Local density approximation and generalized gradient approximation calculations for oxygen and silicon vacancies in silica

Neutral oxygen and silicon vacancies' energies in silica polytypes (alpha-quartz, beta-cristobalite, and stishovite) have been studied using the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange correlation. While the energies of formation of unrelaxed oxygen vacancies are remarkably constant (9.9 +/- 0.1 eV) in the three studied systems in GGA, the relaxation behavior is quite different: A strong Si-Si bond is formed in alpha-quartz, a weak one in beta-cristobalite, while no bond at all is formed in stishovite. In alpha-quartz, peroxyl bridges are formed as the consequences of the relaxation of silicon vacancies. Their energy of formation from O-2 (gas) is low (about 2 eV). (C) 2000 American Institute of Physics. [S0021- 9606(00)70721-5].