New pressure-induced structural transformations in silica obtained by computer simulation

Abstract

TO reproduce and predict crystal structures from first principles has been a longstanding problem in solid-state physics. We have recently shown¹ that a first-principles many-body calculation for clusters can be used to extract effective pairwise interatomic potentials, which were then used in a molecular dynamics study of the stability of crystalline silica (SiO₂). Here we use this approach in a molecular dynamics study of pressure-induced structural transitions at room temperature for various polymorphs of silica. We predict new structural transitions for low-quartz, low-cristobalite and coesite, in which some of the new phases, appearing without the occurrence of diffusion, comprise mixed arrays of fourfold and sixfold Si–O coordinations. Stishovite, the densest known polymorph of silica, persists up to 250 GPa, with deformation to the CaCl₂ structure. Although a recent theoretical calculation predicts a possible polymorph denser than stishovite, which may be of importance in the Earth's interior², this phase is not obtained by (simulated) compression of stishovite at room temperature, presumably because of the potential barrier between the two structures.