Ab initio studies of high-pressure structural transformations in silica

Three successive pressure-induced structural transformations of stishovite (rutile-structure SiO₂) to denser phases are predicted by the first-principles pseudopotential method within the local-density approximation. The transition from the rutile to the orthorhombic CaCl₂ phase occurs at 47 GPa, the transition from the CaCl₂ to the Pnc₂ structure at 98 GPa, and finally the Pnc₂ 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 B1g 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 CaCl₂ phases are determined as a function of pressure. The results are in excellent agreement with available experimental data.