  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.
