STRUCTURAL STUDIES OF PEROVSKITE FERROELECTRIC RELAXORS

Daniel Phelan, Peter Gehring, Qing-Zhen Huang, Zuo-Guang Ye, Chris Stock, Guangyong Xu, Brahim Dkhil, and Pascale Gemeiner

            Solid solutions of the conventional ferroelectric PbTiO₃ and the perovskite relaxors Pb(Mg_1/3Nb_2/3)O₃ and Pb(Zn_1/3Nb_2/3)O₃, known respectively as PMN-xPT and PZN-xPT, have both long-range and locally-ordered structures that vary strongly with composition, temperature, and even applied electric field.  These systems exhibit an intriguing morphotropic phase boundary that separates a Ti-poor monoclinic phase from a Ti-rich tetragonal phase, and it is in the vicinity of this boundary that record-setting piezo-electric coefficients are observed.  This has led to a widespread use of these materials as state-of-the-art components in many technological applications.  In order to develop a full understanding of the exceptional piezoelectric response, it is necessary to obtain a clear picture of the evolution of the true crystalline and local atomic structures. 

It has been shown that these systems exhibit an anomalous ‘skin effect’ in which the presumably strained outer surface layer, or ‘skin,’ differs considerably in structure from the bulk ‘innards’ of the crystal.  This means that structural probes with a limited penetration depth, such as x-rays, may not reveal true bulk structural information.  Realizing that this ‘skin effect’ may account for discrepancies between various studies of the structures of PMN-xPT and PZN-xPT, we have undertaken a systematic neutron diffraction study of a series of powder samples of PMN-xPT grown under different conditions to determine the effect of oxygen annealing on the crystal structure and the ‘skin effect.’  We compare the results of these powder diffraction studies to x-ray diffraction and single crystal neutron diffraction measurements.

For low values of x, both PMN-xPT and PZN-xPT exhibit characteristic diffuse scattering patterns that result from randomly-oriented, nanoscale regions of polarization (aka ‘polar nanoregions’), within which there are atomic displacements from the positions of the average long-range crystal structure.  We have studied the structural phase transitions and the diffuse scattering patterns in PZN-xPT, with x=9% and 12%, by single crystal neutron diffraction in an effort to relate the appearance of polar nanoregions to structural phase transitions in the crystals and to determine how the polar nanoregions change as the morphotropic phase boundary is crossed.

Author Information:

Daniel Phelan
Mentor’s name: Peter Gehring

NIST Center for Neutron Research (610)

National Institute of Standards and Technology
100 Bureau Drive Stop 6102

Gaithersburg, MD 20899-6102

Room:  A113

email: daniel.phelan@nist.gov
Ph:  301-975-4805

Fax : 301-921-9847

Category: Physics

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