  Mg2GeO4 is an analogue for the ultra-high pressure behavior of Mg2SiO4, so we
have investigated magnesium germanate to 275 GPa and over 2000 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 a novel phase with disordered Mg and Ge, in which
germanium adopts eight-fold coordination with oxygen: the cubic Th3P4- type
structure. Simulations using the special quasirandom structure (SQS) method
suggest partial order in the tetragonal I-42d structure, indistinguishable from
I-43d Th3P4 in our experiments. These structures have not been reported before
in any oxide. If applicable to silicates, the formation of this highly
coordinated and intrinsically disordered phase would have important
implications for the interior mineralogy of large, rocky extrasolar planets.
