Structure and properties of defects in amorphous silica: new insights from embedded cluster calculations

We briefly review some of the approaches which have been used to study the. distributions of defect properties in amorphous silica and focus mainly on the implementation of the embedded cluster method. We illustrate some of the results obtained using this method and discuss the remaining problems using the example of oxygen vacancy defects in amorphous SiO2. The neutral vacancies are characterized by a wide distribution of formation energies and structural parameters. Our modelling predicts the two major structural kinds of positively charged vacancies (E' centres): dimers and dangling bond centres. The local structure for both kinds of centres depends on the medium range structure of the surrounding amorphous network. We found that the majority of the dangling bond centres are unpuckered. The optical spectra and electron paramagnetic resonance parameters calculated for all defects are in good agreement with experimental data. The structural criteria which favour the formation of different kinds of centres in the original amorphous structure are formulated in terms of the average Si-O distance of oxygen ion with its two neighbouring silicon ions.