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

• Published in: Proceedings of the National Academy of Sciences - PNAS 2011-01, Vol.108 (4), p.1252-1255
• Main Authors: Tsuchiya, Taku, Tsuchiya, Jun
• Format: Article
• Language: English
• Subjects: Algorithms; Calcium Compounds - chemistry; Crystallization; Dioxides; Earth; Enthalpy; Extrasolar planets; Gas giants; Magnesium Compounds - chemistry; Models, Chemical; Models, Molecular; Phase diagrams; Phase Transition; Physical Sciences; Polyhedrons; Pressure; Pyrites; Silicates - chemistry; Silicon Dioxide - chemistry; Super Earths; Temperature; Thermodynamics; Volume
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1013594108

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.