Abstract

Using the CaIrO3-type structure model (space group Cmcm), lattice parameters and atomic positions of the MgGeO3 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 CaIrO3-type structure model for the pPv phase in MgGeO3 exceeding 1 Mbar and reliability of structural parameters obtained by Rietveld refinements approaching 1 Mbar. The MgGeO3 pPv phase exhibits strong anisotropy in axial compressibility, with the b-axis being most compressible. The polyhedral bulk modulus for the GeO6 octahedron is 1.9× larger than that for the MgO8 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 MgGeO3 pPv with those for MgSiO3, NaMgF3, and CaIrO3 pPv show that MgSiO3 pPv has more similarity with NaMgF3 and MgGeO3 pPv than with CaIrO3 pPv in such parameters as degree of octahedral distortion, implying that both NaMgF3 and MgGeO3 pPv are better analogs to MgSiO3 pPv than CaIrO3 pPv.

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