High-pressure order-disorder transition in Mg2SiO4: Implications for super-Earth mineralogy

$(\mathrm{Mg},\mathrm{Fe})\mathrm{Si}{\mathrm{O}}_{3}$ 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 post-perovskite. In this work, we have explored the high-pressure phase relations in ${\mathrm{Mg}}_{2}\mathrm{Si}{\mathrm{O}}_{4}$ using computations based on density functional theory. We find that a partially disordered $I\overline{4}2d$-type structure would be stable under the conditions expected for the interiors of super-Earth planets. We have explored the mechanism of the phase transition from the ordered ground state and the effect of disordering on the electronic properties of the silicate phase. The discovery of a structure where two very dissimilar cations, ${\mathrm{Mg}}^{2+}$ and ${\mathrm{Si}}^{4+}$, 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.