Ultra-high pressure disordered eight-coordinated phase of Mg$_2$GeO$_4$: Analogue for super-Earth mantles.

High-pressure phases of Mg$_2$GeO$_4$ have been explored as an analogue system for the ultra-high pressure behavior of Mg$_2$SiO$_4$. Using a laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction, we have identified a novel phase in which germanium adopts eight-fold coordination with oxygen. The cubic Th3P4-type phase was synthesized upon heating Mg$_2$GeO$_4$ to ~1600 K at 162 GPa and it remained stable up to ~ 275 GPa and 2020 K. While the Th3P4-type phase is commonly observed in chalcogenides, it has not been reported before in any oxide composition. If applicable to silicates, the formation of this highly coordinated and intrinsically disordered phase or a closely related structure as suggested by theoretical calculations would have important implications for the interior mineralogy of large, rocky extrasolar planets.