the YbCd_5.7 quasicrystal indicate the presence of yet another type of cluster unit besides the triacontahedral unit; this additional cluster unit is of Bergman-type. The presence of this cluster type within the quasicrystals and its coexistence with the triacontahedral clusters is fully compatible with the postulated construction rules and identified building units.

[1] A. P. Tsai, J. Q. Guo, E. Abe, H. Takakura, T. J. Sato, Nature 2000, 408, 537.
[2] J. Q. Guo, E. Abe, A. P. Tsai, Phys. Rev. B 2000, 62, R14605.
[3] C. P. Gómez, S. Lidin, Phys. Rev. B 2003, 68,024203(1).
[4] C. P. Gómez, in Order and disorder in the RE-Cd and related systems, Stockholm University, Stockholm, 2003, pp. 54.
[5] C. P. Gómez, S. Lidin, Angew. Chem., Int. Ed. Engl. 2001, 40, 4037.
[6] H. Takakura, A. Yamamoto, M. de Boissieu, A. P. Tsai, Ferroelectrics 2004, 305, 209.
[7] H. Takakura, C. P. Gómez, A. Yamamoto, M. de Boissieu, A. P. Tsai, in Conf. Proc. ICQ9, submitted, Ames, Iowa State University, USA, 2004.

"Microheterogeneity of some Quasicrystal Forming Alloys in Liquid State"
Pjotr Popel¹, Valery Sidorov¹, Olga Chikova¹, Kirill Grushevsky¹, Irina Brodova
¹Ural State Pedagogical University, 620219 Ekaterinburg Russia
²Institute of Metal Physics, Ural Division of Russian Academy of Sciences, 620219 Ekaterinburg Russia

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
Several physical properties of some quasicrystal forming systems in liquid state have been studied. In particular, temperature dependences of density, viscosity, ultrasound velocity and magnetic susceptibility for Al-Mn, Al-Ni, Zr-Pd melts were registered. Their common feature is branching the curves obtained while heating and subsequent cooling the samples. We link the phenomenon with an irreversible transition of the systems from a metastable microheterogeneous state inherited from solid precursors, to a state of a homogeneous solution. The direct confirmation of the existence of such microheterogeneity in liquid eutectic alloys was obtained by U.Dahlborg and M.Calvo-Dahlborg in the course of small angle neutron scattering experiments. The influence of structural state of liquid precursor on quasicrystal formation has been studied for Al-Mn alloy.

"Dual Role of Disorder and Peculiarities of the Electronic Structure in Icosahedral Quasicrystals"
Alexandre Prekul¹, Nikolay Kuzmin¹, Natalie Shchegolikhina¹
¹Institute of Metal Physics of the RAS, 620219 Ekaterinburg Russia

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
Reconsideration of the experimental data available shows that the structural disorder yields a double effect on the electron system of icosahedral phases – it localizes the electron states and alters the efficient average valency of an alloy. As a result, a specific feature of the electronic structure in the practical icosahedral phases is determined by the following moments. 1) There is a mobility gap, the width of which increases due to an increase of disorder, inside of a fall on the electronic density of states curve. 2) The Fermi-level shifts along with the change of the disorder degree in such a way that it always coincides with a mobility edge. A meaningful consequence of the feature is an emergence of non-degenerated charge carriers at a finite temperature in the presence of degenerated charge carriers at the ground state. Experimental observations of that are various, among them the coexistence of metallic and variable-range hopping conductivities, the empirical regularity that is known as Inverse Matthiessen Rule, the congruence of the temperature parts of the dc-conductivity and magnetic susceptibility, the truncated Drude-like contribution to optical conductivity, are discussed.

"Defects in the Structure of Icosahedral Phase in Rapidly Quenched Quasicrystal-Forming Alloys Al₆₁Cu₂₆Fe₁₃ Subjected to Annealing"
Elizaveta Shalaeva¹, Alexandre Prekul², Robert Martin³
¹Institute of Solid State Chemistry,Ural Div. of RAS, Ekaterinburg, Ural 620219 Russian Federation
²Institute of Metal Physics, Ural Div.of RAS, Ekaterinburg, Ural 620219 Russian Federation
³Department of Physics, Rottenrow Strathclyde University, Glasgow, G40NG UK

Presentation Type: Poster
Topic: Structure

Abstract:
Investigation of structural transformations in quenched two-phase alloys Al₆₁Cu₂₆Fe₁₃ (β(CsCl)-solid solution and i-phase) is informative both for exploring the mechanisms of structural transitions β→i and β-based phase→i and for studying microstructure of i-phase, which is formed as a result of annealing of the quenched alloys. Transmission electron microscopy and probe microanalysis were used to investigate into the phase composition and microstructure of the rapidly quenched alloys Al₆₁Cu₂₆Fe₁₃ after annealing, with the annealing temperature range being 350-650°C. Quenched alloys with the two-phase structure were investigated, when the i-phase forms dendrites and the interdendritic layers, which are considerably copper-enriched, contain β-solid solution that shows signs of massive transformation bcc→fcc. The lattices of i-phase and β-phase demonstrate a typical orientation relationship. Over the whole annealing temperature range the microstructure of the initial icosahedral phase remains nearly unchanged while the major structural changes take place in the interdendritic areas. At an annealing temperature of 450°C in the interdendritic layers a martensite-like phase is formed, with a high concentration of random stacking faults. Annealing at 550-650°C results in the alloys having single-phase multigrain icosahedral structure. It was found that annealing at 550°C leads to the formation of icosahedral structure with a very high concentration of staggered boundaries in the initial interdendritic areas. These boundaries lie in the plains perpendicular to the A5i axes. As a result of the annealing at a temperature of 650°C these defects undergo changes and groups of individual ultra-thin (up to a few nm) interlayers are observed which also lie in the plane perpendicular to A5i axis. The formation of the defects in the icosahedral structure is considered from two points of view: (1) as a result of the mechanism of i-phase formation and the effect of martensite-like phase microstructure and (2) as a result of a strained state due to the phase transformation β-brass→i.

"Quasiperiodic Structures Constructed by Projection in Two Stages"
Alexander Quandt¹, Shelomo Ben-Abraham²
¹Ernst-Moritz-Arndt Universität, Institut für Physik D-17489 Greifswald Germany
²Department of Physics, Ben-Gurion University, POB 653, IL-84105 Beer-Sheba, Israel

Presentation Type: Contributed Talk
Topic: Mathematics

Abstract:
Since the recognition of quasicrystals for what they are a plethora of pentagonal, octagonal, decagonal and dodecagonal structures have been observed in several alloy systems. These structures are quasiperiodic in a plane and periodic in its perpendicular direction. Incommensurate crystals, aperiodic in one direction have, of course been known for decades. At the same time, the mathematical aspects of tilings with the mentioned symmetries and aperiodic chains have been intensively studied as well. It is therefore interesting to study intermediate structures in which the periodic and quasiperiodic directions are intrinsically connected. One way to do so is by projecting a periodic structure in D (>3) dimensions into three-dimensional space in such a way that the second projection be quasiperiodic in plane. We have achieved this earlier in the octagonal case [1] and partly in the dodecagonal case [2]. Here we briefly review these and present an improved dodecagonal version. We also present a new look at the pentagonal, or rather decagonal, case. In the octagonal case we cut and project first the four-dimensional simple cubic lattice Z⁴ into R³ and then into a suitable irrational R². In the pentagonal/decagonal case we start with then five-dimensional simple cubic lattice Z⁵ and in the dodecagonal case with the root lattice D₄ (in the earlier version it was Z⁶).

[1] S.I. Ben-Abraham, Ferroelectrics, 305 (2004) 29-32.
[2] S.I. Ben-Abraham, Y. Lerer, Y. Snapir, J. Non-Cryst. Solids, 334&335 (2004) 71-76.

"Description of the Basic Atomic Clusters in F- and P-Type Prototypic Icosahedral Phases"
Denis Gratias¹, Marianne Quiquandon¹, Jean-Tristan Beauchesne¹
¹LEM/CNRS-ONERA, F-92322 Chatillon cedex France

Presentation Type: Poster
Topic: Structure

Abstract:
Description of the basic atomic clusters in F- and P-type prototypic icosahedral phases

We use the standard cell decomposition of the two basic typical F- and P-icosahedral phases to determine the kind of atomic clusters generated by these models. After the well-known decomposition of the F-type phases in B (Bergman) and M (Mackay) clusters, we present some other kind of clusters (B', extended M and B clusters) and a detailed desciption of their interconnections, overlaps and distribution in the global network for both F- and P-type structures. Emphasis will be made on the comparison between F- and P- icosahedral prototypic structures for simple decorations beyond the KG model.

References:
- D. Gratias, F. Puyraimond and M. Quiquandon, "Atomic clusters in icosahedral F-type quasicrystals", Phys. Rev. B vol 63, 024202-1:16 (2000).
- N. Shramchenko, these de l'universite Paris 7, "Localisation des especes atomiques dans les quasicristaux AlPdMn", 22 octobre 2001.
- F. Puyraimond, these de l'Ecole Polytechnique, "Etude de phases approximantes de structures icosaedriques: de la validite de la methode du cisaillement perpendiculaire", 20 decembre 2001.
- M. Duneau and D. Gratias, "Covering clusters in icosahedral quasicrystals", in Coverings of discrete quasiperiodic sets, Eds P. Kramer and Z. Papadopolos, Springer Tracts in Modern Physics, (2003).

"The Application of Pettifor Maps to Intermetallics and Quasicrystals"
Srinivasa Ranganathan¹
¹Indian Institute of Science, 560012 Bangalore India

Presentation Type: Poster
Topic: Structure

Abstract:
David Pettifor developed a powerful phenomenological method for rationalising the formation of intermetallics. It combines the various atomic bonding factors such as size, electronegativity, valence electrons and the bond orbitals and assigns a number known as Mendeleev Number to each element. In turn this leads to convenient maps, which locate crystalline structures for specific combinations of elements in specific domians. While it was originally developed for binary alloys, it was extended to higher order alloys by considering them as pseudo-binary alloys. The application of this extension to Laves phases and quasicrystals will be emphasised. In the case of ternary Laves phases Zr-Ti-Ni and Hf-Ti-Ni, it will be argued that Zr and Hf occupy the large atom sites, while Ti and Ni occupy the small atom positions. This is in spite of the isoelectronic nature of Ti, Zr and Hf, indicating that the size of the atoms plays a dominnat role. A. Inoue and S. Ranganathan have earlier shown that the rational approximants to quasicrystals can be classified into four types as Bergman, Mackay and Kuo and Tsai approximants. In this scheme all rational approximanrts and by implication quasicrystals can be treated as psedobinary intermetallics with the average Menedeleev number of the elements as the discriminator. The case of Kuo approximant will be compared with the ternary Laves phases with similar compositions but a different occupation schemes. In many ways the Kuo approximant can be regarded as the anti-Laves phase. Both the Laves and anti-Laves phsase have the same stoichimetry of AB2 but the large and the small atom proportions are reversed.

"Recent Results at the Metal-Insulator Transition of Icosahedral AlPdRe"
Ö. Rapp¹, V. Srinivas², S. Poon³
¹KTH, 164 40 Stockholm-Kista Sweden
²Indian Institute of technology, Kharagpur, 721 302 India
³University of Virginia, Charlottesville, Virginia 22901, USA

Presentation Type: Contributed Talk
Topic: Electronic and Magnetic Properties

Abstract:

New results at the metal-insulator transition, MIT, of icosahedral (i-) AlPdRe are discussed. A method to evaluate the localization length, ξ, and the characteristic hopping temperature, T_o temperature from measurements of conductivity and magnetoresistance will be described. Neutron irradiation experiments and subsequent annealings have shown that the resistance ratio R=ρ(295 K)/ρ(4.2 K) can be controlled reversibly. This result rules out that minor undetected sample impurities would control the wide range of observed R-values, and demonstrates that R is an intrinsic parameter, empirically describing the electronic state of i-AlPdRe samples. Consistent results for ξ and T₀ have been obtained. ξ diverges as ~(R-R_c)^-1/3 when approaching the MIT at R_c=20, and T₀ approaches 0 as (R-R_c)^β, with β somewhat smaller than 1. The result for ξ suggests the physical picture that (R-R_c) increases into the insulator as the inverse of the decreasing localization volume, the volume of electrons that hop. By estimating an effective charge carrier density, comparisons with doped semicoductors could be made. The results suggest that quasicrystals and doped semiconductors belong to the same universality class.

"The Interaction of Copper with the Five-Fold Surface of the AlPdMn Quasicrystal"
Dennis Reid¹, Julian Ledieu¹, Jon Hoeft¹, Joseph Smerdon¹, Thomas Lograsso², Amy Ross², Renée Diehl³, Nicola Ferralis³, Vin Dhanak⁴, Ronan McGrath¹
¹Surface Science Research Centre, The University of Liverpool, Liverpool, Merseyside, L69 3BX, UK
²Ames Laboratory, Iowa State University, Ames, IA 50011 USA
³Penn State University, 104 Davey Lab, No. 254, University Park, PA 16802-6300 USA
⁴CCLRC Daresbury Laboratory, Warrington, WA4 4AD, UK

Presentation Type: Poster
Topic: Surfaces

Abstract:
The adsorption of copper on the five–fold surface of the icosahedral Al–Pd–Mn quasicrystal at room temperature has been investigated using scanning tunnelling microscopy (STM), low energy electron diffraction (LEED), and soft X–ray photoemission spectroscopy (SXPS) at the Daresbury Synchrotron Radiation Facility. STM indicates the formation of an overlayer consisting of aperiodically spaced rows of atoms[1]. LEED results obtained at low temperature indicate that Cu atoms are periodically spaced along the row directions with a spacing of 2.50 Å [2]. SXPS was used to probe changes in the electronic structure of the system as a function of Cu dose. Scans of the Al 2p/Cu 3p core level region are consistent with a layer–by–layer growth mode observed in STM. Valence band data show the growth of the Cu d–bands as a function of Cu dose. Shifts are observed in both the position of the d–bands and in the Fermi level position as a function of Cu coverage. In these angle–integrated scans there is no evidence of electron confinement along the Cu rows.

[1] J. Ledieu, J.T. Hoeft, D.E. Reid, J.A. Smerdon, R.D. Diehl, T.A. Lograsso, A.R. Ross and R. McGrath, Phys. Rev. Lett. 92 (13):135507 (2004)
[2] J. Ledieu, J.T. Hoeft, D.E. Reid, J.A. Smerdon, R.D. Diehl, T.A. Lograsso, A.R. Ross and R. McGrath, Submitted for publication

"The Structure of Collagen"
N. Rivier¹, Y. Bouligand², J. Sadoc³
¹Advanced Light Source, Lawrence Berkeley Laboratory, Berkeley CA
²Laboratoire d' Histophysique et Cytophysique, Ecole Pratique des Hautes Etudes, 49100 Angers, France
³Laboratoire de Physique des Solides, Université Paris Sud, 91405 Orsay, France

Presentation Type: Poster
Topic: New Frontiers

Abstract:
Collagen is the principal constituent of extra-cellular, connective tissue, a tightly-packed but flexible bundle of proteins. The protein is here the constituent of a material, rather a biologically active molecule. The structure of collagen, as described by many biologists, seems very complex, but it is periodic, and can be understood by simple and natural rules from material science (close-packing, periodicity, geometric frustration) and from number theory (periodicity and approximants of quasiperiodic structures). The collagen molecule is a triple helix made of three, intertwined polypeptide chains, associated with √3 and its rational convergents. Accordingly, the collagen fibril, a long, periodic bundle of finite collagen molecules, has a transverse structure based on two underlying, perpendicular triangular lattices. It is the Archimedean, square-triangle lattice 3².4.3.4, in both "overlap" and "gap" regions, with small orthorhombic distortions imposed as rational approximants of the two perpendicular triangular lattices, as observed [1]. The three-dimensional structure is a periodic, rotating stack of gap and overlap regions, with a ten-fold rotation symmetry. The Fourier spectrum, obtained experimentally [1,2] and numerically, will be discussed.

[1] Ke. Okuyama, et al., J. Mol. Biol. 152, 427 (1981)
[2] Y. Bouligand, L'assemblage compact des triples héices de collagène, JMC5, poster 2108, Soc. Fr. Phys., Orléans 1997

"Comparative Study of the Wettability by Liquid Pb of Decagonal Al₁₃Co₄ and Cubic Al-Co Thin Films"
C. Bergman¹, G. Clugnet¹, C. Girardeaux¹, C. Perrin-Pellegrino¹, P. Gas¹, D. Chatian², J. Dubois³, N. Rivier⁴
¹L2MP, CNRS-UMR 6137 and Université Paul Cézanne 13397 Marseille, France
²CRMC-N, UPR-CNRS 7251, Campus de Luminy, 13288 Marseille Cedex 9, France
³LSG2M, UMR-CNRS 7584, Parc de Saurupt, 54042 Nancy Cedex, France
⁴IPCMS, Université Louis Pasteur, 67084 Strasbourg Cedex, France

Presentation Type: Poster
Topic: Surfaces

Abstract:
To study the surface properties of quasicrystalline phases, it is necessary to use clean surfaces (without O₂ pollution), of good quasicrystalline quality (if possible without treatments to avoid the necessity of reconstructing the surface). In early work [1], we have demonstrated that it is possible to prepare homogeneous thin films of fixed composition of some intermetallic compounds of the Al/Co system, by successive deposition in high vacuum of the elements and subsequent annealing treatment. Moreover, this method offers the possibility to prepare the samples in the same apparatus where measurements of the wetting properties or characterisations are carried out. This opens the possibility of in situ experiments on thin films of decagonal Al₁₃Co₄ and cubic AlCo phases. Notably, a 300 nm film of Pb has been deposited on the intermetallic substrate, then fragmented into microdroplets (1-10 μm), in situ and under ultra-vacuum conditions. The absence of Al₂O₃ on the surface has been checked by Auger spectroscopy analysis, coupled with electron microscopy measurements, yielding an image of the wetting angle of Pb. The value of the measured wetting angle of liquid Pb is substantially smaller than 90° and there is no significant difference between the values observed on decagonal and cubic films. Further scanning electron and cross-section transmission electron microscopy measurements indicate that there is no anchoring of the liquid drop to the grain boundaries of the films and that there is no reactive wetting.

[1] E. Emeric, C. Bergman, G. Clugnet , P. Gas and M. Audier, Phil. Mag. Lett., 78 77 (1998).

"Electronic Transport Behaviors of Bulk Insulating QC AlPdRe Samples"
Ralph Rosenbaum¹
¹Tel Aviv University, 69978 Ramat Aviv Israel

Presentation Type: Invited Talk
Topic: Electronic and Magnetic Properties

Abstract:
Zero field resistivity and magnetoresistance (MR) data on a 3D insulating QC AlPdRe sample are presented in the temperature range from 300 K to 0.0204 K and in a magnetic field range from 0 T to 18 T. This QC sample had a resistance temperature ratio of 66. From 300 K to 90 K, the conductivity exhibited a simple temperature power law dependence with an exponent z = 1.55. Below 90 K, there was a crossover to a weaker power law having an exponent z = 0.90, valid below 10 K to 0.5 K. Below 0.5 K, there was a second crossover to an activated variable-range hopping (VRH) law having a hopping exponent y = 0.294 and characteristic temperature T₀ = 3.65 K. The presence of a second phase in this sample made a major contribution to the measured conductivity below 1 K; in order to study the QC conductivity contribution, this second phase conductivity was estimated at T = 0 K and its value was subtracted off from the measured conductivity values. At 1 K the MR data can be explained using the saturation conductivity and the forward interference theory of Lien Van Nyugen and the wave function shrinkage model of Ioselevich and Schoepe. The MR behavior below 0.2 K is highly anomalous at very small fields where the MR follows a B^0.63 law dependence; this is to be contrasted to the quadratic B² dependence that is predicted theoretically in many MR theories.
Data on other less insulating AlPdRe QC samples will be presented and compared.
Samples were fabricated using the arc melting technique by Prof. Shui-Tien Lin's group in Tainan, Taiwan. This work was performed at the National High Magnetic Field Lab (NHMFL) under NSF Cooperative Agreement #DMR 9527053 and by the State of Florida.

"Quasicrystalline Order in Binary Dipolar Systems"
Frank Scheffler¹, Philipp Maass², Johannes Roth³, Holger Stark¹
¹Fachbereich Physik, Universität Konstanz, 78457 Konstanz, Germany
²Inst. f. Physik, Technische Universität Ilmenau, 98684 Ilmenau, Germany
³Inst. f. Theoret. u. Angew. Physik, Universität Stuttgart, 70550 Stuttgart, Germany

Presentation Type: Contributed Talk
Topic: Structure

Abstract:
Motivated by recent experimental findings, we investigate the possible occurrence and characteristics of quasicrystalline order in two-dimensional mixtures of point dipoles with two sorts of dipole moments. Despite the fact that the dipolar interaction potential does not exhibit an intrinsic length scale and cannot be tuned a priori to support the formation of quasicrystalline order, we find that configurations with long–range quasicrystallinity yield minima in the potential energy surface of the many particle system. These configurations emanate from an ideal or perturbed ideal decoration of a binary tiling by steepest descent relaxation. Ground state energy calculations of alternative ordered states and parallel tempering Monte-Carlo simulations reveal that the quasicrystalline configurations do not correspond to a thermodynamically stable state. On the other hand, steepest descent relaxations and conventional Monte-Carlo simulations suggest that they are rather robust against fluctuations. Local quasicrystalline order in the disordered equilibrium states can be strong.

"Tilings, Tiling Spaces and Topology"
Lorenzo Sadun¹
¹University of Texas, Austin, TX 78712 USA

Presentation Type: Invited Talk
Topic: Mathematics

Abstract:
To understand an aperiodic tiling (or a quasicrystal modeled on an aperiodic tiling), we construct a space of similar tilings, on which the group of translations (or the Euclidean group) acts naturally. This space is then an (abstract) dynamical system. Dynamical properties of the space (such as mixing, or the spectrum of the translation operator) are closely related to bulk properties of individual tilings (such as the diffraction pattern). The topology of the space of tilings, particularly the Cech cohomology, gives information on how original tiling may be deformed. Tiling spaces can be constructed as inverse limits of branched manifolds. These constructions apply to tilings with infinitely many allowed orientations (like the pinwheel), on which the entire Euclidean group acts, as well as to tilings (like the Penrose tiling) on which only translations act.

"Self-Similarly Quantized Conductance in Quasiperiodic Nano Cables"
Vicenta Sanchez¹, Chumin Wang¹
¹Universidad Nacional Autonoma de Mexico, 04510 Mexico D.F. Mexico

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:

Starting from the Kubo-Greenwood formula [1], the dc and ac electrical conductances of nanocables, such as nanotubes, nanotapes and nanowires, are comparatively studied at zero temperature. For single-wall periodic nanotubes, a uniform distribution of 2G₀ steps are obtained in dc conductance spectra, where G₀=2e²/h is the conductance quantum, as recently observed in carbon nanotubes [2]. This fact could be understood as a consequence of the degeneracy introduced by the periodic boundary condition in the cross section of nanotubes. For the case of two-dimensional periodic nanotapes, regular G₀ steps are observed in their dc conductance spectra. In addition, for three-dimensional wires with a square cross section at nano meter scale, the quantization of their dc conductance is also observed, except that the steps have a variation of height of nG₀ depending on the degeneracy of eigenvalues. These quantized dc-conductance spectra become self-similar stairs if the perpendicular subspace of the system is quasiperiodic. Moreover, the ac conductance reveals a transition from one- to two-dimensional behavior when the cross-section area of nanotubes or nanotapes increases, and a rapid decay with the frequency is also observed when a quasiperiodic order is introduced along the electric field. Finally, a boundary condition analysis is performed on nanotapes without leads, observing a clear splitting of the resonance peaks found in one-dimensional ac-conductance spectra.

[1] V. Sanchez and C. Wang, J. of Non-Crystalline Solids 345, 518 (2004).
[2] A. Urbina, I. Echeverria, A. Perez-Garrido, A. Diaz-Sanchez, and J. Abellan, Phys. Rev. Lett. 90, 106603 (2003).

"Conducting Substrates for Growth of Aligned Carbon Nanotubes"
Gunturi Sastry¹, Saikat Talapatra², Anyuan Cao², Pulickel Ajayan²
¹Banaras Hindu University , Varanasi, UP 221005 India
²Rensselaer Polytechnic Institute, Troy, NY 12180-3590 USA

Presentation Type: Poster
Topic: New Frontiers

Abstract:
We report on the growth of aligned multi-walled carbon nanotubes (AMWNT) on conducting substrates by employing chemical vapor deposition technique (CVD) using ferrocene and xylene. Scanning electron microscopy (SEM) results showed a directional dependence on the nanotube growth. High Resolution Transmission Electron Microscopy (HRTEM) showed the evidence of metal (bcc Iron) filled nanotube structures. The relation with quasiperiodic structure will be broughtout.

"Possible E/T-Spin Fluctuation Spectrum in the P-type Zn-Mg-Ho Quasicrystal"
Taku Sato¹, An Pang Tsai²
¹NSL, ISSP, University of Tokyo, Tokai, Ibaraki 319-1106 Japan, and SORST-JST, Kawaguchi, Japan
²IMRAM, Tohoku University, Sendai 980-8577, Japan, MEL/NIMS, and SORST-JST, Kawaguchi, Japan

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
We will report neutron scattering results on the P-type Zn-Mg-Ho quasicrystal. Powder and single-quasicrystal neutron scattering experiments have been performed using the LAM time-of-flight spectrometers at the KEK pulse neutron source, as well as the GP-TAS spectrometer at the JAERI research reactor. Powder samples were obtained by melting constituent elements, followed by annealing at the elevated temperatures. Single grains were obtained using the Bridgman technique [1]. Scattering intensity was measured for 1.5 < T < 300K, and analyzed in a usual manner to obtain magnetic contributions.

Elastic scattering shows magnetic diffuse scattering, essentially identical to that of the F-type Zn-Mg-Ho quasicrystal [2], indicating similar static short-range spin orders in both the systems. A pronounced feature of the P-type Zn-Mg-Ho appears in its inelastic spectrum. While the scattering intensity for the neutron-energy-gain side (i.e., the E < 0 side) shows clear temperature dependence, the opposite side (E > 0) is surprisingly temperature independent. We should note here that the detailed balance is satisfied for both the sides, and thus, the signal is certainly intrinsic. This temperature-independent signal infers a very special dynamic-scaling function; generalized susceptibility was found to be written as Im &chi(E,T)T^1/3 &prop (E/T)^-1/3Z(E/T), where Z(E/T) &sim tanh(10E/T). The susceptibility is, thus, roughly scaled by E/T. Observation of such E/T-scaling is rare but existing; notable examples may be the heavy-fermion compounds UCu₄Pd [3] and CeCu_5.9Au_0.1 [4], where the scaling is attributed to their possible T=0 phase transitions, i.e., quantum criticality. Hence, it might be presumed that the presently-observed temperature-independence and resulting E/T-scaling infer anomalous spin fluctuations, which might also be due to a possible T=0 transition.

[1] E. Uhrig et al., Phil. Mag. Lett. 83 (2003) 265.
[2] T. J. Sato et al., Phys. Rev. B 61 (2000) 476.
[3] M. C. Aronson et al., Phys. Rev. Lett. 75 (1995) 725.
[4] A. Schroder et al., Nature 407 (2000) 351.

"Influence of Ball Milling on Quasicrystal Formation in Melt-Spun Zr-Based Glassy Ribbons"
Sergio Scudino¹, Jürgen Eckert², Hergen Breitzke³, Klaus Lüders³, Ludwig Schultz¹
¹IFW Dresden, Institut für Metallische Werkstoffe, Dresden, Germany
²FB 11, FG Physikalische Metallkunde, Technische Universitüt Darmstadt, Darmstadt, Germany
³Fachbereich Physik, Freie Universitüt Berlin, Berlin, Germany

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
The devitrification of the melt-spun Zr₅₇Ti₈Nb_2.5Cu_13.9Ni_11.1Al_7.5 glassy ribbon is characterized by the formation of a metastable icosahedral quasicrystalline phase during the first stage of the crystallization process. To investigate the effect of the mechanical deformation on the formation of quasicrystals, the glassy ribbon was submitted to ball milling. The mechanical treatment drastically affects both the thermal stability and the microstructure evolution upon heating. In particular, primary quasicrystal formation is progressively suppressed with increasing milling time. A ribbon produced from the previously milled ribbon no longer shows quasicrystal formation indicating that the variation of the quenched-in short-range order during milling is not the only reason for the lack of quasicrystal formation. The investigation of the effect of composition suggests that oxygen, which is introduced during milling, is responsible for the modified crystallization behavior by selectively reacting with zirconium and, consequently, by changing the chemical composition of the glassy phase. The oxidation problem can be bypassed and the quasicrystalline phase can be formed by the addition of the appropriate amount of zirconium. This indicates that quasicrystal formation in the ball-milled ribbon is crucially linked to the contamination of the material rather than to the question whether there is a special quenched-in short-range order.

"Inter-Layer Spacing Rule for the Stability of Quasicrystal Surfaces: Experimental Evidence"
Hem Sharma¹, Vincent Fournée¹, Masahiko Shimoda¹, An Pang Tsai¹, Akiji Yamamoto¹
¹National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan

Presentation Type: Contributed Talk
Topic: Surfaces

Abstract:
Despite their complex aperiodic structure, quasicrystals under suitable preparation conditions yield atomically flat terraced surfaces. We have recently proposed a rule, called 'inter-layer spacing rule,' to explain the appearance of such terraced surface [1]. In this contribution, we will mainly focus on this rule. We employed scanning tunneling microscopy (STM) to investigate the fivefold surface of various quasicrystals of icosahedral (i) Al-Pd-Mn family. These include i-Al-Cu-Fe and i-Al-Cu-Ru. STM images of the i-Al-Cu-Fe surface show steps of different heights. All observed step heights can be obtained by linear combination of two basic heights of 0.37 nm (= L) and 0.21 (= S = L/τ). An analysis based on the recently proposed refined structure of the iso-structural i-Al-Pd-Mn reveals that both the observed step-height distribution and high-resolution STM images on terraces are consistent with those derived from the bulk truncations at the positions where blocks of atomic layers are separated by larger inter-layer spacings (gaps).

The step-height distribution is found to evolve with increasing annealing temperature. At higher annealing temperatures, steps of S+L- and S+2L-height are observed more frequently, in contrast to L and S+L-step configurations obtained at lower annealing temperature as reported by other research groups [3]. We find that the steps prefer to be formed with Fibonacci number of S and L steps, with heights up to 3S+5L. Our recent results on the i-Al-Cu-Ru surface prepared at higher annealing temperatures also show step-height distribution similar to that of the i-Al-Cu-Fe surface. The origin of the higher steps can be explained by surface terminations at larger gaps above regions of high atomic density or high Al-concentration. Finally, we will discuss the validity of the inter-layer spacing rule for the stability of non-Al based quasicrystals.

[1] Sharma et al., Phys. Rev. Lett. 93, 165502 (2004).
[2] Yamamoto et al., Phys. Rev. B 68, 094201 (2003).
[3] Cai et al., Surf. Sci. 495, 19 (2001).

"Partial Phonon Density of States of Dy atoms in Icosahedoral Quasicrystal i-Zn₆₀Mg₃₁Dy₉ and Related Crystal h-Zn₆₅Mg₂₈Dy₇ Measured by ¹⁶¹Dy Nuclear Resonant Scattering"
Kaoru Shibata¹, Marc de Boissieu², Alfred Baron³, Satoshi Tsutsui³, Taku Sato⁴, Anpang Tsai⁵
¹Neutron Science Research Center, Japan Atomic Energy Research Institute, 319-1195 Ibaraki-ken Japan
²LTPCM, UMR CNRS 5614, ENSEEG-INPG, BP 75, 38402 Saint Martin d'Hères, France
³Japan Synchrotron Radiation Research Institute (JASRI), Sayo, Hyogo 679-5198, Japan
⁴Institute for Solid State Physics (ISSP), University of Tokyo, Tokai, Ibaraki 319-1106, Japan
⁵Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Sendai 980-8577, Japan

Presentation Type: Poster
Topic: Thermal and Dynamical Properties

Abstract:
The ZnMgRe icosahedral phase is particularly interesting for the study of dynamical properties; its atomic structure is completely different from that other icosahedoral phases. Thus we have obtained one of the most detailed results of dispersion relations for i-ZnMgY [1]. For larger wavevectors we observed a broad distribution of modes, which can be split into three dispersionless optical bands located at 8, 12 and 17 meV. The 8 and 12 meV bands are associated with the crossing of pseudo-zone boundaries by the acoustic branch. Therefore, in this experiment we have investigated the origin of these dispersionless optical bands in the ZnMgRe icosahedral phase on the i-ZnMgDy and related crystal h-ZnMgDy by mean of the 25.65 keV ¹⁶¹Dy nuclear resonant scattering (NRS) method. In this NRS experiments, the powdered i-Zn₆₀Mg₃₁Dy₉ and h-Zn₆₅Mg₂₈Dy₇ samples used natural Dy atom (¹⁶¹Dy : 19% natural abundance), have been used. Approximate sizes of samples are 3.0 × 5.0 × 6.0 mm³ with polished surfaces.The NRS experiments have been carried out at BL35XU undulator beam line at SPring-8. Thus the measurements of samples have been performed at ambient temperature. We have measured the corresponding optical modes.

Reference.
[1] K. Shibata, et al., J. Phys. Condens. Matter 14 (2002) 1847.

"Investigations of Thin Films on Quasicrystalline Surfaces: Sn Film Growth on the 5-Fold Surface of i-AlCuFe"
Masahiko Shimoda¹, Hem Sharma², An Pang Tsai³
¹National Institute for Materials Science, Tsukuba, Ibaraki 305-0047 Japan
²Japan Society for the Promotion of Science, Japan
³Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Japan

Presentation Type: Poster
Topic: Surfaces

Abstract:
For the purposes of producing single element quasicrystals, several attempts of epitaxial film growth have been performed mainly by vapor deposition of metals on well defined clean quasicrystalline surfaces. We believe that single element quasicrystals, as much simpler systems than well-known binary or ternary quasicrystalline alloys, provide us the opportunity of studying the nature of quasicrystals, especially the relation between the exotic structure and unusual physical properties. Until now, however, only a few successful results have been reported by Franke et al. on Bi and Sb films on the 10-fold surface of d-AlNiCo and the 5-fold surface of i-AlPdMn [1] and by Ledieu on Cu multi layers on the 5-fold surface of i-AlPdMn [2]. Our previous study on the Sn single layer film prepared by using surface diffusion was not very successful, because of the difficulty of controlling the film formation [3]. In this report, we present results of thin film growth of Sn by vapor deposition on the 5-fold surface of i-AlCuFe quasicrystal, which has been proven to be another example of single element quasicrystals by analyzing STM images [4].

This work was partially supported by JST Solution-Oriented Research for Science and Technology (SORST), JSPS Grants-in-Aid for Scientific Research and Special Coordination Funds of MEXT, Japan.

References
[1] K.J. Franke, H.R. Sharma, W. Theis, P. Gille, Ph. Ebert, K.H. Rieder, Phys. Rev. Lett. 89 (2002) 156104.
[2] J. Ledieu, J. T. Hoeft, D. E. Reid, J. Smerdon, R. D. Diehl, T. A. Lograsso, A. R. Ross, and R. McGrath, Phys. Rev. Lett. 92, (2004) 135507.
[3] M. Shimoda, Progress in Surface Science 75, (2004) 87-108.
[4] H. R. Sharma, M. Shimoda, A. R. Ross, T. A. Lograsso and A. P. Tsai, to be submitted.

"Chemical Short-Range Order in AlMn Liquids Probed by EXAFS"
Virginie Simonet¹, Françoise Hippert², Marc Audier², Jean-Louis Hazemann³, Robert Bellissent⁴, Agnés Traverse⁵, Armando Sole⁶
¹Laboratoire Louis Néel, CNRS, BP166, F-38042 Grenoble Cedex 9 France
²LMGP, ENSPG, BP46, F-38042 Saint Martin d'Hères Cedex, France
³Laboratoire de Cristallographie, CNRS, BP166, F-38042 Grenoble Cedex 9 France
⁴DRFMC/SPSM-MDN, CEA-Grenoble, F-38042 Grenoble Cedex, France
⁵LURE, B?t 209D Centre Universitaire, BP34 , F-91898 Orsay Cedex, France
⁶ESRF, BP220, F-38043 Grenoble Cedex 9, France

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
The study of the short-range order in liquids in equilibrium with quasicrystals (QC) or approximant phases yields information about the chemical affinities of the different atomic species when the atomic positions are not yet constrained by (quasi)-periodicity. This information can in turn be related to properties of solid phases and may help in understanding the driving mechanisms leading to the formation of atomic arrangements presenting an icosahedral symmetry. In the case of Al-Mn liquids, an additional interest comes from their peculiar magnetic properties since magnetic moments appear on Mn atoms on melting, whereas the equilibrium solid phases are almost non-magnetic. This paramagnetism increases with the temperature in the liquid state. Previous neutron-scattering studies on Al₈₀Mn₂₀ (Maret et al. J. Phys. (France) 50, 295 (1989)) and Al_88.5Mn_11.5 (Simonet et al., Phys. Rev. B, 65, 24203 (2001)) liquid alloys were performed using an isomorphous substitution property of Mn by Cr or by a mixture of Fe and Cr in order to extract the partial pair distribution functions. This complex analysis revealed a topological icosahedral order and a Al-transition metal atom affinity. These results were recently confirmed by Molecular Dynamics simulations (Jakse et al., Phys. Rev. Lett. 93, 207801 (2004)). A more direct way to probe the chemical local order is to carry out Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The Mn-K edge EXAFS spectra of liquid Al_88.5Mn_11.5 and of various known solid phases in the Al-Mn system, from simple ones to complex approximants, were measured. The analysis of the solid phase spectra was used as a reference work to validate the analysis of the EXAFS oscillations in the liquid state at several temperatures ranging from 1223 to 1773 K. The results confirm the Al-Mn affinities correlated to a Mn-Mn repulsion. The chemical order is found to evolve with temperature, with a progressive increase of the number of Mn-Mn first neighbors with increasing temperature. These unusual features are discussed in relation with the magnetic properties.

"Strengthening Effects of Icosahedral Phase in Magnesium Alloys"
Alok Singh¹, Masaki Watanabe², Akira Kato², An-Pang Tsai³
¹National Institute for Materials Science, Tsukuba, Ibaraki 305-0047 Japan
²Toyota Motor Corp., Japan
³National Institute for Materials Science, Tsukuba; Tohoku University, Sendai

Presentation Type: Contributed Talk
Topic: Mechanical Properties and Applications

Abstract:
Recently icosahedral quasicrystalline phase has been applied as strengthening phase in magnesium alloys. The icosahedral phase exists in equilibrium with the magnesium matrix in the Mg-Zn-Y system, forming in interdendritic spaces in the dilute alloys. Thermomechanical processing such as hot rolling [1] and extrusion [2] has been applied to disperse this phase finely as nanoparticles in the matrix. Thus the quasicrystal-containing alloys form a class of wrought magnesium alloys for structural applications.

We have studied microstructure and mechanical properties of two alloys Mg₉₅Zn_4.2Y_0.8 (Alloy 1) and Mg_92.5Zn_6.5Y (Alloy 2) after extrusion at two temperatures of 250°C and 400°C, in as-extruded and heat-treatment conditions. Tensile strength was evaluated at room temperature (RT), 200°C and 300°C. Hot extrusion at 400°C produced fine recrystallized grains of 2-5μm size, and a consequently higher yield strength of 260MPa. The cold extrusion at 250°C produced grain size 12-15μm with surprisingly no cold worked structures and better higher temperature strengths. The icosahedral phase has a strong grain boundary pinning effect. Annealing at 400°C stabilizes the grain size at ~15μm. It also interacts strongly with dislocations. Alloy 2 shows consistently higher strength due to a higher volume fraction of the icosahedral phase. The alloys show very good ductility at all temperatures; ~20% elongation at RT, 40% at 200°C and over 80% at 300°C.

The icosahedral phase forms strong interfaces with the matrix in several possible orientations due to its high symmetry. Annealing at 400°C results in reprecipitation of finer phases in the matrix in Alloy 1, while in Alloy 2 it results in formation of a nano-composite with a hexagonal phase H. Both of these phenomenon result in higher strengths. The alloys show very good strengths at 200°C. At 300°C deformation of the icosahedral phase occurs. The strength falls drastically at this temperature.

[1] Bae et al., Acta Mater. 50 (2002) 2343.
[2] Singh et al., Mater. Sci. Engg. A 385 (2004) 382.

"Interfaces of Icosahedral Phase with Magnesium Matrix and other Intermetallic Phases in Mg-Zn-Y Alloys"
Alok Singh¹, Masaki Watanabe², Akira Kato², An-Pang Tsai³
¹National Institute for Materials Science, Tsukuba, Ibaraki 305-0047 Japan
²Toyota Motor Corp., Japan
³National Institute for Materials Science, Tsukuba; Tohoku University, Sendai

Presentation Type: Poster
Topic: Surfaces

Abstract:
The icosahedral phase can make a variety of interfaces with other phases. Its interfaces with the matrix are especially important with respect to its strengthening effects. These interfaces are studied by transmission electron microscopy and high resolution electron microscopy in Mg₉₇Zn_2.5Y_0.5 and Mg_92.5Zn_6.5Y alloys.

Icosahedral phase can show several orientation relationships (ORs) with the hexagonal magnesium matrix. In the most commonly observed OR (OR1), the Mg hexagonal axis occurs along an icosahedral twofold, and two of the fivefold axes are along hexagonal <210>. OR2 is a variant of this, where the common hexagonal and twofold axes are roated by 90°. A fivefold axis occurs along the hexagonal axis in case of OR3. All these ORs can be related to each other by a {102} hexagonal twin. These ORs can also be shown to be related to each other through twinning of the icosahedral phase, by which further ORs can be derived, some of which with low symmetry but observed nevertheless. Experimental evidences are shown. The interfaces are always planar, predominantly on icosahedral fivefold and twofold planes, and preferably on hexagonal basal and prismatic planes.

The icosahedral phase forms on cubic W-Mg₃Zn₃Y₂ phase with three twofold axes along three <111> axes and three fivefold axes along three <110> axes. The interface is curved. This unique OR results in the icosahedral phase making OR1 and OR3 with the (Mg) matrix.

The icosahedral phase can nucleate on a hexagonal H-Mg₃ZnY phase (a=0.92 nm, c=0.95 nm), which is identified as the pre-eutectic phase in this alloy, with OR such that an icosahedral twofold axis is parallel to the hexagonal axis, while two of the fivefold axes are along two <100> axes. The interfaces are planar, with ledged structures.

"Scanning Tunnelling Microscopy of an Ultrathin Magnetic Film on an i-AlPdMn Quasicrystal Template"
Joseph Smerdon¹, Julian Ledieu¹, Jon Hoeft¹, Lisa Wearing¹, Dennis Reid¹, Amy Ross², Thomas Lograsso², Ronan McGrath¹
¹Surface Science Research Centre, University of Liverpool, Liverpool, Merseyside L69 3BX UK
²Ames Laboratory, Iowa State University, Ames, IA 50011 USA

Presentation Type: Poster
Topic: Surfaces

Abstract:
Recently there has been much interest in systems involving the use of quasicrystalline substrates as nanotemplates to impose aperiodic order on deposited elemental ultrathin films¹. An investigation into the possibility of growing an aperiodically ordered magnetic film on i-AlPdMn is currently being undertaken, and scanning tunnelling microscopy (STM), Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED) results from this study will be presented. Results gathered so far reveal that when deposited on i-AlPdMn, Co grows layer-by-layer and a new, ten-fold diffraction pattern is obtained.

[1] Ledieu et al., Phys. Rev. Lett. 92 (2004) 135507

"Growth of Aperiodic and Periodic Cu Thin Films on an i-AlPdMn Quasicrystal Studied with Medium-Energy Ion Scattering"
Joseph Smerdon¹, Tim C. Noakes², Paul Bailey², Julian Ledieu³, Chris McConville⁴, Charles Parkinson⁴, Amy Ross⁵, Thomas Lograsso⁵, Ronan McGrath³
¹University of Liverpool, Liverpool, Merseyside L69 3BX UK
²CCLRC Daresbury Laboratory, Daresbury, Warrington, WA 4AD, UK
³Surface Science Research Centre, University of Liverpool, Liverpool, L69 3GH, UK
⁴Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
⁵Ames Laboratory, Iowa State University, Ames, IA 50011 USA

Presentation Type: Poster
Topic: Surfaces

Abstract:
Recently there has been much interest in the use of icosahedral quasicrystalline surfaces as nanotemplates for the growth of overlayers also exhibiting aperiodic order. The surface of i-AlPdMn has been shown to essentially be a bulk-like truncation of the icosahedral structure¹. STM images reveal the characteristic 'Penrose tiling' type patterns associated with two-dimensional aperiodic order². Cu deposited on the i-AlPdMn surface adopts a long-range aperiodic structure for coverages greater than 4ML, which transforms into a periodic structure upon annealing³. STM reveals that the aperiodic structure is formed from rows of atoms spaced according to a one-dimensional Fibonacci sequence. Medium energy ion scattering (MEIS) measurements have been made using 100keV He+ ions to probe the surface and near surface composition and structure of both periodic (obtained after annealing) and aperiodic Cu layers deposited onto the five-fold surface of i-AlPdMn. These MEIS data reveal that for the aperiodic structure a pure Cu film is present, whereas for the periodic structure, the surface is composed of a quaternary alloy of Al, Mn, Pd, and Cu. Simulations of the structure of the periodic film indicated that it is composed of 5 domains of (110) oriented cubic materials. Analysis of the aperiodic structure is somewhat more complicated since as with the clean surface, large models are required to account for the lack of periodicity in the film. A range of models have been tried, with the current best fit given by rows of a (100) Cu structure aperiodically spaced according to Pd positions in the bulk quasicrystal model.

[1] Gierer et al., Phys. Rev. Lett. 78 (1997) 467
[2] Ledieu et al., Surf. Sci. Lett. 492 (2001) L729
[3] Ledieu et al., Phys. Rev. Lett. 92 (2004) 135507

"Formation of 1/1 Cubic Approximant Phase in Sc-Based Alloys"
Yeong Gi So¹, Aya Shimizu², Yasushi Kamimura¹, Keiichi Edagawa³, Shin Takeuchi²
¹IIS, Univ. Tokyo, Komaba, Meguro-ku, Tokyo 153-8505 Japan
²Dept. of Mat. Sci. and Tech., Science Univ. of Tokyo, Noda, Chiba 278-0022 Japan

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
In 1985, Cenzual et al. [1] showed that the intermetallic compounds with the composition Sc₅₇M₁₃ (M = Rh, Ru, Pt, Ir) have the structure consisting of a bcc stacking of an icosahedral cluster, which has the lattice parameter of about 14Å and whose space group is Pm3. Subsequently, Ja et al. [2] reported that Sc₂₉Fe₆ yields the phase isostructural with them. The structure can be interpreted as a 1/1 cubic approximant to the icosahedral quasicrystal. In this study, we have attempted to produce the approximant phase in Sc based ternary alloys with addition of various transition metal elements, and determined the formation area of the approximant phase in each ternary system. Sc-Rh-TM and Sc-Fe-TM (TM : Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ag, Au, Ru, Pd and Pt) alloys with various compositions were prepared from the elemental constituents by arc-melting under an argon atmosphere. After sealed in an evacuated quartz ampoule, the alloys were annealed with various conditions. The samples were characterized by X-ray diffraction measurements and by electron diffraction experiments. For the Sc-Rh-TM ternary systems, the shape of the formation area of the 1/1 phase in the ternary phase diagram varies, depending on the TM element added. The shapes of the formation area can be classified into four groups. The first group consists of TM=Ti, V and Cr, for which the formation area lies approximately along a Rh-constant line. The second group consists of TM = Mn, Co, Ni, Au, Ag and Cu, for which it lies along a Sc-constant line. The third group, which consists of TM = Fe and Ru, shows relatively wide composition area extending two-dimensionally in the phase diagram. The fourth group is of TM = Y, whose behavior is almost the same as that of the group 1, but slightly different; the formation area in the group 4 lies approximately along a Rh constant line but it deviates toward high Rh concentration with increasing Y concentration. For the Sc-Fe-TM ternary systems, we found that TM = Ni, Pd and Pt stabilize the 1/1 phase. Of them, TM = Pt yields the 1/1 phase of the highest quality with very sharp diffraction peaks.

Reference
[1] K. Cenzual et al., Acta Cryst. C41 (1985) 313.
[2] B. Ja et al., Dopovidi Akademii Nauk Ukrainsíkoi RSR (1986) 29.

"Structure Peculiarities of Hydride-forming Ti –Zr–Ni Alloys Prepared by Sintering"
Myroslav Karpets¹, Yuriy Solonin¹
¹Frantsevich Inst. for Problems of Material Science, 03680 Kyiv Ukraine

Presentation Type: Poster
Topic: Mechanical Properties and Applications

Abstract:
The structure and phase transformations in Ti–Zr–Ni alloys in temperature range 293 - 873 K have been 'in-situ' studied with use of a high temperature X-Ray diffractometer. The initial and hydrogenated alloys were investigated. The alloys were prepared by two methods: (i) by arc melting of the mixtures of pure components; (ii) by sintering of the pressed tablets, consisting of the mixtures of the Ti, Zr, and Ni powders at temperatures 1073 -1173 K in vacuum. Hydrogenation was performed both in gas hydrogen at 530 K, 4,4 MPa and electrochemically in 5 M KOH electrolyte. Arc melted samples in initial state comprised two phases: hexagonal Laves phase C14 and Ti–Zr solid solution. After annealing of these alloys at 773 - 873 K a quasicrystalline phase with icosahedral symmetry or a cubic approximant W (TiZrNi) - phase were observed. Hydrogenation of the annealed Ti₄₀Zr₄₀Ni₂₀ alloy, containing W–phase, leads to hydrogen absorption up to 2,4 mass %. As a result the specific volume of the unit cell increases from 2,9224 nm³ to 3,3209 nm³. The samples of the Ti–Zr–Ni alloys prepared by powder metallurgy method in initial state contained mixture of a quasicrystalline i–phase and crystalline phase with structure type Ti₂Ni. The hydrogenation-dehydrogenation conditions for alloys, prepared by both methods, were studied. It was shown that use of the powder metallurgy method allows to lower the preparation temperatures of the hydride-forming alloys in system Ti–Zr–Ni.

"Contrasting Devitrification Behavior of Amorphous Zr-Pd and Zr-Pt Structures Obtained by Liquid Quenching and Vapor Deposition"
Daniel Sordelet¹, Xiaoyun Yang¹, Min Ha Lee¹, Matthew Besser¹, Matthew Kramer¹
¹Ames Laboratory, Ames, IA 50011 USA

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
Vitrifrication (i.e., quenching a liquid to form an amorphous solid) of near-eutectic Zr-Pd and Zr-Pt alloys produces a structure that upon heating forms a metastable quasicrystal as the first devitrification product. This behavior is often speculated to be linked to the existence of icosahedral short-range order in the amorphous state that previously existed in the starting liquid and was 'frozen-in' during vitrification. However, no unambiguous experimental proof exists to confirm this speculation, although numerous studies provide various results that support the idea that the short-range order of the amorphous structure has structural similarities to the metastable icosahedral phase that forms upon heating.

In the current study, a new route for producing amorphous Zr-Pd and Zr-Pt structures is examined. Physical vapor deposition (i.e., dc magnetron sputtering) was used to prepare amorphous films using targets prepared by (i) alloying constituents by arc melting and (ii) mixing of elemental powders. While some differences in film structure and composition occur with the different targets, a consistent observation is that the amorphous films follow different devitrification pathways compared to melt spun ribbons of the same composition. Specifically, the sputtered films do not form a metastable quasicrystal phase and instead devitrify initially into the stable structures predicted by the equilibrium phase diagrams. Examinations of short-range and medium range structural characteristics of sputtered films and melt spun ribbons were performed, respectively, by high-energy synchrotron X-radiation and energy-filtered transmission electron microscopy. The relationship between different amorphous structures and devitrification pathways will be discussed.

"Linkage Between Oxygen Content and Metastable Phase Formation in Melt Spun Zr₈₀Pt₂₀ Ribbons"
Daniel Sordelet¹, Xiaoyun Yang¹, Matthew Kramer¹
¹Ames Laboratory, Ames, IA 50011 USA

Presentation Type: Poster
Topic: Structure

Abstract:
The as-quenched structure and devitrification behavior of Zr₈₀Pt₂₀ melt spun ribbons containing measured oxygen contents ranging form 184 to 4737 ppm mass were studied. Ribbons containing less than 500 ppm mass oxygen are fully crystallized and consist predominately of a metastable ordered β-Zr phase with significant solution of Pt. Increasing oxygen content to 1053 and 1547 ppm mass produces a transition to an apparently amorphous and to mixed amorphous and quasicrystalline structures, respectively. Samples with further oxygen additions of 2690 and 4737 ppm mass are comprised of quasicrystalline and crystalline phases in an amorphous matrix; the latter has a Fd3m space group that form a large unit cell cubic structure and contains clusters of icosahedral symmetry. This phase is often referred to as the big cube phase.

The results from this study suggest a critical level of oxygen may be required to promote glass formation in Zr₈₀Pt₂₀ melt spun ribbons produced at a specific quench rate. However, thermal analysis of Zr₈₀Pt₂₀ melt spun ribbons that appear to be amorphous by laboratory X-ray diffraction indicates that they contain nuclei or clusters of icosahedral symmetry that coarsen during heating, as opposed to forming by nucleation from a fully amorphous structure. Thus, a sequence of metastable structures beginning with a β-Zr superstructure transitioning to a QC phase and ultimately to the big cube structure as oxygen is increased from around 100 ppm to 5000 ppm mass. The structural relationships between these metastable phases will be discussed in two corresponding posters during ICQ9.

"Mössbauer Effect Study of the Decagonal Quasicrystal Al₆₅Co₁₅Cu₂₀"
Zbigniew Stadnik¹, Guowei Zhang²
¹Department of Physics, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
²Department of Radiation Oncology, University of Maryland, Baltimore, MD 21201 USA

Presentation Type: Poster
Topic: Structure

Abstract:
We present a systematic ⁵⁷Fe Mössbauer effect study in varyingtemperature range between 5.0 and 297 K and in an external magnetic field of9.0 T on a high-quality stable decagonal quasicrystal Al₆₅Co₁₅Cu_19.9Fe_0.1. The iron atoms are shown to be located in two distinct classes of sites. Thevalues of the principal component of the electric field gradient tensor and theasymmetry parameter at these sites are, respectively, -1.898(77) ↔ 10²¹V/m², 0.97(15) and -3.952(91)↔ 10²¹ V/m²,0.00(16). This is discussed in terms of the local atomic structure of theAl-Co-Cu decagonal alloy. The average quadrupole splitting decreases withtemperature as T^3/2. The vibrations of the Fe atoms are well described by a Debye model, with the Debye temperature of 546(7) K.

"X-ray Diffraction of Co-Rich decagonal Al–Co–Ni, its Closely Related W-Approximant and of the Ternary X Phase"
Sergiy Katrych¹, Volker Gramlich¹, Walter Steurer¹
¹Laboratory of Crystallography, ETH-Zurich, 8093 Zurich, Switzerland

Presentation Type: Poster
Topic: Structure

Abstract:
The X-ray difraction study was performed on single- and polycrystalline samples annealed for 60 h at 900°C. The existence of the W approximant as stable phase has been confirmed at slightly different conditions from those reported earlier [1]. The crystal structure has been redetermined on good quality crystals with almost no diffuse scattering observable at standard conditions. The intensity distribution of the reconstructed reciprocal layers shows close similarity to those of the Co-rich decagonal phase [2]. The Bragg layers h₂=2n+1 of the W phase correspond to the diffuse interlayers in decagonal Co-rich quasicrystal with h₅=(2n+1)/2. Both show much weaker intensity than the other layers. Consequently, the superstructure along the pseudotenfold direction should be very similar to that observed in the decagonal phase. The diffuse layers in the Co-rich decagonal phase are rather sharp, i.e. the diffuse scattering is condensed in almost Bragg like peaks.
The phase diagram was also explored around the reported stability range of the X phase [3]. The previously unknown crystal structure of the X phase has been determined. It is monoclinic with lattice parameters a=12.145Å, b=4.0766Å, c=7.6565Å, &beta=105.87 Å.

[1]Hiraga, K.; Ohsuna, T.; Nishimura, S.: A New Crystalline Phase Related
to an Al-Ni-Co Decagonal Phase. J. Alloys Comp. 325 (2001) 145.
[2]Katrych S., Steurer W.: X-ray Diffraction Study of Decagonal Al-Co-Ni as a Function of Composition, Z. Kristallogr. 219 (2004) 606.
[3]Gôdecke T., Scheffer M., Lück R., Ritsch S., Beeli C.: Isothermal Sections of Phase equilibria in the Al-AlCo-AlNi System. Z.Metallkd. 89 (1998) 687.

"Ordered and Disordered Structural States for the Al-Ni-Ru Decagonal Quasicrystal with 0.4 nm Periodicity"
Wei Sun¹
¹Beijing University of Technology, 100022 Beijing China

Presentation Type: Poster
Topic: Structure

Abstract:

A kind of disordered Al-Ni-Ru decagonal quasicrystal with 0.4 nm periodicity was observed to coexist with the highly ordered superlattice decagonal quasicrystal with the same periodicity in the as-cast Al₇₀Ni₂₀Ru₁₀ alloy. The remarkable feature of the tenfold symmetry electron diffraction pattern taken from this disordered decagonal quasicrystal is that there exists strong diffuse scattering around the main reflections. The structure of the Al-Ni-Ru superlattice decagonal quasicrystals is characterized by highly ordered arrangement of two types of atom clusters with fivefold symmetry in a long range, while the basic decagonal quasicrystal by a mixture of atom clusters with and without fivefold symmetry. In comparison with the supperlattice and basic decagonal quasicrystals, the disordered decagonal quasicrystal is considered to have a transition structural state laying in between the supperlatice Al-Ni-Ru decagonal quasicrystal and the corresponding basic decagonal quasicrystal. By means of a combination of high-resolution electron microscopy and high-angle annular dark-field imaging techniques, the structural characteristic of this disordered decagonal quasicrystal has been clearly revealed.

"Surface Composition of Thin Films of i-AlCuFe"
Orest Symko¹, Ehab Abdel-Rahman²
¹University of Utah, Salt Lake City, UT 84112-0830 U.S.
²Helwan University, Department of Physics, Ain Helwan, Cairo, Egypt

Presentation Type: Poster
Topic: Surfaces

Abstract:
Some of the unusual properties of quasicrystals can be related to the peculiar symmetry of this class of materials. Their surface properties are especially interesting as they can be used in all sorts of applications. Of special interest is the composition of the surface, its relation to the composition and phase of the bulk sample, and its effect on sample properties. This was investigated by spectroscopic studies of the surface of thin films of i-AlCuFe. Thin films of quasicrystal phase were formed on silicon and sapphire substrates using magnetron sputtering techniques followed by heat treatment. The quality of the quasicrystal films was determined using X-rays, TEM, and electrical resistivity. The surface composition of the samples was then studied using X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectroscopy (ToF - SIMS). As expected there was oxidation at the surface. This was not uniform. Some sites at the film surface indicated the presence of pure aluminum. Elemental composition was studied as the surface was penetrated and the oxide presence was followed down into the sample. Argon ions at 1 keV were used to sputter down in thickness the quasicrystal films. Concentration profile was determined. Aluminum oxide signals disappeared at a depth of approximately 2 nm below the surface and bulk quasicrystal behavior was observed. The presence of oxides at the film surface contributes to the surface energy of the films and affects the magnitude of the pseudogap. This will be discussed and its role in the surface properties of the thin films will be evaluated. The results presented here are very important in the application of thin quasicrystal films to coatings developed for their surface properties.

"The Study of the Binary Quasicrystal Cd₁₇Ca₃ and its 1/1-Approximant Cd₆Ca by Positron Annihilation"
Yoshiki Takagiwa¹, Tetsuji Akiyama¹, Kiminori Sato¹, Ikuzo Kanazawa¹, Hideoki Murakami¹, Yoshinori Kobayashi², Ryuji Tamura³, Shin Takeuchi³
¹Tokyo Gakugei University, 184-0051 Tokyo Japan
²National Institute of Advanced Industrial Science and Technology, 305-8565 Ibaraki Japan
³Tokyo University of Science, 278-8501 Chiba Japan

Presentation Type: Poster
Topic: Structure

Abstract:
A binary icosahedral quasicrystal Cd₁₇Ca₃ and its cubic 1/1-approximant crystal Cd₆Ca are investigated by positron annihilation measurement. The positron annihilation measurement is the most significant method to detect vacancy-type defects in the icosahedral quasicrystal and its approximant. Our group has already indicated structural vacancy-type sites in many kinds of stable icosahedral quasicrystal and approximant. The positron annihilation lifetime measurement was carried out at room temperature, we changed the measurement temperature from 50K to 300K. According to Tamura et al., an order-disorder transition occurs at 100K in Cd₆Ca crystalline approximant. Our objection was also to measure the positron lifetime change around the order-disorder transition temperature.
The observed positron annihilation lifetime spectra for both specimens were composed of a single component, and the lifetimes at room temperature were 270±2(psec) and 268±2(psec) for quasicrystal and its cubic approximant. The positron lifetimes for both specimens flatly increased from 50K to 300K because of the thermal expansion. However, there was no change around the order-disorder transition temperature, that is, 100K. Both specimens have very close positron lifetimes, so this suggests that the quasicrystal is composed of the same local cluster units as that in the approximant. We have estimated the positron lifetimes in free state for Cd₁₇Ca₃ quasicrystal and Cd₆Ca approximant from compositionally weighted average of the constituent element values (Cd:190psec and Ca:297psec). The values are 207 and 206 psec for Cd₁₇Ca₃ quasicrystal and Cd₆Ca approximant, respectively, which are significantly shorter than the measured lifetimes. Therefore positrons are not annihilated in the defect–free region of both specimens. We have considered that positrons are most likely to be localized around the 4Cd tetrahedral inside the dodecahedral cluster of 20Cd in which half of the eight cubic Cd sites are unoccupied, and they annihilate therein.

"Heat Capacity of Cd-Yb, Cd-Ca and In-Ag-Yb Quasicrystals and Their Cubic Approximant Crystals"
Akira Inaba¹, Hiroyuki Takakura¹, An Pang Tsai²
¹RCMT, Graduate School of Science Osaka University, 560-0043 Osaka, Japan
²IMRAM, Tohoku University, Sendai 980-8577, Japan

Presentation Type: Poster
Topic: Thermal and Dynamical Properties

Abstract:
Heat capacity measurements have been performed in the temperature range from 0.4 K to 310 K for quasicrystals of Cd-Yb, Cd-Ca and In-Ag-Yb alloys and their cubic approximant crystals. None of those quasicrystals exhibited any phase transition, whereas the 1/1 crystal approximants of Cd-Yb and Cd-Ca showed a structural phase transition at 109 K and at 98 K, respectively. The entropy change obtained, 0.075 JK^-1mol^-1 for Cd-Yb and 0.058 JK^-1mol^-1 for Cd-Ca, is explained reasonably by an order/disorder mechanism, where the Cd₄ units are involved. An excess heat capacity was also observed in the lowest temperature region for the both approximant crystals. This may be attributed to either a further ordering or some other low-energy excitation. The 1/1 and 2/1 crystal approximants of In-Ag-Yb, both of which are supposed to be isomorphous to those of Cd-Yb and Cd-Ca, showed no phase transition because of the possible chemical disorder introduced into the crystal lattice.

The electronic contribution to the heat capacity is essentially the same between the quasicrystal and the crystal approximant for all the alloys. The feature of the vibrational density of states obtained from the lattice heat capacity is also very similar, which is distinctly different from the feature of ordinary simple crystals. Such similarity in the electronic and vibrational properties seems to be consistent with the structural similarity between the quasicrystal and the crystal approximant in this particular system.

"Ideal Six-Dimensional Structure of Icosahedral Cd-Yb Quasicrystals"
Hiroyuki Takakura¹, Cesar Gomez², Akiji Yamamoto³, Marc de Boissieu⁴, An Pang Tsai²
¹RCMT, Graduate school of Science, Osaka University, Toyonaka 560-0043, Japan
²IMRAM, Tohoku University, Sendai 980-8577, Japan
³Advanced Materials Laboratory, NIMS, Tsukuba 305-0044, Japan
⁴LTPCM/ENSEEG, UMR CNRS 5614, INPG, UJF, PB75, 38402, St.Martin d'Hères Cedex, France

Presentation Type: Contributed Talk
Topic: Structure

Abstract:
Recent detailed structural studies of the related approximants of icosahedral Cd-Yb (i-Cd-Yb) quasicrystals have led to an identification of two fundamental building units of the approximants, which consist of a rhombic triacontahedral (RT) cluster and an acute rhombohedon (AR) [1,2]. On the other hand, the reconstructed density of an i-Cd-Yb by the low-density elimination method convinces us that the structure is composed of the same building units of the approximants [3]. In this paper, we propose an ideal six-dimensional model for i-Cd-Yb that is the completed version of our previous model [3]. In the model, a RT cluster or an AR has an edge length of a = 5.69 Å that corresponds to the icosahedral lattice constant of i-Cd-Yb. The RT clusters are disposed on twelvefold vertex sites of the three-dimensional Penrose tiling with an edge length of a*τ^-2 (τ = (1+√5)/2 is the golden mean). Supposing that a deterministic rule, unrealistic short separation of a pair of RT clusters along their common fivefold axis is excluded. The model is fully compatible with a local construction rule of real approximants [4]. That is two adjacent RT clusters overlap each other along their common threefold axis by sharing an obtuse rhombohedron, and share a rhombus face along their common twofold axis. In the model, the ARs fill the interstice of resultant arrangement of the RT clusters. Hence space is filled completely by using the building units. Decorations of the building units are unique except for the variation of overlap scheme of the RT clusters, and some minor positional disorder of Cd atoms in the both building units. The density and composition predicted by the model fit to the experimental values.

[1] C. P. Gomez, S. Lidin, Phys. Rev. B 2003, 68, 024203(1).
[2] C. P. Gomez, S. Lidin, Angew, Chem., Int. Ed. Engl. 2001, 40, 4037.
[3] H.Takakura, A. Yamamoto, M. de Boissieu, A. P. Tsai, Ferroelectrics, 2004, 305, 209.
[4] C.P.Gomez, Proc. ICQ9 submitted.

"Ab Initio Reconstruction of P-Type Icosahedral Zn-Mg-Ho Quasicrystal Structures"
Hiroyuki Takakura¹, Akiji Yamamoto², Taku Sato³, An Pang Tsai⁴, Yoshiki Ozawa⁵, Nobuhiro Yasuda⁶, Koushiro Toriumi⁵
¹RCMT, Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan
²Advanced Materials Laboratory, NIMS, Tsukuba 305-0044, Japan
³Neutron Science Laboratory, ISSP, University of Tokyo, Tokai 319-1106, Japan
⁴IMRAM, Tohoku University, Sendai 980-8577, Japan
⁵Graduate School of Materials Science, University of Hyogo, Kamigori-cho 678-1297, Japan
⁶Japan Synchrotron Radiation Research Institue (JASRI), Mikazuki-cho 679-5198, Japan

Presentation Type: Poster
Topic: Structure

Abstract:
We present reconstructed six-dimensional (6D) electron densities of a p-type icosahedral Zn-Mg-Ho quasicrystal (QC) using synchrotron radiation x-ray intensity data. Diffraction experiments at room temperature have been performed on a low temperature vacuum camera equipped with a cylindrical Image-Plate installed at Spring8 BL02B1 under the condition of an incident x-ray energy 20.04 keV. A spherical single quasicrystal sample with a diameter of 100 μm, which has a composition of Zn₇₅Mg₁₆Ho₉, was used. More than 3000 independent reflections were obtained after a suitable data reduction, and corrections, namely Lorentz and polarization factors. Of these 1427 reflections were utilized for subsequent phase reconstruction of structure amplitudes by the 6D LDE method. A probable structure solution has been obtained. That is the reconstructed density that gives the most reasonable electron density distribution both in the internal and external spaces, among one hundred reconstructed densities. The density indicates probable characteristic of the p-type Zn-Mg-Ho structure in particular the location, size and shape of occupation domains (ODs), as well as the distribution of heavy element — Ho. It is shown that three large ODs would characterize the 6D structure of the p-type icosahedral Zn-Mg-Ho. Their positions in the 6D primitive unit cell are V: (0,0,0,0,0,0), B: (1/2,1/2,1/2,1/2,1/2,1/2) and E: (1/2,0,0,0,0,0). The arrangement of the ODs inferred from the density reveals that the QC structure is composed of Bergman-type clusters. This can be verified by a simple geometric consideration of the density, by taking a section of the external space centered at V. Because the electron density distribution corresponding to the OD at V is low at the center, it is considered that the Bergman-type cluster has an empty core. It is also considered that the OD at B contains Ho atoms, which mainly distribute around its threefold directions. Based on the knowledge obtained by the reconstructed density, we discuss the structure of the p-type icosahedral Zn-Mg-Ho QC.

"Homologous Temperature Dependence of the Yield Stress of Icosahedral Quasicrystals and its Implication"
Shin Takeuchi¹
¹Tokyo University of Science, Noda, , Chiba 278-8510 Japan

Presentation Type: Poster
Topic: Mechanical Properties and Applications

Abstract:
Temperature dependence of the yield stress of icosahedral quasicrystals differs from one type of icosahedral quasicrystal to another. However, when we plot the yield stress vs. temperature relations with respect to the normalized stress of σ/G and the normalized temperture of T/(Ga³/k_B) (G: the shear modulus, a: the average atomic diameter, k_B: the Boltzmann constant), the normalized yield stress vs. the normalized temperature relations for different icosahedral quasicrystals come close to each other, i.e., a homologous relation holds approximately. This fact implies that a common deformation mechanism is operating for any icosahedral quasicrystal. The activation enthalpy of deformaiton at low stress level is estimated to be about 0.4Ga³. Assuming that the deformation is carried by dislocation climb, it is concluded that the half of this activation enthaply value is due to the jog-pair energy and tha other half due to the activation energy of vacancy diffusion.

"Atomic Structure and Electron Transport Properties of the Al-Pd-Mn-Re-Si 1/1-Cubic Approximant"
Tsunehiro Takeuchi¹, HIroki Sakagami², Uichiro Mizutani²
¹EcoTopia Science Institute, Nagoya University, Nagoya, 464-8903 464-8603 JAPAN
²Department of Crystalline Materials Science, Nagoya University, Nagoya, Aichi 464-8603 JAPAN

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
1/1-cubic approximant was reported to exist as a thermodynamically stable phase in the Al-Pd-Mn-Si alloy system. Since Mn and Re stay in the same column in the periodic table, it is natural to consider that Re can be substituted for Mn in the Al-Pd-Mn-Si 1/1-cubic approximant and that the Al-Pd-Re-Si approximant, that is expected to possess a large electrical resistivity as the corresponding Al-Pd-Re icosahedral quasicrystal, could be obtained.

In this study, we investigate the formation area the the Al-Pd-Mn-Re-Si 1/1-cubic approximants by partially substituting Re for Mn in the Al-Pd-Mn-Si 1/1-cubic approximant. The electrical resistivity and thermoelectric power were measured and discussed in terms of the electronic structure near the Fermi level and the local atomic arrangements determined by the synchrotron radiation Rietveld analysis. We found that the Mn atoms are not fully substituted by Re atoms, and that the Al-Pd-Re-Si 1/1-cubic approximant could not be obtained. Structure analysis on the Al-Pd-Mn-Re-Si 1/1-cubic approximants revealed that one of the Mn sites in the Al-Pd-Mn-Si 1/1-cubic approximant is too small to be occupied by the large Re atoms. We also found that the number of atoms in the unit cell decreases with increasing Re concentration. Induced vacancies cause a variation in the number of electrons in the unit cell, and the stabilization mechanism associated with the carrier concentration is strongly affected. As a consequence, the Al-Pd-Mn-Re-Si 1/1-cubic approximant is destabilized by the Re substitution. The Re concentration dependence of the electrical resistivity and thermoelectric power can be also accounted for by considering the Fermi level moving across the pseudogap in relation with the variation in the carrier concentration.

"Thermal Conductivity of the Al-TM-Si 1/1-Cubic Approximants"
Tsunehiro Takeuchi¹, Toshio Otagiri², Hiroki Sakagami², Shinji Okui³, Uichiro Mizutani²
¹EcoTopia Science Institute, Nagoya University, Nagoya, Aichi 464-8603 JAPAN
²Department of Crystalline Materials Science, Nagoya University, Nagoya, Aichi 464-8603 JAPAN
³Department of Applied Physics, Nagoya University, Nagoya, Aichi 464-8603 JAPAN

Presentation Type: Contributed Talk
Topic: Thermal and Dynamical Properties

Abstract:
The possession of the low thermal conductivity as low as 1 W/m⋅K is one of the most pronounced characteristics of the icosahedral quasicrystals. Together with the large thermoelectric power up to 100 μV/K, it encourages us to utilize the quasicrystals as a practical thermoelectric material. The origin of the low thermal conductivity, however, has not been fully investigated yet, so that the guiding principle to utilize the quasicrystals as a thermoelectric material has not been established yet. Deep investigation into the mechanism leading to the low thermal conductivity in quasicrystals has been strongly required.

Recently, we have successfully revealed the mechanism leading to the large thermoelectric power and large electrical resistivity of the icosahedral quasicrystals by using the approximants that possess essentially the same properties as those of the corresponding quasicrystals. The use of approximants as substitutes for the quasicrystals effectively worked in our previous works because structure analyses and band calculations well developed for the crystalline materials were easily applied to the approximants in sharp contrast to their difficulty with the quasicrystals. In this study, thus, we employed again the approximants rather than using the quasicrystals to investigate the mechanism for the low thermal conductivity of the icosahedral quasicrystals and their corresponding approximants.

Al-TM-Si (TM = Cu, Fe, Mn, Re) 1/1-cubic approximants were selected and their atomic arrangements were investigated by the synchrotron radiation Rietveld analysis. We found that the 1/1-cubic approximants possess low thermal conductivity less than 4 W/m⋅K at room temperature regardless of the compositions selected. This is surprising result because the structure analysis revealed that some of the 1/1-cubic approximants have a nearly perfect periodicity. Very low thermal conductivity in the perfectly ordered structure is discussed in terms of the lowering of the optical phonon branches associated with the large lattice constant and its relation to the quasiperiodicity.

"Experimental Investigations on the Electronic Structure and the Low-Temperature Stability of Cd-Based Quasicrystals and their 1/1, 2/1 Cubic Approximants"
Ryuji Tamura¹, Ken Minoda¹, Shin Takeuchi¹, Tsunehiro Takeuchi², Keiichi Edagawa³, Takayuki Kiss⁴, Takayoshi Yokoya⁴, Shiku Shin⁴
¹Tokyo University of Science, Noda, Chiba 278-8510 Japan
²RCAWEM, Nagoya Univ., Nagoya 464-8603, Japan
³IIS, Univ. of Tokyo, Meguro-ku, Tokyo 153-8505, Japan
⁴ISSP, Univ. of Tokyo, Kashiwa, Chiba 277-8581, Japan

Presentation Type: Invited Talk
Topic: Structure

Abstract:
Valence band spectra near the Fermi edge were investigated with ultrahigh energy resolution down to 7 meV for icosahedral (i) phases, 2/1 and 1/1 cubic approximants in the Cd-Yb and Cd-Ca systems. For all the studied compounds, a sharp Fermi edge, indicative of sizable density of states at the Fermi level, was clearly observed and the Fermi levels are found to be located not at the bottom of the pseudogap but at its shoulder. In addition, a peak structure appears across the Fermi level for the Cd-Ca system, which is also obtained for the Cd-Yb system when subtracting strong Yb 4f derived peaks from the spectra. Ca 2p-3d resonant photoemission spectroscopies on the Cd-Ca compounds reveal contributions from Ca 3d states at the Fermi level and they were successfully separated, which means that the Ca 3d states are descended below the Fermi level as a result of hybridization with the Cd sp states. The above features are in a satisfactory agreement with the LMTO calculations performed on the Cd₆Ca 1/1 approximant.

The low-temperature stability of the Cd-based compounds was probed by various techniques such as electrical resistivity, magnetic susceptibility, specific heat measurements, X-ray and electron diffraction experiments. They all exhibit an occurrence of a phase transition at 110K and 100K for Cd₆Yb and Cd₆Ca, respectively. In addition, the transition temperatures are found to increase substantially under pressure up to 2 GPa for both compounds, indicating that the low-temperature phases have a higher atomic density. Formation of a superlattice with slight tetragonal-type distortion of the lattice was observed. The transitions can be understood by an orientational ordering of Cd tetrahedron located at the center of the icosahedral clusters. Moreover, a similar transition has been also observed in a number of Cd₆RE (RE=rare earth metals) compounds, suggesting that it is a common feature of the Cd₆M-type compounds with disordered cluster cores. The stability of the 2/1 and i phases at low temperatures will be also reported in the presentation.

"Epitaxy between Materials of Periodic and Aperiodic Order"
Katharina Franke¹, Wolfgang Theis¹, Peter Gille², Karl-Heinz Rieder³
¹Freie Universiät Berlin, 14195 Berlin, Germany
²Ludwig-Maximilians-Universität München, 80333 München, Germany

Presentation Type: Contributed Talk
Topic: Surfaces

Abstract:
Epitaxial interfaces between periodic crystals can be characterized by a common interface unit cell. Since quasicrystals lack periodicity, their interfaces cannot exhibit such repeating unit cells. But, is this an essential feature of epitaxy or just a by-product of periodic as opposed to aperiodic long-range order? Analyzing the orientation and unit cell parameters gained by SPA-LEED and He-atom scattering experiments from AlAs films grown on decagonal Al_71.8Ni_14.8Co_13.4, we find that interfaces develop which can lock into registry, just as in periodic crystals. The characteristic feature of the interface is a matching of the reciprocal lattice structure which is achieved by a moderate strain in the AlAs film. We discuss the implications of the match in reciprocal space for the atomic arrangement at the interface and show how a lowering of the interface energy is achieved. The study of interfaces including quasicrystals reveals that the essence of epitaxy lies in the matching of the two reciprocal lattices.

"Quantum Well States in Thin Silver Films with Quasiperiodic Intrinsic Step Structure"
Paolo Moras¹, Luisa Ferrari¹, Sandra Gardonio¹, Carlo Carbone¹, Jun Fuji², Wolfgang Theis³, Karsten Horn⁴
¹Istituto di Struttura della Materia, Consiglio Nazionale delle Ricerche, Trieste, Italy
²Laboratorio TASC, I.N.F.M., Trieste, Italty
³Freie Universität, Berlin, Germany
⁴Fritz-Haber-Institut der MPG, Berlin, Germany

Presentation Type: Poster
Topic: Surfaces

Abstract:
Ag films grown on GaAs(110) present in high-resolution STM images a quasi-periodic distribution of stripes running along the GaAs[110]-direction with two characteristic separations of 17 Å and 12 Å. The resulting one-dimensional superstructure can be described by a golden-mean Fibonacci sequence with a coherence length of about 125 Å. Angle-resolved photoemission has been used to examine the spectral functions of the sp-derived Ag electronic states as a function of the photoelectron momentum. While the dispersion of the Ag sp–quantum well states along the stripe direction is close to a free–electron–like parabola, it displays flatter bands and a complex branching structure perpendicularly to them. A close correspondence is found between the incommensurate wave vectors derived from the LEED pattern and those characterizing the Fermi contours of the Ag films in the two-dimensional reciprocal space.

"Wandering and Faceting of Steps on Quasicrystalline Surfaces and on Related Crystalline Surfaces"
J. Ledieu¹, V. Fournée¹, E. Cox², R. McGrath², N. Richardson³, T. Lograsso⁴, K. Caspersen⁵, B. Unal⁴, J. Evans⁶, P. Thiel⁵
¹LSG2M, CNRS-UMR 7584, Parc de Saurupt, F-54042 Nancy, France
²Surface Science Research Centre, The University of Liverpool, Liverpool L69 3BX, United Kingdom
³School of Chemistry, University of St. Andrews, North Haugh, KY16 9ST, United Kingdom
⁴Department of Materials Science and Engineering and Ames Laboratory, Iowa State University, Ames, IA 50011, USA
⁵Department of Chemistry and Ames Laboratory, Iowa State University, Ames, IA 50011, USA
⁶Department of Mathematics and Ames Laboratory, Iowa State University, Ames, IA 50011, USA

Presentation Type: Poster
Topic: Surfaces

Abstract:
We compare step morphologies on surfaces of Al-rich metallic alloys, both quasicrystalline and crystalline. The steps are observed after quenching from some elevated temperature (which depends on the alloy) to room temperature. On a large-scale, the steps on the quasicrystals are relatively rough, i.e., have high diffusivity, compared to those on crystalline surfaces. On a shorter scale, the steps on all of the Al-rich alloys tend to exhibit local facets (short linear segments), with facets being shorter on the quasicrystals than on the crystalline surfaces. Facets are shortest and most difficult to identify for the fivefold quasicrystal surface. We propose that the large-scale roughness is locked in at a higher temperature, during cooling, than the shorter-range faceting, and that faceting tends to expose steps of highest atomic density.

"Quasiperiodic States in Linear Surface-Wave Experiments"
M. Torres¹, J. Adrados¹, P. Cobo², G. Chiappe³, E. Louis³, J. Miralles³, J. Vergès⁴, J. Aragòn⁵
¹Instituto de Física Aplicada, CSIC, Serrano 144, 28006 Madrid, Spain
²Instituto de Acústica, CSIC, Serrano 144, 28006 Madrid, Spain
³Departamento de Física Aplicada and Unidad Asociada of the CSIC, Universidad de Alicante, San Vicente del Raspeig, 03690 Alicante, Spain
⁴Departamento de Teoría de la Materia Condensada, Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain
⁵Centro de Física Aplicada y Tecnología Avanzada, UNAM, Apartado Postal 1-1010, Querétaro 76000, México

Presentation Type: Invited Talk
Topic: New Frontiers

Abstract:
A subtle procedure to confine quasiperiodic hydrodynamic modes on the free surface of a fluid is presented here. The experiment consists of a square vessel with an immersed square central well vibrating vertically so that the surface waves generated by the meniscus at the vessel boundary interfere with the bound states of the well. This is a classical analogy of a quantum well where some fundamental phenomena, such as bonding of states and interference between free waves and bound states, can be visualized and controlled. The above mentioned interference leads to an hydrodynamic transition from quasiperiodic to periodic patterns for the first time. As it will be shown, the quantum analogs of the present experiment suggest that our results could be transferred to design quantum confinements exhibiting electronic quasiperiodic surface states and their rational approximants. On the other hand, Bloch-like surface waves associated with a quasiperiodic structure are observed in a revisited classic wave propagation experiment which consists of pulse propagation with a shallow fluid covering a quasiperiodically drilled bottom. We show that a transversal pulse propagates as a plane wave with quasiperiodic modulation, emphasizing the idea that analogous concepts to Bloch functions can be applied to hydrodynamic quasicrystals under certain circumstances.

"Crack Propagation in Quasicrystals and Complex Crystals"
Frohmut Rösch¹, Hans-Rainer Trebin¹, Peter Gumbsch²
¹ITAP, Universität Stuttgart, 70550 Stuttgart, Germany
²IZBS, Universität Karlsruhe, 76131 Karlsruhe, Germany

Presentation Type: Poster
Topic: Mechanical Properties and Applications

Abstract:
We present numerical experiments on the propagation of mode I cracks in a three dimensional icosahedral model quasicrystal. In particular, the dependence on the plane structure and the influence of clusters have been investigated. For this purpose samples in different orientations are endowed with atomically sharp seed cracks and subsequently loaded by linear scaling of the displacement field. The response of the system is then monitored by molecular dynamics simulations. Brittle fracture without any crack tip plasticity is observed.

The fracture surfaces turn out to be rough on the scale of the clusters. These are not strictly avoided, but to some extent intersected by the dynamic crack. However, compared to the flat seed cracks the clusters are cut less frequent. Hence the roughness of the crack surfaces can be attributed to the clusters, whereas the constant average heights of the fracture surfaces reflect the plane structure of the quasicrystal. A detailed analysis shows that the distribution of the clusters governs the brittle fracture behaviour. We may thus conclude that the clusters are indeed physical entities, not only structural units.

In order to overcome the limitations of simple model potentials, more realistic potentials of EAM type are required, but are presently not available. Such potentials are necessary especially for situations of low coordination, which occur at the crack surface. As a first step to resolve this issue, EAM potentials for a complex crystal phase, the C15 Laves phase of NbCr₂, have been constructed by using the force matching method. These and other EAM potentials from the literature are used in a study of C15NbCr₂, and the results are compared. It is planned to develop similar potentials also for quasicrystals.

"Investigations of Phason Statics and Dynamics"
Hans-Rainer Trebin¹, Ulrich Koschella¹, Mazahiro Umezaki², Takashi Odagaki²
¹ITAP, Stuttgart University, D-70550 Stuttgart Germany
²Department of Physics, Faculty of Science, Kyushu University, Fukuoka 812, Japan

Presentation Type: Invited Talk
Topic: Thermal and Dynamical Properties

Abstract:
The experimental observation of phason fluctuations, both static and dynamic, is well established and has been interpreted within the frame of a hydrodynamical theory. Theoretically, it is still open in what temperature range a hydrodynamical description of phasons is applicable. It has been proposed in the literature that towards low temperatures the hydrodynamical regime is terminated because the form of the phason elastic energy changes from harmonic to nonanalytical. Here we report on two contributions to the field: First, for a two-dimensional binary model quasicrystal at zero temperature we have investigated the relation between atomic two-body potentials and the phason elastic energy. It turns out that there are indeed certain potentials that can induce a nonanalytic phason elastic energy. But these must have very peculiar features in their long range part rendering their existence highly improbable. Second, one solution of the hydrodynamical equations is a phasonic excitation riding a phonon. We have studied the atomistic counterpart via a nonlinear chain with double well potentials, which resembles a dynamical fibonacci sequence. We see correlated phason flips propagating with phonon excitations and can extract observables like waiting times of the particles in the wells.

"Phase Diagrams Involving Quasicrystal and Crystal Growth in the In-Ag-Yb System"
Jun Hasegawa¹, Shin Takeuchi¹, Satoshi Ohhashi¹, Anpang Tsai¹
¹Institute of Multidisplinary Research for Advanced, Sendai, Miyagi prefe 980-8577 Japan

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
Partial phase diagrams including quasicrystalline phase in In-Ag-Yb system have been studied. In addition to the quasicrystalline phase, an 1/1 and a 2/1 cubic approximants were found to form in the vicinity of stoichiometry of quasicrystalline phase i.e., In₄₂Ag₄₂Yb₁₆. Like the quasicrystal in Cd-Yb system, the quasicrystal in In-Ag-Yb system exhibits congruent melting. Based on the phase diagram, we have grown a single quasicrystal with a size around cm order and single grains of approximants with sizes around few mm. Details will be given inn the presentation.

"Phason Dynamics in a Dynamical Fibonacci Chain"
Masahiro Umezaki¹, Hans-Rainer Trebin², Takashi Odagaki¹
¹Department of Physics, Faculty of Science, Kyushu University, 812-8581 Fukuoka Japan
²ITAP, Stuttgart University, D-70550 Stuttgart Germany

Presentation Type: Poster
Topic: Thermal and Dynamical Properties

Abstract:
The dynamical properties are investigated for a dynamical Fibonacci chain which is constructed to support phason flips.By a constant-temperature molecular dynamics simulation the dynamical structure factor is obtained at various temperatures for a wide range of frequencies and wave vectors. A detailed analysis of the dynamical structure factor is presented for the transition from phonon-dominant to phason-dominant dynamics. Also the spatial evolution of phason flips is observed when one end of the chain is forced to oscillate. Pinning and depinning of the phason flips is shown to play an important role for phason dynamics.

"Terrace Equilibration Process at an Icosahedral Quasicrystal Surface"
Baris Unal¹, Cynthia Jenks², Thomas Lograsso³, Amy Ross³, Patricia Thiel²
¹Departments of Materials Science and Engineering , Iowa State University, Ames, IA 50011 USA
²Department of Chemistry and Ames Laboratory, Iowa State University. Ames. IA 50011 USA
³Ames Laboratory, Iowa State University. Ames. IA 50011 USA

Presentation Type: Poster
Topic: Surfaces

Abstract:
We investigate the structure of a fivefold surface of icosahedral Al–Pd–Mn using scanning tunneling microscopy. Our studies show that after annealing at 900-915 K, the terraces appear that are rich in shallow voids. Upon annealing at 925-950 K, the voids are gone. The void-rich terminations have a distinctive local atomic configuration, very similar to 'rings' identified in the model of Papadopolos and Kasner [1]. Due to high density of voids, a new termination becomes exposed, which also results in a change in the step heights at the edges of the terraces. We propose that the shallow steps associated with the voids, and the ring configuration, are signals of a surface that is in an intermediate stage of structural equilibration.
[1] Papadopolos, Z., et al., Phys. Rev. B 66, 184207 (2002)

"Self-Assembled Micellar Liquid Quasicrystal"
X.-B. Zeng¹, G. Ungar¹, Y. Liu¹, V. Percec², A. Dulcey²
¹University of Sheffield, Department of Engineering Materials, Sheffield, U.K.
²University of Pennsylvania, Department of Chemistry, Philadelphia, U.S.A.

Presentation Type: Invited Talk
Topic: New Frontiers

Abstract:
Taper-shaped molecules, such as benzylether dendrons with n-alkyl terminal groups, self-organize in a variety of bulk thermotropic mesophases with periodicities in the range 3-30 nm. The molecules can assemble into spheres ("micelles", or supramolecular dendrimers), which then pack on three periodic 3-d lattices with symmetries Im3m(bcc)¹, Pm3n(cubic)² and the recently discovered P4₂/mnm (tetragonal).³ The tetragonal and the m3n phases belong to tetrahedrally close packed (tcp) structures, or Frank-Kasper phases⁴, which are widespread in transition metal alloys. This indicates the commonality of transition metal atoms and supramolecular dendrimers. Here we report on the first quasicrystalline phase in a supramolecular systems, a scaled-up "micellar" version of metallic quasicrystals, presenting a new mode of organization of soft matter.

The quasicrystalline (the fourth "micellar") phase has been observed in a number of dendrimers. We have succeeded in growing monodomains of this liquid crystal phase. On the basis of small-angle X-ray diffraction of such a domain and backed by AFM imaging, it was possible to establish that the structure possesses 12-fold rotational symmetry, and is thus a dodecagonal quasicrystal.⁵ We have constructed a model of the structure and the simulated diffraction pattern gives a reasonable match with that recorded for all orientations. Due to their exceptionally high symmetry, the prospect of growing self-assembled quasicrystals with "periodicities" on the optical scale (still some way off) opens exciting possibilities for generating materials with a wide 3-d photonic bandgap through supramolecular self-assembly.

¹ Duan, H., Hudson, S. D., Ungar, G., Holerca M. N. and Percec, V. Chem. Eur. J., 7, 4134 (2001).
² Balagurusamy, V. S. K., Ungar, G., Percec, V., Johansson, G. J. Am. Chem. Soc., 119, 1539 (1997).
³ Ungar, G., Liu, Y. S., Zeng, X. B., Percec, V., Cho, W.-D. Science, 299, 1208 (2003).
⁴ Frank, F. C., Kasper, J. S. Acta Cryst., 11, 184 (1958).
⁵ Zeng, X.-B., Ungar, G., Liu, Y., Percec, V., Dulcey, A. E., Hobbs J. K. Nature, 428, 157 (2004).

"From Harmonically Modulated Structures to Quasicrystals"
Grzegorz Urban¹, Janusz Wolny¹
¹AGH University of Science and Technology, 30-059 Krakow Poland

Presentation Type: Poster
Topic: Structure

Abstract:
The Fibonacci sequence is a model structure for one-dimensional quasicrystals. It can be obtained by 'cut and projection method' from higher-dimensions. However, it is also an example of modulated structure with higher-order modulations. Two wave vectors: the first one describing the positions of the main reflections and the second one for the satellite reflections, fully index the diffraction pattern of the Fibonacci sequence. The diffraction pattern of the chain is analysed in an average unit cell approach, where the statistical distribution of atomic distances in respect to the reference grid is calculated. It is well known [1] that the average unit cell for harmonically modulated structure has a universal shape. One can also calculate the universal self-correlation function, which is the so called average Patterson function (ordinary Patterson function reduced to the average unit cell). For Fibonacci sequence the average unit cell looks very simple [2]: it has a rectangular shape and has non-zero value only along a certain line. Performing Fourier analysis of rectangular distribution function one gets the amplitudes of harmonic components for quasicrystalline structure. The universal average Patterson functions are calculated and compared for different modulated structures, including quasicrystals. This function is obtained directly from the measured diffraction pattern by Fourier transform and reduction to the average unit cell. Its shape is compared with the universal shapes of differently modulated structures and can be used for distinguishing between the structures.

[1] G. Urban, J. Wolny, Statistical approach to the multi-q modulated structures. Average Patterson analysis, Philosophical Magazine 84 (2004) 2905n17.
[2] B. Kozakowski, J. Wolny, Structure factor for quasicrystals n 1D case, Acta Physica Polonica A 104 (2003) 55-66.

"Feature of the Structure and Properties of Quasicrystalline Coatings deposited from the Vapor Phase"
Anatolii Ustinov¹, Sergey Polischuk¹
¹E.O.Paton Electric Welding Institute of National A, 03150 Kiev Ukraine

Presentation Type: Poster
Topic: Mechanical Properties and Applications

Abstract:
It has been found in our previous work that thick (from 10 up to 200 μm) coatings of Al-Cu-Fe system with a quasicrystalline structure were formed by high-rate electron beam stationary evaporation from one source and deposition of the vapour phase of alloy of a specified chemical composition. The developed deposition procedure opens up new possibilities for controlling the microstructure parameters and properties of quasicrystalline coatings, which is of theoretical and practical interest. In this connection we have studied features of the structure and properties of Al-Cu-Fe quasicrystalline condensates differing each from other in chemical composition. It is shown that even under the conditions of a high deposition rate (up to 100 nm/s) in a broad range of substrate temperatures (from 200 to 800 °C) and considerable deviation of the condensate composition from the stoichiometric (Al_62.5Cu₂₅Fe_12.5) during condensation of the vapour phase consisting of a mixture of pure elements (Al, Fe and Cu) a quasicrystalline structure with a high degree of ordering is formed. It is shown that grain sizes can be varied in a broad range (from micro- to nanosized scale) by variation of substrate temperature. Deformation curves of the condensates with a quasicrystalline structure constructed by using the method of microindentation revealed extended areas of work hardening, which are nearly twice as large as that for bulk quasicrystalline specimens. The observed concentration conditions of formation of a quasicrystalline structure from the vapour phase and unusual mechanical properties of quasicrystalline condensates may be linked to the presence of a higher concentration of vacancy-type defects in the icosahedral structure.

"On the Question of Insulating Phase Formation in Al-Pd-Ir System"
Srinivas Veeturi¹, T. Takeuchi², U. Mizutani²
¹Indian Institute of Technology, Kharagput, West Bengal 721 302 India
²Department of Crystalline Materials Science, Nagoya University, Japan
Presentation Type: Contributed Talk
Topic: Electronic and Magnetic Properties

Abstract:
Recent experiments suggested a possibility of metal-insulator transition in Al-Pd-Re quasicrystals (QCs), which has now become a challenging and controversial subject [1]. Quasicrystalline materials posses a complex structure with a large number of atoms in the unit cell, while the atomic structure of corresponding lower order approximants (crystalline counter parts) is believed to be less complicated and easier to understand. On the theoretical front, recent band calculations based on a complex atomic structural model suggested possible Al-based semiconducting 1/1-1/1-1/1 approximants with a band gap of 0.52 eV [2]. However, so far there are no experimental investigations to verify such predictions. In the present work we report our recent experimental results on the phase formation domain in Al-Pd-Ir system and on the electrical properties, to verify theoretically predicted results.

Alloy ingots of Al-Pd-Ir with various compositions were prepared and a ternary phase diagram of this system has been constructed. The diffraction pattern of 1/0-1/0-1/0 approximant phase was also simulated from the known crystal structure of Al₃Ir (cP32, Pm3n, a = 0.7660 Å). From these results the Al₆₉Pd₁₀Ir₂₁ sample was identified as a single phase 1/0-1/0-1/0 approximant with a lattice constant of about 7 Å and is highly stable, as it could be obtained in the as-cast state. The electrical resistivity measurements as a function of temperature for few compositions has been done. Although the magnitude of resistivity is more than 500 mΩcm at room temperature, the temperature coefficient of resistivity (TCR) is observed to be positive. These results are consistent with the binary 1/0-1/0-1/0 approximants Al₇₃Ir₂₇ and Al₇₃Rh₂₇ with an exception that larger values of resistivity are obtained in the present case. Such large resistivity values have been reported in various approximants [3] and consistently interpreted in terms of the degrees of chemical disorder and depth of the pseudogap at the Fermi level within the context of the Boltzmann electron transport mechanism [4]. A positive TCR coupled with a fairly high resistivity value observed in the present Al-Pd-Ir 1/0-1/0-1/0 approximant can be understood as a mean free path effect in a system where a deep pseudogap exists at the Fermi level. The Al-Pd-Ir 1/0-1/0-1/0 approximant is believed to be typical of a deep pseudogap system. Therefore the binary Al-Ir and ternary Al-Pd-Ir 1/0-1/0-1/0 approximants are definitely in a metallic regime and do not exhibit any insulating behavior, as opposed to the prediction by Krajci and Hafner for their theoretically constructed 1/1-1/1-1/1 approximant [2].
References:
V. Srinivas et al. PRB 63 (2001) 172202; T. Su et al PRB66 (2002) 054438.
M. Krajci and J. Hafner, Proc. Int. Conf. on Quasicrystals (Bangalore India) 2002.
T. Takeuchi and U. Mizutani, PRB 52 (1995) 9300.
U. Mizutani, Mater. Sci & Engr 294-296 (2000) 464.
N.F. Mott, Phil. Mag. 26 (1972) 1015.

--------------------------------------------------------------------------------
* JSPS fellow. Permanent address: Physics and Meteorology, IIT, Kharagpur-721 302, India

"Magnetic Properties of Rapidly Solidified Fe Substituted AlMnCuGe Quasicrystals"
Srinivas Veeturi¹, S. Yu², Richard Dunlap³
¹Indian Institute of Technology, Kharagpur, W.B 721 302 India
²Dept. of Physics, Chungbuk National University, Cheongju 361-763, South Korea
³Dept. of Physics, Dalhousie Uversity, Nova Scotia, Canada B3H 3J5

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
Quasicrystals exhibit a range of magnetic properties starting diamagnetism in thermodynamically stable quasicrystals to weak ferromagnetic behavior in rapidly solidified alloys. However, it is still not clear whether these unusual properties are intrinsic to their structure or otherwise. Low concentration Si,Ge samples exhibit spin glass like behavior while at higher concentration they show ferromagnetic behavior. However, the magnetization values of these quasicrystalline alloys are very small even at low temperatures, in spite of their high Curie temperatures. The aim of this work is to clarify the magnetic properties of Fe substituted Al₄₀Mn₂₅Cu₁₀Ge₂₅ alloys and compare with the properties of their crystalline counterparts. This in turn will shine light on the question whether the magnetic properties observed in these systems are intrinsic to the QC structure or not. Rapidly solidified Al₄₀Mn₂₅Cu_10-xFe_xGe₂₅(x=0-10) alloys were prepared and the quasicrystalline nature of the samples was confirmed. Subsequently magnetization as a function of field and temperature was measured for this series. At room temperature they exhibit hysterisis and no saturation has been observed at the field of 1 Tesla. This indicates that the presence hard magnetic phase in the alloy system. The value of magnetization at 1Tesla (M_1T) and coercivity (H_C)) decrease as Fe concentration increases. Magnetization data as a function of temperature shows that magnetic transition temperature decreases with increasing Fe content. It appears that as Fe concentration increased the AFM coupling increases. These results will be compared with their crystalline counterparts T-AlMnGe and detailed magnetic properties will be presented. We will also further clarify the magnetic state of these quasicrystalline alloys.

"Low temperature electrical transport in AlPdRe Quasicrystals"
Srinivas Veeturi¹, Östen Rapp², S. Poon³
¹I.I.T, Kharagpur, W.B 721 302 India
²Solid State Physics, KTH-Electrum 229, 164 40 Stockholm-Kista, Sweden
³Dept. of Physics, University of Virginia, Charlottesville Va 22901, USA

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
The low temperature electrical transport in icosahedral (i-) AlPdRe is a subject of recent interest. Both experimental and theoretical studies indicate the existence of an electrically insulating state in this system. While a stronger proof stems from magnetoresistance (MR) results in the temperature range 1-10 K, transport properties at lower temperatures are poorly understood [1,2]. The ultra low temperature conductivity shows a saturation behavior in conductivity (i.e, s(0) ?I^ 0), and the characteristic minimum in the variable range hopping MR vanishes. Now it is well established that the saturation conductance is related to the resistance ratio R [=r(295 K)/r(4.2 K)]. However, the physical origin of s(0) is not well understood. Low temperature MR and conductivity data of different R samples can provide information on such questions. In the present investigation we report the low temperature conductivity and MR results on Al_70.5Pd₂₁Re_8.5 with R-values in the range of 2-200. These studies indicate that s(0) not only decreases with R but also on application of magnetic field. This suggests that an external field drives the system deeper into the insulating regime. Highly resistive samples exhibit large MR (34%) values at close to 10 mK in 6 Tesla applied magnetic field. Detailed analysis of conductivity and effect of magnetic field on saturation conductivity will be discussed to understand the origin of saturation conductance in these systems.

References:
1. V. Srinivas et al., Phys. Rev. B65 (2002) 094206.
2. R. Rosenbaum et al J. Phys. Cond. Matter. 16 (2204) 821.

"Evaluation of Nano-Quasicrystalline Phase in Al-Cu-Fe Powders."
Srinivas Veeturi¹, Pranabesh Barua¹, B. Murty¹
¹Indian Institute of Technology, Kharagpur, West Bengal 721 302 India

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
Recently, MA technique has been exploited to synthesize nanoquasicrystalline Al-Cu-Fe alloys [1,2]. Though the process of MA leads to formation of disordered i-phase, an optimum thermal treatment results in stable QC phase in the vicinity of composition Al₆₅Cu₂₀Fe₁₅. An important question that was not addressed in earlier reports is that, whether a QC phase could be obtained by direct MA (without post milling thermal treatment)? Recently there were attempts to investigate the single i-phase formation domain in Al-Cu-Fe system. These results indicate that the single i-phase formation domain can be extended through MA processes. In addition to this, one can even obtain quasicrystalline materials in nanaophase. In the present article we report the evolution of nanoquasicrystalline phase Al-rich Al-Cu-Fe alloys. Further from structural and thermal stability we propose phase formation mechanism in Al-rich region of Al-Cu-Fe.
Elemental blends of Al, Cu and Fe powders of <45 mm size corresponding to the nominal compositions of Al₇₅Cu₁₅Fe₁₀, Al₈₀Cu₁₂Fe₈, Al₆₅Cu₁₅Fe₂₀, Al₆₅Cu₂₀Fe₁₅, , Al₇₀Cu₂₀Fe₁₀, and Al₇₀Cu₁₀Fe₂₀, have been mechanically alloyed in a high-energy planetary ball mill (Fritsch Pulverisette P-5) with tungsten carbide milling media (10mm diameter balls) in toluene. Present observations indicate the volume fraction of i-phase is quite large in the as milled Al₇₅Cu₁₅Fe₁₀, composition compared to the other compositions. The i-phase appears to be sensitive to Al, TM ratio and also to Cu, Fe ratio. No traces of i-phase have been observed in the compositions Al₆₅Cu₁₅Fe₂₀, Al₇₀Cu₁₀Fe₂₀, and Al₈₀Cu₁₂Fe₈. Icosahedral phase formed during MA in Al₇₀Cu₂₀Fe₁₀, composition, transforms into almost single i-phase quasicrystal with F-type structure after additional heating at elevated temperatures. It is observed that i-phase is stable up to 900 °C for 40 hr milled Al₇₅Cu₁₅Fe₁₀ alloy while 30 hr milled sample transforms to w-phase on heat treatment. From a detailed microstructure and thermal stability study we propose a phase formation mechanism that is consistent with our earlier reports.
1. P. Barua et al., J. Appl. Phys. 91 5353 (2002)References therein.
2. V.V. Tcherdyntsev et al., J. Non.Cryst. Sol. 312-314 (2002) 522.

"Transport Properties in Multidimensional Fibonacci Lattices"
Chumin Wang¹, Vicenta Sanchez¹, Fernando Salazar¹
¹Universidad Nacional Autonoma de Mexico, 04510 Mexico D.F. Mexico

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:

In this work, the Kubo-Greenwood formula is used to investigate the electronic and phononic transport properties in macroscopic quasiperiodic systems by means of an exact renormalization method [1]. The convolution technique is employed in the analysis of two- and three-dimensional Fibonacci lattices [2]. The dc electrical conductance spectra of multidimensional systems exhibit a quantized behavior when the electric field is applied along a periodically arranged atomic direction, and it becomes a devil's stair if the perpendicular subspace of the system is quasiperiodic. This conductance maintains a constant value for small imaginary parts (η) of the energy and decays when η>η_c, where η_c is proportional to the inverse of the system length. The spectrally averaged conductance shows a power-law decay as the system length grows, neither constant as in periodic systems nor exponential decays occurred in randomly disordered lattices, revealing the critical localization nature of the eigenstates in quasicrystals. Finally, the thermal conductance by phonons shows a linear temperature dependence and an asymptotic constant behaviour at low and high temperature regions, respectively.

[1] V. Sanchez, L.A. Pérez, R. Oviedo-Roa, and C. Wang, Phys. Rev. B 64, 174205 (2001).
[2] V. Sanchez and C. Wang, Phys. Rev. B 70, 144207 (2004).

"Effect of Grain Size on Plastic Deformation of Al-Cu-Fe-(B) Icosahedral Poly-Quasicrystal at Elevated Temperature"
Jianbo Wang¹, Renhui Wang¹
¹Department of Physics, Wuhan University, 430072 Wuhan, China

Presentation Type: Poster
Topic: Mechanical Properties and Applications

Abstract:
It is accepted now that the dislocation motion is a predominant process mediating the high-temperature plastic deformation of QCs. The scenario of the dislocation motion in the plastic deformation in single QCs may be expressed as follows: The phonon component of a dislocation on a densely packed plane of a QC may climb and/or glide under applied stress leaving a phasonic stacking fault behind. Owing to the strong intermetallic bonds in clusters in a QC and large stacking fault energies, the critical stress for dislocation movement is very high. Hence almost all the QCs are brittle at room temperature. At elevated temperature, thermal activation helps moving dislocations circumvent or cut these clusters in the course of plastic deformation. The phasonic stacking fault left by a moving dislocation broadens and then disappears due to intense diffusion at high temperature. Then, the partial dislocation becomes a perfect dislocation.
There are two competing processes in the course of plastic deformation of QCs: strain-hardening and disorder-softening. Owing to the high structural symmetry, plastic deformation in IQCs is usually carried out by multiple slip which enhances the strain hardening process, showing an initial hardening stage with high hardening rate. On the other hand, the phason type strain left behind by the moving dislocations is rather strong and its recovery can not be sufficient. This causes a disordering and hence a softening of the obstacles to dislocation motion, leading to a decrease of the flow stress with increasing plastic strain.

It is necessary to obtain information of mechanical properties of poly-grained IQCs at high temperature in order to utilize them eventually. We perform mechanical testing at different temperature, with different strain rate, on poly-grained Al-Cu-Fe-(B) IQCs, which are prepared by powder metallurgy. From the experimental results, we can draw the following conclusions. Due to the rapid recovery caused by intense diffusion in small-sized grains, the quasicrystal lattice reorders rapidly from disordering, which inhibits the disordering-softening effect. We can decrease the reordering effect, when we reduce the diffusion through lowering the testing temperature and/or increasing the strain rate. The diffusion is stronger when the grain size is smaller. It is obvious that grain size plays a very important role in the plastic deformation of the poly-grained IQCs.

Acknowledgements:
The author (JW) thanks Prof. K. Urban, Dr. M. Feuerbacher, Dr. M. Heggen for their help and hospitality during his stay at Institut für Festkôrperforschung, Forschungszentrum Jülich, Germany. The authors are grateful to Dr. D. J. Sordelet at Ames Laboratory of USA for providing part of the samples for the mechanical testing.

"Structures of Different Modifications in Al-Co-Ni Decagonal Quasicrystals"
Renhui Wang¹, Supeng Song¹, Dongxia Xiong¹, Jianbo Wang¹, Jianian Gui¹
¹Department of Physics, Wuhan University, 430072 Wuhan China

Presentation Type: Poster
Topic: Structure

Abstract:
Among decagonal quasicrystals (DQCs), Al-Co-Ni DQCs are the ones which received most thorough investigation. There are seven different modifications: b-Ni (basic Ni-rich phase), b-Co (basic Co-rich phase), type I (type I superstructure), type II (type II superstructure), S1 (S1-type superstructure), 5f (5-fold phase) and 5f HT (5f high temperature phase) states. Each of the states has characteristic features in the parallel beam electron diffraction pattern (EDP), in the atomic structure of the atomic cluster for the unit cell, and in the tiling describing the ways by which the unit cells connected into the DQC. In the present paper several experimental techniques, including atomic resolution Z-contrast high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM) technique, and theoretical considerations, including first-principles calculation, are applied to investigate systematically the characteristic features of the atomic structures of each modification. The present studies show some new results compared with ones in the literature. For example, when studying the Al_72.7Co_19.3Ni_8.0 alloy annealed at 900 °C and then at 700 °C for 48 hours, our HAADF-STEM observation reveals two types of the unit cells of 2 nm in diameter. One type of the atomic clusters has the pentagonal contrast possessing the same orientation at its center. The other type has a star shaped contrast in its center. Such a contrast may be interpreted by an atomic model based on the τ² –inflated Al₁₃Co₄ approximant phase.

"Phasonic Disorder in Decagonal Al-Co-Ni"
Thomas Weber¹, Miroslav Kobas¹, Walter Steurer¹
¹ETH Zurich, CH-8093 Zurich Switzerland

Presentation Type: Poster
Topic: Structure

Abstract:
Diffuse scattering coming from phasonic disorder carries information about the local structure and about crystal chemical building and stabilization principles of quasicrystals. In this contribution, we employ the hydrodynamic theory of phasonic disorder to describe the local structure of a decagonal Al-Co-Ni quasicrystal ('Edagawa-phase'). The calculations allow distinguishing different cluster types published so far. The best agreement with experimental data could be achieved with the mirror symmetric Abe-cluster. Modeling of phason diffuse scattering associated with the S1 and S2 superstructure reflections indicate disorder of superclusters. The former represent strong inter-cluster correlations inside quasiperiodic layers, while the latter are due to intra- and inter-cluster correlations, both between adjacent and inside quasiperiodic layers. Further, it is demonstrated that phasonic disorder and fivefold orientational disorder of single clusters are an equivalent description of local disorder phenomena.

"LT-STM-Spectroscopy of the Icosahedral AlPdMn Surface"
Roland Widmer¹, Pascal Ruffieux¹, Oliver Groening¹, Michael Bielmann¹, Pierangelo Groening¹
¹Empa Thun, 3602 Thun Switzerland

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
One of the many consequences of the lack of a translational symmetry in quasicrystals is that the description of the valence electronic structure can not be based, as in ordinary crystals on Bloch-States anymore. In this context the question arises whether quasicrystalline structure leads as theoretically predicted to the localization of states and whether this localization is at the origin of the peculiar electronic transport properties with a strongly reduced conductivity as compared to the constituting elements and a positive temperature coefficient of the conductivity.

In order to discuss the presence of localized states and to observe the spiky DOS as predicted by theory [1] we will present low-temperature (5K) scanning tunneling microscopy/spectroscopy (STM/STS) results obtained on the bare 5-fold surface of an i-Al₇₀Pd₂₀Mn₁₀ QC. By averaging tunneling spectra taken homogeneously spaced over several nm² we could reproduce the line shape observed by Escudero et al. [2] in point contact spectroscopy on oxidized surfaces. The averaged spectra show a large parabolic gap structure with a sharp dip of 30meV FWHM at the Fermi energy. In accordance with Escudero we also observed characteristic shoulders on both sides of the dip. Whereas the averaged spectrum is smooth and featureless, with exception of the sharp dip structure, the individual spectra show a rich structure of peaks mainly situated in the unoccupied local density of states (LDOS). The FWHM of these peaks is typically between 30meV and 100meV. The spectral features vary rapidly with tip position, where the spatial extend of a peak type spectral feature is of the order of 5-10Å. We will present the LT-STS data in combination with high resolution STM images to correlate the LDOS with topographic features of the 5-fold i-AlPdMn surface. Further we will discuss the problem that the spiky DOS may only be observed on the sub-nm level and that in methods averaging over a larger surface areas (>10x10nm²) like photoelectron spectroscopy the spiky character of the DOS is not present anymore.

[1] J. Hafner, and M. Krajci, Phys. Rev. B. 47, 11 (1993)
[2] R. Escudero et al., J. Phys.: Condens. Matter 11, 383 (1999)

"Structural Model of i-CaCd Based on Canonical Cells and Supported by Total Energy Calculations"
Michael Widom¹, Marek Mihalkovic²
¹Carnegie Mellon University, Pittsburgh, PA 15213 USA
²Slovakian Academy of Sciences

Presentation Type: Contributed Talk
Topic: Structure

Abstract:
Cadmium-based quasicrystals (Cd-Ca and Cd-Yb) were the first binary alloys discovered to form thermodynamically stable quasicrystals. As a binary alloy, and with a strong size difference between atomic species, they are ideal systems for structural and thermodynamic analysis. Observed quasicrystal approximants with crystallographically determined structures can be interpreted as decorations of Henley's canonical cells. We use first-principles total energy calculations to resolve details of the most favorable cell decorations. These decorated cells define an ensemble of icosahedrally symmetric quasiperiodic structures whose energies lie just slightly above the energies of the known crystalline structures. Prior experience with the alloy system B-Mg-Ru suggests that the configurational entropy of this ensemble may be sufficient to stabilize the quasicrystal phase at elevated temperatures.

"Cohesive Energies and Stacking Periodicities of Al-Co-Ni Structures"
Marek Mihalkovic¹, Michael Widom²
¹Slovakian Academy of Sciences
²Carnegie Mellon University, Pittsburgh, PA 15213 USA

Presentation Type: Poster
Topic: Structure

Abstract:
We employ first-principles total energy calculations to investigate the stability of quasicrystals, approximants and ordinary crystals in the binary alloy system Al-Co and the ternary Al-Co-Ni. In the case of Al-Co, we generally confirm the experimentally reported phase diagrams except in the composition range Al₁₃Co₄-Al₃Co, where there is even no consensus among experimental studies. Indeed, we find that the reported Al₁₃Co₄ structures (Pearson types mC102 and oP102, both are decagonal approximants) are most likely unstable at low temperature. Instead, they are stabilized at high temperature owing in part to vacancy entropy. At composition Al₃Co we find that the structure of Pearson type oI96, with a 12 Å periodicity, is more stable than the structure with Pearson type mC32, with 4 Å periodicity. Similarly, in the ternary alloy system the best structural models for the "basic Ni-rich" decagonal phase with 4 Å periodicity are unstable by about 40 meV/atom. Introducing tiling flips between adjacent atomic layers reduces the energy by about 20 meV/atom while converting the structure to an 8 Å periodicity. In the Co-rich quasicrystal-forming limit the quasicrystal has an 8 Å stacking periodicity. The recently solved approximant W-AlCoNi (Sugiyama) reveals that the 8 Å periodicity results from tile flips between adjoining layers, analagous to what we found in the Ni-rich limit but with an additional "binary tiling" constraint. Presumably the 4 Å periodicity in the Ni-rich limit results from randomly flipping or not between adjacent layers, implying configurational disorder in the Ni-rich phase that may be linked with the experimentally observed layers of diffuse scattering that appear at low temperature.

"Decorated 2D Quasicrystals—Structure Factor and Average Patterson Analysis"
Janusz Wolny¹, Barlomiej Kozakowski¹, Aleksandra Dabrowska¹
¹AGH University of Science and Technology, 30-059 Krakow Poland

Presentation Type: Poster
Topic: Structure

Abstract:
There are many different approaches to the structural analysis of 2D quasicrystals. Among them are: higher-dimensional analysis using 'cut and projection' method or an average unit cell approach operating in physical space only. There are also some other concepts based on tiling or covering by clusters. The way of getting the structure factor for all of those approaches is the calculation of the statistical distribution of atomic positions in respect to some reference grid.
In this paper we are showing that the atomic surface used in the higher-dimensional approach is an average unit cell risen up to higher-dimension. On the other way one can get the average unit cell by an oblique projection of an atomic surface onto physical space. The two different approaches are completely equivalent for perfect structures. The great advantage of the physical space calculations can be seen for decorated and also defected structures. What is rather simply and obvious in physical space looks rather difficult in higher dimensions.

We have obtained an analytical expression for the structure factor of arbitrary decorated Penrose tiling. This structure factor can be used for refinement of the atomic positions from the measured diffraction pattern. It also allows to include any disorder comes from phasons or phonons and also some other defects. For perfect Penrose tiling the atomic distribution has a triangular shape. The Fourier transforms of these distribution functions added with some appropriate phases of decorating atoms lead to the requested structure factor for the analysed structure. Calculations have been performed for the decoration of two rhombuses: thick and thin and also for different clusters: the Gummelt's cluster of 33 atoms and the kite-cluster of 17 atoms. For cluster analysis numbers of independent atomic positions, were also found.
The average Patterson function was defined and successfully used for the structure refinement. Such Patterson function can be calculated by simple reduction of normal Patterson function to the average unit cell or by convolution of probability distributions of atomic positions in respect to the reference lattice.

"Fine Structure in Cd-Yb Hexagonal Approximant Cd₅₁Yb₁₄"
Dongmei Wu¹, Ozan Ugurlu¹, Scott Chumbley¹, Matthew Kramer¹, Thomas Lograsso¹
¹Ames Laboratory, Ames, IA 50011 USA

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
Synthesis of large, bulk single grains of the hexagonal approximant crystalline phase Cd₅₁Yb₁₄ has been accomplished using the Bridgman method. Growth was carried out under controlled solidification conditions in sealed Ta crucibles which maintain compositional integrity by eliminating evaporative losses of Cd and reaction with the containment material. Compositional analysis of the as-grown phase indicates no evidence of macrosegregation along the length of the crystal, confirming the congruent nature of melting. High-resolution TEM characterization of the hexagonal phase is hampered by the enhanced atmospheric reactivity of the higher Yb content of the hexagonal phase. Such a high sensitivity to ambient exposure was not observed during the recent synthesis and TEM studies of the quasicrystalline Cd₈₄Yb₁₄ or cubic approximate phase Cd₈₆Yb₁₄. TEM and x-ray transmission powder diffraction results taken at the Advanced Photon Source at Argonne National Laboratory show the presence of a second phase. The precipitates have no crystallographic relationship to the hexagonal phase and appear to be randomly oriented throughout the matrix. This phase distribution suggests the phase exists in the bulk and did not result from atmospheric exposure during the preparation of the specimen.

"Correlations in the Local and Long-Range Order between the Single Grain Cd-Yb Quasicrystalline and Its Cubic Approximant Phase"
YaQiao Wu¹, M. Kramer¹, Dongmei Wu¹, T. Lograsso¹
¹Iowa State University, Ames, IA 50011 US

Presentation Type: Contributed Talk
Topic: Structure

Abstract:
Understanding the unique structure of QCs and what gives rise to the aperiodic structure has been an open question. The general simplicity of a binary system presents numerous advantages in both computational and experimental investigations of this structure. In the present paper, large single crystals of binary icosahedral Cd₈₄Yb₁₆ quasicrystalline and its corresponding b.c.c. approximant Cd₆Yb phase were studied at the atomic level using high resolution transmission electron microcopy (HRTEM) and computer simulation techniques, to study the correlations between local and long-range order in the QC phase and its corresponding approximant phase. Both quasicrystalline Cd₈₄Yb₁₆ and b.c.c. Cd₆Yb single crystals were synthesized by using Bridgman method. Foils for HRTEM were electron discharge machined from the larger grains, on the order of 0.75 cm ³, and mechanically thinned to ~200 μm before being electropolished. Through-focal HRTEM images were obtained along two-fold and five-fold directions of QC, and the corresponding [001] and [530] orientations of the approximant Cd₆Yb, respectively. HRTEM investigations revealed a high degree of structural perfection and the remarkable similarity between the local atomic structure of QC and its approximant phase. Image reconstruction and computer simulation of the HRTEM images and cluster model are being preformed to obtain a 3D structure model aimed at understanding the formation of the edge-sharing and overlapping of the aperiodic-packing of the four-shell clusters previously suggested for the primitive icosahedral Cd₈₄Yb₁₆ phase.

"Study of Diffraction Patterns from Quasicrystals with Eight-Rotational Symmetry"
Jietian Zhang¹, Qinglan Rang¹, Tianbao Xie²
¹College of Physics, Peking University, Beijing, China 100871
²Linfield College, McMinnville, Oregon 97128 USA

Presentation Type: Poster
Topic: Structure

Abstract:
Computer simulations on quasicrystal samples with eight-rotational symmetry have been conducted using self-developed software. The quasicrystal samples have been investigated in this research were formed by randomly close packed squares and rhombuses. The length of each side of the squares and the rhombuses was unit and the acute angle of the rhombus was 45°. The study showed as far as there were good numbers of these two units and their space distributions were relatively the same in all directions, the spectra from these samples were eight-rotational symmetry patterns with the coordinator origin as their center.

"Influence of Pd on Formation of Amorphous and Quasicrystal Phases in Rapidly Quenched Zr₂Cu_xPd_(1-x)"
Min Xu¹, Y Ye¹, J. Morris², M. Kramer¹
¹Iowa State University, Ames, IA 50011 US
²Meatals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115, USA

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
Amorphous Zr₂Cu_xPd_(1-x) (x = 0, 0.25, 0.5, 0.75 and 1) alloys were prepared by melt spinning to investigate the role of Pd in the formation of metallic glass and the metastable icosahedral quasicrystalline phase (i-phase). The effect of Pd on thermal behavior and kinetics of i-phase formation in Zr₂Cu_xPd_(1-x) alloy system was determined using differential scanning calorimetry (DSC). Amorphous to i-phase transition was not observed in Zr-Cu binary alloys during devitrification process. Partial or total replacement of Cu by Pd in Zr₂Cu_xPd_(1-x) alloy system has been shown to result in i-phase formation. For the same heat treatment, varying the Pd alters the initial nucleation and phase selection during devitrification. The increasing Pd/Cu ratio in the alloy composition tends to delay the onset time and increase the onset temperature of the transformation from glass toi-phase. The thermal analysis indicate that the activation energy for the formation of the i-phase Zr₂Cu_xPd_(1-x) alloy system has a maximum exothermic event when x = 0.5. The difference in activation energy among different Zr₂Cu_xPd_(1-x) alloys could be due to the effect of Pd on the amorphous short-range order (SRO) and i-phase microstructure. Ab initio calculations suggest that the role of the Pd is to lower the electronic density of states at the Fermi surface. This could suggest that the effect of the ternary additions is an electric structure effect rather than an atomic size effect. Total scattering factors, S(Q), as a function of time and their reduced radial distribution functions G(r) will be determined and compared using various Pd/Cu ratios. This should help us to understand the role SRO of the as quenched amorphous alloys and its effect on the phase selection process.

Thisresearch has been sponsored by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy under contract DE-AC05-00OR-22725 with UT-Battelle and contract W-7405-ENG-82 with Iowa State University of Science and Technology.

"Short Range Order in Al-Pd-Mn Quasicrystalline Melt Studied by Small Angle X-ray Scattering"
T. Yamada¹, J. Okada, Y. Yokoyama³, Y. Watanabe¹, S. Kohara⁴, M. Inui⁵, K. Matsuda⁶, S. Nanao¹
¹Institute of Industrial Science, The University of Tokyo, Meguro-ku, Tokyo 153-8505, Japan
²Department of Advanced Materials Science, The University of Tokyo, Kashiwa-shi, Chiba 277-8561, Japan
³Materials Science and Engineering, University of Hyogo, Himeji 671-2201, Japan
⁴Japan Synchrotron Radiation Research Institute, Hyogo 679-5198, Japan
⁵Faculty of Integrated Arts and Science, Hiroshima University, Higashi-Hiroshima 739-8521, Japan
⁶Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
It is a matter of great interest how quasicrystals are formed. In order to understand the formation process of quasicrystals, the structure of quasicrystal melts should be illuminated. Since an icosahedral cluster is a structure unit of icosahedral quasicrystals, it can be surmised that the cluster is present in the quasicrystal liquids. Previously, it has turned out that the viscosity of Al₇₂Pd₂₀Mn₈ melt at 1573 K is dozens times as high as that of aluminum and the viscosity rises steeply near the melting point. These results suggest the possibility that a cluster-like-structure exists in the melt.

We have carried out small angle X-ray scattering (SAXS) measurements using synchrotron radiation for Al₇₂Pd₂₀Mn₈ single quasicrystals, in which a stable icosahedral quasicrystalline phase is formed from a liquid phase, right above the melting point to ascertain the existence of icosahedral clusters in the melt and understand those behavior. The attempt has never yet been performed.

SAXS measurements were conducted using a high energy X-ray source (37.8 keV) from synchrotron radiation on the beamline BL04B2 at SPring-8. The measured temperatures were 1340 K and 1223 K. As a result, we have succeeded in observation of SAXS spectra for the quasicrystal liquid and it has revealed that the liquid includes the cluster. Applying the Guinier approximation to the spectra, the radius of gyration, R_g, was estimated. R_g is the value deeply related to the cluster size. The obtained radius of gyration at 1223 K is about 35 Å, which is 10% larger than that at 1340 K. This suggests that the cluster size is expanded with temperature descent and the quasicrystalline cluster is gradually formed in the liquid.

"Recent Development in Structure Determination of Quasicrystals"
Akiji Yamamoto¹, Hiroyuki Takakura²
¹Advanced Materials Laboratory, NIMS, Tsukuba, Ibaraki 305-0044 Japan
²Research Center for Molecular Thermodynamics, Osaka Univ.

Presentation Type: Invited Talk
Topic: Structure

Abstract:
An accurate structure determination of quasicrystal structures based on a higher-dimensional approach is made by two steps.[1] They are a model building and a structure refinement. This paper describes a recent progress in both stages.

So far, the information for an initial model in x-ray structure refinements has been given by high-resolution electron microscopy (HRTEM) images for decagonal quasicrystals (d-QCs) together with their crystal approximant (CA) structures. On the other hand, for icosahedral quasicrystals (i-QCs), it is mainly obtained by the low-density elimination method (LDEM) [2] and CA structures, since it is difficult to extract the location of atoms or clusters from HRTEM images because of the non-periodicity of i-QCs. Recently, it was clarified that the LDEM is applicable to any kinds of aperiodic crystals, including d-QCs and modulated structures. The applications of the LDEM to a d-QC and an incommensurate crystallographic shear (CS) structure[3] together with a primitive i-QC [4] will be demonstrated as examples. It will be shown that incommensurate CS structures have diffraction patterns similar to those of quasicrystals with incommensurate linear phason strain and can be described by occupation domains as well as quasicrystals. The LDEM gives a rough positions and sizes of ODs and the distribution of atoms within ODs.

In the structure refinement, each OD is subdivided into smaller ones which are occupied by different atoms or same atoms but in different local environments. In the latter case, there exists an ambiguity for the choice of subdivided ODs. There are two ways for the choice. Since quasicrystals have no period in two dimensional subspace or entire three-dimensional space, there exists no atom positions which have exactly same local environments. In one approximation, if some atoms are surrounded by similar near neighbor atoms, they are regarded as the same local environment. This leads to subdivided small ODs, which is parallel to the internal space. This method was applied to several quasicrystal structure refinements and its efficiency was verified.[5] However, this requires smaller ODs if we take into account the difference in local environments in farther neighbor. In order to prevent the increase of the number of ODs, it may be efficient to introduce continuously modulated ODs. The merit and demerit of this method will be discussed.

[1] A. Yamamoto Acra Crystallogr, A52 (1996) 509.
[2] H. Takakura et al. Phys. Rev. Lett. 86 (2001) 236.
[3] Y. Michiue, et al. Acta Crystallogr. (2004) submitted.
[4] H. Takakura Proc. ICQ9 submitted.
[5] A. Yamamoto et al. Feroelectrics, 305 (2004) 223.

"Competing ε-Zr₆Pt₃O and QC Phases in Rapidly Solidified Zr₈₀Pt₂₀ Eutectic Alloys with Controlled Oxygen Contents"
Xiaoyun Yang¹, Matt Kramer¹, Dan Sordelet¹
¹Ames Laboratory (USDOE), Iowa State University, Ames, Iowa 50011 USA

Presentation Type: Poster
Topic: Structure

Abstract:

Epitaxial growth of fcc non-stoichiometric ε-Zr₆Pt₃O (Fd m, Ti₂Ni-type structure) with QC has been observed in rapidly solidified Zr₈₀Pt₂₀ eutectic alloys with approximately 4737 ppm oxygen. The detailed experimental approach is described in another corresponding paper submitted to the conference by D. J. Sordelet et al. In the present paper, a crystallographic relationship between ε-Zr₆Pt₃O and QC is revealed: icosahedral twofold axes coincide with <110>, <111>, <112> and <113> axes of ε-Zr₆Pt₃O; the space group Fd m preclude ε-Zr₆Pt₃O from an approximant to QC; both the morphology and XRD results suggest that ε-Zr₆Pt₃O forms first. In this way, the fcc ε-Zr₆Pt₃O and QC are quite similar to β-Zr(Pt) superstructure (Im3m) and QC in the ultra-low oxygen content eutectic ribbons (another report by X. Y. Yang et al. submitted to the conference). Oxygen is considered to play a crucial role to the phase competition occurred because of its strong affinity with Zr. The QC phase acts as an intermediate structure and is stabilized at intermediate oxygen level. When the oxygen level is decreased (e.g., 184 ppm mass O) or increased (e.g., 4737 ppm mass O), the formation of QC is not preferred or stabilized; whereas, two competing crystalline, β-Zr(Pt) superstructure (in 184 ppm mass O ribbons) and non-stoichiometric &epsilon-Zr₆Pt₃O (in 4737 ppm mass O ribbons), are predominant, respectively, in each of the case. It is important to consider the oxygen level gradients through the thickness of the ribbon. Quite similar bond-length exists between ε-Zr₆Pt₃O, β-Zr(Pt) and QC phase, which should give rise to their structural correlations. A structural model for ε-Zr₆Pt₃O is proposed and verified by theoretical ab initio calculations.

"Quasicrystal Formation in Gas Atomized Zr₈₀Pt₂₀ Powders"
Xiaoyun Yang¹, Elena Rozhkova¹, Dan Sordelet¹
¹Ames Laboratory (USDOE), Iowa State University, Ames, Iowa 50011 USA

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
Different solidification pathways give rise to different microstructures. In the past years, the nano-icosahedral phase formation was well reported in the Zr₈₀Pt₂₀ melt-spun ribbons. Here in the present paper, high-pressure gas atomization (HPGA) techniques are utilized to synthesize Zr₈₀Pt₂₀ powders to investigate the structure formation due to the high cooling rates involved in this process; helium was used as the atomization gas. Powders were first screened to below 106&mum;, and then sieved in the size ranges of <20, 20-45, 45-76, 76-106 microns. X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning and transmission electron microscopy (SEM and TEM) characterization was performed on each of the different powder size fractions to examine their structural and crystallization differences. Both the TEM and XRD results show that the predominant phase for all these size range powders is the icosahedral phase (QC), a very small amount of a nanocrystalline phase is also detected. The QC grain size is typically several microns, and slightly varies with different powder sizes. In the as-solidified state, the QC grains are distorted, as evidenced by asymmetries in the TEM electron diffraction patterns. DSC results show that two exothermic reactions occurred for <20 microns powder, suggesting the existence of a residual amorphous phase. This was further confirmed by TEM analysis. For coarser grains, their thermal behaviors are quite similar, one single exothermic reaction occurred at almost the same peak temperature, corresponding to the transformation from meta-stable QC to stable Zr-Pt crystalline phases. In addition, comparisons between gas-atomized Zr₈₀Pt₂₀ powder and melt-spun Zr₈₀Pt₂₀ ribbon are made, aimed at understanding the structural difference formed via different solidification pathways. Microstructure evolution mechanism involved in this gas atomization process is discussed.

"Coincident Lattice Sites between Cubic β-Zr(Pt) and an Isochemical QC Phase in Rapidly Solidified Zr₈₀Pt₂₀ Eutectic Alloys with Ultra-Low Oxygen Content"
Xiaoyun Yang¹, Matt Kramer¹, Elena Rozhkova¹, Dan Sordelet¹
¹Ames Laboratory (USDOE), Iowa State University, Ames, Iowa 50011 USA

Presentation Type: Poster
Topic: Structure

Abstract:
Coexistence of a bcc hyperstoichiometric β-Zr(Pt) (Im3m) superstructure and a QC phase in rapidly solidified Zr₈₀Pt₂₀ alloys containing the ultra-low oxygen content (approximately 184 ppm mass O) has been studied by TEM. The detailed experimental approach is described in another corresponding paper submitted to the ICQ9 conference by D. J. Sordelet et al. In the present paper, a thorough TEM analysis of the crystallographic relationship between beta-Zr(Pt) and QC is described. Results show the beta -Zr(Pt) and QC phase have a well-defined orientation relationship, close crystallographic match and nearly identical stoichiometry; the orientation relationship between the two phases can be described as: two-fold axes of QC are nearly parallel to <110>, <111>, <112> and <113> axes in beta -Zr(Pt). In all these observed orientation relationships, {110}_beta-Zr(Pt) planes register with icosahedral {211111} five-fold or {221001} two-fold planes. The space group Im3m for the β-Zr(Pt) and the observed orientation relationship preclude β-Zr(Pt) as an approximant to QC; (3) both the β-Zr(Pt) and QC are distorted. The β-Zr(Pt) maintain a basic β-Zr Bravais lattice with an aperiodic superlattice; (4) morphologies and growth habit suggest that QC forms first, followed by an easy nucleation of the β-Zr(Pt) on the surfaces of QC.

"The Relations of Hexagonal Phases in i-ZnMgRE Quasicrystal by Electron Crystallography"
Hong Zhang¹, Peter Oleynikov¹, Xiaodong Zou¹, Sven Hovmöller¹
¹Structural Chemistry, Stockholm University, SE-191 45 Stockholm Sweden

Presentation Type: Poster
Topic: Structure

Abstract:
The relationship between a quasicrystal and its approximant can be determined from the electron diffraction pattern. Since the crystallographic phases are lost in diffraction patterns, they must be deduced in order to solve the structure of a crystal. Comparing structure factor phases of related approximants, we found that the strong reflections have identical phases. This brings up a new approach for solving the structures of approximants. We applied this approach to decagonal approximants [1-2]. Here we applied it also to icosahedral quasicrystals. There are three related hexagonal phases &mu3, &mu5 and &mu7 in the vicinity of the stoichiometry i-Zn₆Mg₃RE. The unit cell parameters a are related as 3:5:7 (14.62, 23.50 and 33.56 ?, respectively) while the c axes are almost the same (8.71, 8.60 and 8.87 ?). All have space group P63/mmc.

These hexagonal phases are key crystalline phases for understanding the structure of i-ZnMgRE quasicrystal [3-5]. The distribution of the strong reflections in this series of approximants is very similar. We could relate the strong reflections from μ7 (solved by X-ray diffraction) to μ5 and μ3 and deduce their structures. Compared with the structure models of μ5 and μ3 obtained from X-ray diffraction and TEM, only one out of 23 (for μ5) and 11 (for μ3) unique atoms did not agree. This shows that our new approach is general and can be applied to various quasicrystals and approximants in different chemical systems for solving unknown structures from a known one.

[1] J. Christensen, P. Oleynikov, S. Hovmöler and X.D. Zou, Ferroelectrics, 305, 273, 2004
[2] X.D. Zou, J. Christensen, H. Zhang, P. Oleynikov, S. Hovmöller, A new approach for solving quasicrystal structures, 22nd European crystallographic meeting, 26-31 August, 2004, Budapest, Hungary.
[3] E. Abe, H. Takakura, A. Singh and A.P. Tsai, J. Alloys and Comp. 283, 169, 1999.
[4] K. Sugiyama, K. Yasuda, T. Ohsuna and K. Hiraga, Z. Acta Krist. 213, 537, 1998.
[5] K. Sugiyama, K. Yasuda, Y. Horikawa, T. Ohsuna and K. Hiraga, J. Alloys and Comp. 285, 172, 1999.

"The Evaluation of Phase Transformations Involving Icosahedral AlCuFe Quasicrystals"
Liming Zhang¹, Reinhard Lueck²
¹Ames Lab, Iowa State University, Ames, IA 50011 USA
²Max-Planck Institute for Metal Research

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
Phase equilibria in the vicinity of the AlCuFe icosahedral quasicrystals have been studied [1]. The icosahedral phase (i–phase) forms via a ternary peritectic reaction at 882 °C from liquid and two primary phases termed λ and β. At lower temperatures, The i–phase is reaction partner with liquid, λ, cubic β and tetragonal ω in two different transition reactions (U–type). Solid–state phase transformations involving the i–phase have been investigated recently. A quasi–binary eutectoid decomposition was found to link the i– and φ–Al₁₀Cu₁₀Fe₁ phase, which also leads to direct phase equilibria between the i– phase and another cubic phase with low Fe content (β). The crystal structure of the φ–phase was determined from powder diffraction data using routines for indexing DICVOL, intensity extraction and ab–initio structure determination EXPO and for Rietveld refinement FULPROF. A face–centered orthorhombic cell (a = 8.1530(3)Å, b = 14.1370(4)Å, c = 10.0736(4)Å, Vol = 1161.0(7), Z = 4, at room temperature) with a starting structural model of ξ₁–Al₃Cu₄ (space group Fmm2, Z = 4) approximates the observed pattern of powder samples quenched from 620 °C fairly well. The isothermal phase fields of the i–, φ– and β– phases were constructed based on metallographic investigations and structural analysis as well as on isopleths. These phase fields were found to have a common feature, which extend towards an orientated compositional direction referred to a constant valence electron concentration. It indicates the phase stability of these phases is strongly influenced by their electronic structures.

[1] L.M. Zhang, R. Lueck. Phase diagram of the Al– Cu– Fe quasicrystal–forming alloy system. Z. Metallkd. 94 (2003) 91, 98, 108, 341, 774.

"Understanding Czochralski Growth of Decagonal AlCoCu"
Götz Meisterernst¹, Liming Zhang¹, Peter Gille¹
¹Crystallography Section, Dept. of Earth and Environmental Sciences, LMU München, Germany

Presentation Type: Poster
Topic: Structural Evolution and Phase Stability

Abstract:
Contrary to all experience gained with quasicrystal growth in the Al-Co-Ni system, Czochralski growth of decagonal AlCoCu single crystals is affected by various features that make the preparation of large crystals much more difficult.

(i) The relevant part of the Al-Co-Cu ternary phase diagram has been known in more detail only recently [1]. In the solid-liquid equilibria used for the crystallization of the decagonal phase, the tie-lines are directed almost along Al isopleth lines whereas with AlCoNi there is a remarkable concentration shift with respect to the Al content only.
(ii) The stability region of the decagonal AlCoCu phase has a much stronger temperature dependence compared to the AlCoNi phase, leading sometimes to phase decomposition during cooling.
(iii) Primary nucleation of the AlCoCu phase at some ceramic tip is always followed by a break-down of supercooling which leads to a spontaneous formation of decagonal needles, i. e. the use of a seed seems to be imperative.
(iv) The use of AlCoNi seeds instead of native seeds has proved possible [2], but there has been some phase decomposition in the quaternary region that had been formed by interdiffusion.
(v) Crystal growth kinetics of decagonal AlCoCu seems to be much more anisotropic than in the Al-Co-Ni system. Growth in quasiperiodic directions is even slower, but increasing the supersaturation results in a sudden crystallization of a lot of needles.

Using extremely stable growth conditions, we have succeeded in growing cm-size single-grain AlCoCu quasicrystals. Structural perfection and other properties will be compared with results known from AlCoNi growth experiments.

[1] L. M. Zhang, P. Gille, J. Alloys & Compounds 370 (2004), 198.
[2] P. Gille, P. Dreier, R.-U. Barz, J. Alloys & Compounds 342 (2002), 7.

"New Insights in the Origin of the DOS Pseudogap and the Effective Negative Valence of Mn in i-AlPdMn - An Ab Initio Study of the Original and a Modified Quandt-Elser Model"
Eeuwe Zijlstra¹, Shyamal Bose¹
¹Brock University, St. Catharines, Ontario L2S 3A1 Canada

Presentation Type: Poster
Topic: Electronic and Magnetic Properties

Abstract:
Empirically the occurrence of narrow regions of stability in the phase diagrams of icosahedral Al–based quasicrystals can be explained by the so–called Hume–Rothery rules. Theoretically this explanation corresponds to a picture where a pseudogap in the electronic density of states (DOS) stabilizes the quasiperiodic structure. The negative valences of Mn, Fe, and Re atoms explain why certain substitutions preserve the quasiperiodic order, while other substitutions lead away from the above–mentioned narrow regions of quasiperiodic stability. Although this picture appears to work quite well in general, details, such as, which interactions cause the pseudogap in the DOS (i.e., Fermi surface–Jones zone interactions or hybridization, or a combination of both) and what exactly causes the (effective) negative valence of the Mn, Re, and Fe atoms, are still debated. In this talk a review of the main arguments used in this debate will be presented. New insights are obtained by a detailed study of the effects on the electronic DOS and the electronic charge density of various atomic substitutions in a modified version of the Quandt–Elser model of i–AlPdMn. To obtain our results we used the full–potential linear augmented plane wave (FLAPW) method, which is one of the most accurate ab initio schemes currently available. Our results suggest that the pseudogap in i–AlPdMn is caused by hybridization. The negative valence seems to originate from a charge transfer. Our results for i–AlPdMn will be compared to results for the i–AlPdRe, i–AlCuFe, and i–CdYb quasicrystals, which we have also studied.

"A New Approach for Solving Quasicrystal Structures from their Approximants"
Xiaodong Zou¹, Peter Oleynikov¹, Sven Hovmöller¹
¹Stockholm University, 10691 Stockholm Sweden

Presentation Type: Poster
Topic: Structure

Abstract:
Since the first report of quasicrystals, great efforts have been made to find out the nature of quasicrystals. Different approaches are made to determine the structures of quasicrystals. One approach of solving quasicrystal structures is from their approximants, since it was found by HREM that quasicrystals and their approximants have very similar building clusters. In many cases, a series of approximants is related to a quasicrystal. The unit cell dimensions of different approximants within the same series increase by inflations. At the end of such a series, when the unit cell dimensions become infinite, a quasicrystal is found. Based on this, a few structure models have been proposed for different quasicrystals. However, there is no general procedure for solving quasicrystal structures from their approximants. Recently we have derived a new general approach for solving structures of one approximant from another approximant by scaling. We found that not only the intensity distributions of the strong reflections are similar within a series of approximants, the structure factor phases of those reflections are also similar, when a common origin is used. The similarity of the intensity distributions indicates that the approximants are built from similar clusters, while the phase information tells us how the clusters are arranged in the structure. Here we apply this simple approach to solve the basic structure of the d-AlCoNi quasicrystal. We used several known structures which are related to the d-AlCoNi quasicrystal and determined by X-ray diffraction [1-3] as starting points. The structure model obtained from our approach is compared with the structure model refined by X-ray diffraction[4]. Some general rules of the cluster arrangement are found. This method is general and can be applied to all types of quasicrystals, including decagonal and icosahedral quasicrystals.

[1] C. Freiburg, B. Grushko, W. Wittenberg, and W. Reichert, Mater. Sci. Forum. 228-231(1996) 583.
[2] S. Katrych and W. Steurer, private communication.
[3] P. Oleynikov, J. Christensen, X.D. Zou, S. Hovmöller, M. Döbling, B. Grushko, unpublished.
[4] W. Steurer, Z. Kristallogr. 219 (2004) 391.