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September 7, 2005

NaMgF₃ perovskite under pressure: Octahedral tilting evolution and phase transition

H. Liu¹, J. Chen², J. Hu¹, C.D. Martin², D.J. Weidner², D. Häusermann¹, and H.-k. Mao^1
1High Pressure Collaborative Access Team, Carnegie Institution of Washington, Advanced Photon Source, Argonne National Laboratory, Argonne, IL; ²Mineral Physics Institute, Stony Brook University, Stony Brook, NY

A high-pressure test of an analog for a common deep-Earth mineral may allow researchers to estimate the physical properties of materials in the planet's lower mantle. We induced pressures greater than 16 gigapascals (GPa), more than 160,000 times the normal atmospheric pressure on the Earth's surface, on a perovskite material with a similar makeup as the ubiquitous deep-mantle perovskite MgSiO₃ and analyzed the change in its physical makeup under the stress. We observed slight changes to its chemical bonds beginning at 6 GPa and a compression that destroyed the perovskite crystal structure at pressures approaching 20 GPa.

Jingzhu Hu (left) and Haozhe Liu.

Jiuhua Chen

Geophysical interest in the perovskite family of materials dates from the 1960s when geologist A.E. Ringwood proposed that the Earth's lower mantle is dominated by iron-bearing MgSiO₃ perovskite. A fundamental understanding of the behavior of selected representatives of the perovskite family of structures to high-pressure/high-temperature conditions is valuable for establishing possible behaviors of this family in general. We have chosen to study NaMgF₃ perovskite (neighborite) as it has gained significant attention as an analogue material of MgSiO₃ perovskite since O'Keeffe et al pointed out their structural similarity in 1970s. Neighborite is isoelectronic with MgSiO₃, and they are isostructural, possessing the same type of distortion from the ideal cubic perovskite structure to crystallize in space group Pbnm.

Figure 1. Crystalline structure of NaMgF3 under pressure of 10.1GPa.

We used a diamond anvil cell high-pressure device and synchrotron x-ray micro-diffraction to in situ study the crystalline change of the sample under high-pressure conditions. A series of experiments were carried out using various pressure-transmitting medium, the focusing beam size and grain size of the sample (at NSLS beamline X17C and the High Pressure Collaborative Access Team facility at the Advanced Photon Source) to optimize the quality of our diffraction data. The structural evolution of NaMgF₃ under high pressure, therefore, could be analyzed from the Rietveld refinement of the high-quality x-ray diffraction patterns. The atomic positions could be obtained under high pressure; for example, Figure 1 demonstrates the crystalline structure of neighborite under 10.1 GPa when helium was used as the pressure medium.

The centrosymmetrically distorted orthorhombic perovskite with space group Pbnm is distorted by two independent octahedral tilting angles Θ and Φ, where Θ is an anti-phase tilt and Φ is an in-phase tilt. It can also be conceived as the tilting Φ about the threefold <111>pc axes of the regular octahedra (see Figure 2 insert). The octahedral tilting of the NaMgF₃ perovskite could be quantitatively derived from the cell parameters (macro-approach) as well as from the positional parameters of atoms (micro-approach), as shown in Figure 2. The overall trends of the octahedral tilting angles, i.e. increasing with increasing pressure, are similar for both the macro and micro approaches. The volumetric compression was dominated by the shortening of the octahedral Mg-F bond at the beginning of compression below 6 GPa. In the 6-12 GPa pressure range, the contribution from the octahedral tilting matches that of the bond length compression. This is followed by an increasing contribution from the octahedral tilting above 12 GPa.

Figure 2. Pressure evolution of the octahedral tilting angles of NaMgF3 perovskite as derived from lattice parameters (macro-approach), and atomic positions (micro-approach) plotted as solid and open symbols, respectively. Insertion shows the octahedral tilting angles referred to an ideal cubic Pm3m perovskite.

The octahedral tilting, increasing with pressure, finally destroys the perovskite structure. An experiment was carried out using silicone oil as the pressure medium, which could generate more shear stress to accelerate the phase transition. We observed a phase transition at about 19.4 GPa, and the patterns above this pressure can no longer be indexed by the Pbnm perovskite structure, but can be indexed with the layering-type post-perovskite structural model with space group Cmcm. This distinct material created under these extremely high-pressure conditions should be further studied to establish its properties. Such research could help scientists better understand the mechanisms of perovskite, including the temperatures and pressures required for it to stabilize near the Earth's core.

BEAMLINE
X17C

FUNDING
National Science Foundation
U.S. Department of Energy
U.S. Department of Defense - Tank-automotive and Armaments Command

PUBLICATIONS
Liu, H.-Z., J. Chen, J. Hu, C. D. Martin, D. J. Weidner, D. Häusermann, and H.-K. Mao, Octahedral tilting evolution and phase transition in orthorhombic NaMgF₃ perovskite under pressure, Geophys. Res. Lett., 32, L04304, (2005).

FOR MORE INFORMATION
Dr. Haozhe Liu
HPCAT, Bldg. 434E
Advanced Photon Source
Argonne National Laboratory
Argonne, IL
Email: hliu@hpcat.aps.anl.gov