Prediction of a hexagonal SiO₂ phase affecting stabilities of MgSiO₃ and CaSiO₃ at multimegabar pressures

Ultrahigh-pressure phase relationship of SiO₂ silica in multimegabar pressure condition is still quite unclear. Here, we report a theoretical prediction on a previously uncharacterized stable structure of silica with an unexpected hexagonal Fe₂P-type form. This phase, more stable than the cotunnite-type structure, a previously postulated postpyrite phase, was discovered to stabilize at 640 GPa through a careful structure search by means of ab initio density functional computations over various structure models. This is the first evidential result of the pressure-induced phase transition to the Fe₂P-type structure among all dioxide compounds. The crystal structure consists of closely packed, fairly regular SiO₉ tricapped trigonal prisms with a significantly compact lattice. Additional investigation further elucidates large effects of this phase change in SiO₂ on the stability of MgSiO₃ and CaSiO₃ at multimegabar pressures. A postperovskite phase of MgSiO₃ breaks down at 1.04 TPa along an assumed adiabat of super-Earths and yields Fe₂P-type SiO₂ and CsCl (B2)-type MgO. CaSiO₃ perovskite, on the other hand, directly dissociates into SiO₂ and metallic CaO, skipping a postperovskite polymorph. Predicted ultrahigh-pressure and temperature phase diagrams of SiO₂, MgSiO₃, and CaSiO₃ indicate that the Fe₂P-type SiO₂ could be one of the dominant components in the deep mantles of terrestrial exoplanets and the cores of gas giants.