Rietveld structure refinement of MgGeO[subscript 3] post-perovskite phase to 1Mbar

Journal Article · Thu Aug 07 04:00:00 EDT 2008 · Am. Mineral.

DOI:https://doi.org/10.2138/am.2008.2691· OSTI ID:1006717

Kubo, A; Kiefer, B; Shim, S -H; Shen, G; Prakapenka, V B; Duffy, T S ^[1]

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1. Princeton

Using the CaIrO{sub 3}-type structure model (space group Cmcm), lattice parameters and atomic positions of the MgGeO{sub 3} post-perovskite (pPv) phase were determined based on Rietveld refinements at 78--109 GPa and first-principles calculations based on density functional theory. The reproducibility of structural parameters obtained for different samples, consistency with theoretical calculations, and good agreement with expected bond lengths based on structurally similar materials all provide evidence for both validity of CaIrO{sub 3}-type structure model for the pPv phase in MgGeO{sub 3} exceeding 1 Mbar and reliability of structural parameters obtained by Rietveld refinements approaching 1 Mbar. The MgGeO{sub 3} pPv phase exhibits strong anisotropy in axial compressibility, with the b-axis being most compressible. The polyhedral bulk modulus for the GeO{sub 6} octahedron is 1.9x larger than that for the MgO{sub 8} hendecahedron. Examination of neighboring O-O distances shows that the O-O distance aligned along the a direction is one of the longest and that aligned along c is one of the shortest, and these may be related to the lower compressibility along c compared with a. Comparison of structural features of MgGeO{sub 3} pPv with those for MgSiO{sub 3}, NaMgF{sub 3}, and CaIrO{sub 3} pPv show that MgSiO{sub 3} pPv has more similarity with NaMgF{sub 3} and MgGeO{sub 3} pPv than with CaIrO{sub 3} pPv in such parameters as degree of octahedral distortion, implying that both NaMgF{sub 3} and MgGeO{sub 3} pPv are better analogs to MgSiO{sub 3} pPv than CaIrO{sub 3} pPv.

• Research Organization: Advanced Photon Source (APS), Argonne National Laboratory (ANL), Argonne, IL (US)
• Sponsoring Organization: USDOE
• OSTI ID: 1006717
• Journal Information: Am. Mineral., Journal Name: Am. Mineral. Journal Issue: (5-6) ; 07, 2008 Vol. 93; ISSN AMMIAY; ISSN 0003-004X
• Country of Publication: United States
• Language: ENGLISH

Related Subjects

36 MATERIALS SCIENCE
58 GEOSCIENCES
ANISOTROPY
BOND LENGTHS
COMPRESSIBILITY
DENSITY FUNCTIONAL METHOD
GERMANIUM OXIDES
LATTICE PARAMETERS
MAGNESIUM OXIDES
PEROVSKITES
PRESSURE RANGE GIGA PA
X-RAY DIFFRACTION