Mixed Coordination Silica at Megabar Pressure

Silica (${\mathrm{SiO}}_{2}$), as a raw material of silicon, glass, ceramics, abrasive, and refractory substances, etc., is of significant importance in industrial applications and fundamental research such as electronics and planetary science. Here, using a crystal structure searching method and first-principles calculations, we predicted that a ground state crystalline phase of silica with $R\overline{3}$ symmetry is stable at around 645--890 GPa, which contains six-, eight-, and nine-coordinated silicon atoms and results in an average coordination number of eight. This mixed-coordination silica fills in the density, electronic band gap, and coordination number gaps between the previously known sixfold pyrite-type and ninefold ${\mathrm{Fe}}_{2}\mathrm{P}$-type phases, and may appear in the core or mantle of super-Earth exoplanets, or even the solar giant planets such as the Neptune. In addition, we also found that some silicon superoxides, Cmcm ${\mathrm{SiO}}_{3}$ and Ccce ${\mathrm{SiO}}_{6}$, are stable in this pressure range and may appear in an oxygen-rich environment. Our finding enriches the high-pressure phase diagram of silicon oxides and improves understanding of the interior structure of giant planets in our solar system.