The high-pressure phase diagram of $\mathrm{Si}{\mathrm{O}}_2$ has been calculated using density-functional perturbation theory. We find that phase transitions of silica do not correspond to any observed seismic discontinuities within the Earth. We analyze the breakdown of close packing occurring in $\mathrm{Si}{\mathrm{O}}_2$ above $200\mathrm{GPa}$ and conclude that it is due to asphericity and deformability of ions, which can generally lead to the formation on non-close-packed structures at high pressure. Our calculations indicate that pyrite-structured $\mathrm{Si}{\mathrm{O}}_2$, stable above $200\mathrm{GPa}$, has no soft modes down to ambient pressure and could, therefore, be quenchable. Above $750\mathrm{GPa}$ a cotunnite-type phase is stable.

• Received 11 October 2004

DOI:https://doi.org/10.1103/PhysRevB.71.064104