The mechanism for the pressure-induced transformation of cristobalite to stishovite and post-stishovite phases has been obtained from constant pressure ab initio molecular dynamics simulations. The cristobalite to stishovite transformation is found to be a two step process where ${\mathrm{SiO}}_4$ tetrahedra first rotate followed by a lattice distortion to yield the six-coordinated stishovite structure. Further compression of stishovite yields the ${\mathrm{CaCl}}_2$ structure and is followed by another six-coordinated structure with symmetry ${P2\mathrm{}}_1/n$ (at 11 Mbars) which remains stable to a pressure of about 14 Mbars and then transforms into a nine-coordinated ${P2\mathrm{}}_1/m$ structure.

• Received 7 December 2000

DOI:https://doi.org/10.1103/PhysRevB.63.104106