STRUCTURAL-PROPERTIES OF 9 SILICA POLYMORPHS

It is well known that silica exists in amorphous or glassy phases as well as in numerous crystalline polymorphs. We present a comprehensive overview of the structural and elastic properties of crystalline polymorphs of silica, including both fourfold- and higher-coordinated structures. With an ab initio approach, we have examined alpha-quartz, stishovite and the CaCl2 structure, and the fluorite and the Pa3BAR structures. These are representative of fourfold, sixfold, and eightfold cation coordination, respectively. Using a pairwise interatomic force-field model, we have examined these forms and have extended the work to include other polymorphs like beta-quartz, alpha- and beta-cristobalites, and beta-tridymite. We have determined the variation of lattice parameters and internal coordinates as a function of pressure for all these polymorphs and presented the equations of state for all these structures. We have compared our predictions with available experimental data. Our calculations can be used to obtain information about pressure-induced transformations between various polymorphs. For our quantum-mechanical calculations, we have used "soft" pseudopotentials constructed within the local-density approximation. The classical calculations are based on a two-body potential that has been recently developed with the use of self-consistent Hartree-Fock calculations.