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(B-2)-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. _ 1