Prediction of a hexagonal SiO_2 phase affecting stabilities of MgSiO_3 and CaSiO_3 at multimegabar pressures

摘要

Ultrahigh-pressure phase relationship of SiO_2 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 Fe_2P-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 Fe_2P-type structure among all dioxide compounds. The crystal structure consists of closely packed, fairly regular SiO_9 tricapped trigonal prisms with a significantly compact lattice. Additional investigation fuerther elucidates large effects of this phase change in SiO_2 on the stability of MgSiO_3 and CaSiO_3 at multimegabar pressures. A postperovskite phase of MgSiO_3 breaks down at 1.04 TPa along an assumed adiabat of super-Earths and yields Fe_2P-type SiO_2 and CsCI (B2)-type MgO. CaSiO_3 perovskite, on the other hand, directly dissociates into SiO_2 and metallic CaO, skipping a postperovskite polymorph. Predicted ultrahigh-pressure and temperature phase diagrams of SiO_2, MgSiO_3, and CaSiO_3 indicate that the Fe_2P-type SiO_2 could be one of the dominant components in the deep mantles of terrestrial exoplanets and the cores of gas giants.