Quantum mechanical ab initio study of mixed SiO2-GeO2 crystals as reference models for Ge-doped silica glasses

In the present work. we have applied first principles quantum mechanical techniques to the study of periodic models of four silica polymorphs containing different proportion,, of germanium as a Substitutional species: alpha-quartz, beta-quartz, alpha-cristobalite, and stishovite (rutile) except for beta-quartz, which is considered for the sake of completeness, these phases exist both in the pure silicon and Pure germanium forms. Computations have been performed using both the Hartree-Fock (HF) approximation and the gradient corrected density functional PWGGA technique. A triple-zeta plus polarization basis set has been used. All geometries have been fully optimized at the HF level. Equilibrium structure, stability. and electronic properties of the mixed phases are analyzed with reference to the pure systems. It is shown that the two species are completely miscible for all crystalline structures in all proportions. The properties of the mixed crystals vary smoothly with the impurity content. No localized defect states associated with substitutional impurities are found either in the occupied or the virtual manifold. The relevance of the present results for simulation studies of germanium-containing silica glasses is discussed.