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

Abstract

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} pPvmore » 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.« less