First-principles calculations, including electron correlation and cluster models, were made of the formation energies of a series of defects in the bulk oxide and in silica glasses which contained Ge impurities. The defects which were considered included O vacancies, =Si-Si=, =Si-O-Si=, =Si-O-O•, =Si-O•, =Si•, E' centers, =Si•+Si=, double O vacancies, =Si-Si-Si=, Frenkel pairs, =Si-Si-O-O-Si=,  et cetera. The corresponding analogues, in which Ge replaced network Si atoms, were also considered. It was found that the formation of a single O vacancy, defined as being the energy required to remove and take to infinity a neutral O atom, was 8.5eV. This was consistent with recent thermodynamic estimates. It was shown that, in the presence of Ge impurities, the formation of O-deficient centers occurred at lower energies.

G.Pacchioni, G.Ieranò: Physical Review B, 1997, 56[12], 7304-12. See also: Physical Review B, 1998, 57[2], 818-32