Vibrational spectra of vitreous SiO2 and vitreous GeO2 from first principles

Abstract

Using a density-functional approach, we calculate the principal vibrational spectra of vitreousSiO2 andvitreous GeO2 and discuss their analogies and differences. For both glasses, we generate model structuresconsisting of a random network of corner-sharing tetrahedra and differing only by theirpacking density. The comparison between calculated and measured neutron structurefactors supports the validity of our model structures. Our investigation then extends to thevibrational properties, including the inelastic-neutron, infrared, and Raman spectra. Forthese spectra, good agreement with experiment is also found. Our results supportthe picture that silica and germania are constituted by a continuous randomnetwork of corner-sharing tetrahedra. In particular, the good agreement withexperiment for the Raman spectra supports the average intertetrahedral angles of148° and135° found in ourmodels of vitreous SiO2 and vitreous GeO2, respectively. The concentration of small ring structures in these glasses is also discussed.