Ultrahigh-pressure Disordered Eight-coordinated Phase of MgGeO: Analogue for Super-Earth Mantles

MgGeO is important as an analog for the ultrahigh-pressure behavior of MgSiO, a major component of planetary interiors. In this study, we have investigated magnesium germanate to 275 GPa and over 2,000 K using a laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction and density functional theory (DFT) computations. The experimental results are consistent with the formation of a phase with disordered Mg and Ge, in which germanium adopts eightfold coordination with oxygen: the cubic, ThP-type structure. DFT computations suggest partial Mg-Ge order, resulting in a tetragonal [Formula: see text] structure indistinguishable from [Formula: see text] ThP in our experiments. If applicable to silicates, the formation of this highly coordinated and intrinsically disordered phase may have important implications for the interior mineralogy of large, rocky extrasolar planets.