Intrinsic and Extrinsic Point Defects in a-SiO2

Point defects in amorphous silicon dioxide (a-SiO₂) can be considered as any local deviations from the “perfect” glass structure, which for the sake of discussion will be taken to be a continuous random network of Si (0_½)₄ tetrahedra joined at the corners. Thus, for example, a vacancy-interstitial pair defined on such an otherwise perfect network would be the analogue of a Frenkel pair in crystalline solid. Since in our ideal glass each silicon has four bonds to neighboring atoms while each oxygen has but two, Frenkel pairs involving oxygen atoms (fig. 1a) would seem more probable than those involving silicons. Other possibilities include over or under-coordinated atoms, substitutional or interstitial impurities, or bonds between like atoms. Note in fig. 1a that a neutral oxygen vacancy is tantamount to a Si-Si homo bond.