Ab initio molecular dynamics study of the pressure-induced phase transformations in cristobalite

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 SiO₄ tetrahedra first rotate followed by a lattice distortion to yield the six-coordinated stishovite structure. Further compression of stishovite yields the CaCl₂ structure and is followed by another six-coordinated structure with symmetry P2₁/n (at 11 Mbars) which remains stable to a pressure of about 14 Mbars and then transforms into a nine-coordinated P2₁/m structure.