  (Mg, Fe)SiO$_3$ post-perovskite is the highest pressure silicate mineral
phase in the Earth's interior. The extreme pressure and temperature conditions
inside large extrasolar planets will likely lead to phase transitions beyond
pPv. In this work we have explored the high-pressure phase relations in
Mg$_2$SiO$_4$ using computations based on density functional theory. We find
that a partially disordered I-42d type structure would be stable in the
interiors of these super-Earth planets. The discovery of a structure where two
very dissimilar cations, Mg$^{2+}$ and Si$^{4+}$ occupy the same
crystallographic site opens up a domain of interesting crystal chemistry and
provides a foundation for other silicates and oxides with mixed occupancy. We
have explored the mechanism of the phase transition from the ordered ground
state and the effect of the disordering on electronic properties of the
silicate phase.
