PERIODIC HARTREE-FOCK STUDY OF MINERALS - HEXACOORDINATED SIO2 AND GEO2 POLYMORPHS

The structure and stability of the rutile structure of SiO2 and GeO2 a nd of three other putative hexacoordinated polymorphs, namely the anat ase, pyrite and alpha-PbO2 structures, have been investigated by the P eriodic Hartree-Fock method. A universal basis set allowing accurate c alculations on both silica and magnesiosilicates has been designed for this purpose. The optimized structural parameters and equation-of-sta te coefficients of these four polymorphs are reported. The calculated values for the rutile structure are in good agreement with the availab le experimental data. It is shown from the calculated equations of sta te that, for SiO2, only the stishovite phase is stable in the 0-200 GP a pressure range, whereas, for GeO2, the phases with Pbcn and Pa3BAR s ymmetry become stable above 60 GPa.