AB-INITIO STUDIES OF HIGH-PRESSURE STRUCTURAL TRANSFORMATIONS IN SILICA

Three successive pressure-induced structural transformations of stisho vite (rutile-structure SiO2) to denser phases are predicted by the fir st-principles pseudopotential method within the local-density approxim ation. The transition from the rutile to the orthorhombic CaCl2 phase occurs at 47 GPa, the transition from the CaCl2 to the Pnc(2) structur e at 98 GPa, and finally the Pnc(2) phase transforms to the pyrite pha se at 226 GPa. It is clearly illustrated that the first transition is associated with an elastic instability which arises from the strong co upling between elastic constants and the softening rutile B-1g mode. T he fully optimized structures of the four polymorphs of silica are obt ained as a function of pressure. In addition, all zone-center transver se optic modes of the rutile and CaCl2 phases are determined as a func tion of pressure. The results are in excellent agreement with availabl e experimental data.