Structural properties of SiO₂ in the stishovite structure

In addition to the familiar family of fourfold-coordinated polymorphs, silica exists in sixfold-coordinated structures, among which stishovite is the most important one. Here we predict the lattice parameters and structural properties of stishovite using two approaches: direct quantum-mechanical calculations and classical interatomic potentials. Our quantum-mechanical calculations are based on ‘‘soft’’ pseudopotentials constructed using the local-density approximation. For the interatomic potential calculations, we have used a recently developed two-body potential extracted from Hartree-Fock self-consistent calculations of the total energy of silica clusters. The results of these two types of calculations are compared and contrasted. Unlike similar comparisons for fourfold-coordinated polymorphs, we find that both approaches agree very well with experimental data. We attribute this difference to fewer internal degrees of freedom in stishovite. Angular, or many-body forces play a less significant role in this structure as contrasted to open structures such as quartz.