Structure and stability of several high-pressure crystalline polymorphs of silica

The structure and stability of several proposed high-pressure crystalline polymorphs of silica have been studied with a first-principles pseudopotential total-energy method within the local-density approximation. The monoclinic I2/a phase predicted recently from molecular-dynamics calculations on highly compressed amorphous silica or α quartz [J.S. Tse, D.D. Klug, and Y. LePage, Phys. Rev. Lett. 69, 3647 (1992)] is shown to be energetically competitive with the well-known stishovite structure and is substantially lower in energy than the cubic fluorite Pa3¯ structure up to 170 GPa. The results are in full accordance with previous molecular-dynamics calculations thus confirming that the empirical potential proposed by van Beest et al. is applicable to the study of the structure and dynamics of high-pressure phases of silica.