Ultrahigh-pressure disordered eight-coordinated phase of Mg₂GeO: Analogue for super-Earth mantles

Mg₂GeO₄ is important as an analog for the ultrahigh-pressure behavior of Mg₂SiO₄, 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, Th₃P₄-type structure. DFT computations suggest partial Mg-Ge order, resulting in a tetragonal I 4 ¯ 2 d structure indistinguishable from I 4 ¯ 3 d Th₃P₄ 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.