Defects in amorphous SiO2:: Valence alternation pair model

The structure of the intrinsic coordination defects in amorphous SiO2-namely, a nonbridging oxygen and a threefold-coordinated silicon-is studied using a modern ab initio method, aiming at elucidating their actual structure and possibly their contribution to mass transport. We show that the silicon part adopts a planar configuration and does not make any new bonds. Conversely the nonbridging oxygen develops spontaneously a new bond with an already fourfold-coordinated silicon, which becomes now fivefold coordinated. We discuss the energetics of this defect, including the energy gain due to the formation of the fivefold-coordinated silicon, as well as the possible contribution it could give to oxygen self-diffusion.