Structural stability of silica at high pressures and temperatures

The high-pressure phase diagram Of SiO2 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 SiO2 above 200 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 SiO2, stable above 200 GPa, has no soft modes down to ambient pressure and could, therefore, be quenchable. Above 750 GPa a cotunnite-type phase is stable.