Periodic Hartree-Fock study of minerals: Hexacoordinated SiO2 and GeO2 polymorphs

Abstract

The structure and stability of the rutile structure of SiO 2 and GeO 2 and of three other putative hexacoordinated polymorphs, namely the anatase, pyrite and α-PbO 2 structures, have been investigated by the Periodic Hartree-Fock method. A universal basis set allowing accurate calculations on both silica and magnesiosilicates has been designed for this purpose. The optimized structural parameters and equation-of-state coefficients of these four polymorphs are reported. The calculated values for the rutile structure are in good agreement with the available experimental data. It is shown from the calculated equations of state that, for SiO 2 , only the stishovite phase is stable in the 0-200 GPa pressure range, whereas, for GeO 2 , the phases with Pbcn and Pa3 symmetry become stable above 60 GPa