Ab initio studies of high-pressure structural transformations in silica

Three successive pressure-induced structural transformations of stishovite (rutile-structure ${\mathrm{SiO}}_2$) to denser phases are predicted by the first-principles pseudopotential method within the local-density approximation. The transition from the rutile to the orthorhombic ${\mathrm{CaCl}}_2$ phase occurs at 47 GPa, the transition from the ${\mathrm{CaCl}}_2$ to the ${\mathrm{Pnc}}_2$ structure at 98 GPa, and finally the ${\mathrm{Pnc}}_2$ phase transforms to the pyrite phase at 226 GPa. It is clearly illustrated that the first transition is associated with an elastic instability which arises from the strong coupling between elastic constants and the softening rutile ${\mathrm{B}}_{1\mathrm{g}}$ mode. The fully optimized structures of the four polymorphs of silica are obtained as a function of pressure. In addition, all zone-center transverse optic modes of the rutile and ${\mathrm{CaCl}}_2$ phases are determined as a function of pressure. The results are in excellent agreement with available experimental data.