Defects and defect processes in silicon dioxide

Silicon dioxide has two outstanding characteristics: it is of great technological importance in several forms, and it involves physical phenomena of considerable scientific interest. In this paper we review several issues regarding defects and defect processes in SiO2. We consider issues associated with oxygen vacancies in different charge states in both alpha-quartz and silica glass, and ways that hydrogen can interact with vacancies. Interstitials are discussed, with particular focus on interstitial hydrogen. We reach two conclusions: first, that basic chemical notions of bonding seem to work well in describing processes in silicon dioxide, a relatively unconstrained material that can relax to yield chemically stable structures; and second, that considerable progress in understanding many of the questions that still surround defects in silica glass could be attained from a careful analysis of local structures in high-quality, many-atom structural models of silica.