Rietveld structure refinement of MgGeO₃ post-perovskite phase to 1 Mbar

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 CalrO₃-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 1.9x 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.