Correlation between the atomic structure, formation energies, and optical absorption of neutral oxygen vacancies in amorphous silica

We have calculated the distributions of structural parameters, formation energies, and defect levels of neutral oxygen vacancies (NOV) in amorphous silica (a-SiO2). All oxygen sites in the amorphous structure were considered as possible candidates for vacancy formation in these calculations. The electronic structure of NOV configurations at 75 selected sites were studied using an embedded cluster method. The formation energies correlate with the Si-Si distance in relaxed vacancies and with vacancy relaxation energies. We carried out classical molecular dynamics calculations to test the possible effect of high temperature annealing on predictions from static calculations and found that it affects the high formation energy NOV configurations. Using classical atomistic simulations we then calculated the structure and formation energies of NOV in 220 different sites. For the 23 low formation energy NOV configurations obtained in classical calculations we calculated the optical absorption spectrum of NOV. We found that the sigma ->sigma* transitions determine the low energy tail of the optical absorption spectrum and are strongly affected by the Si-Si distance in the vacancies. Therefore the red part of the NOV optical absorption spectrum should depend strongly on sample preparation and any further treatment which can create neutral oxygen vacancies. The results demonstrate how a statistical approach based on the embedded cluster method can be effectively applied to studying the properties of defects in amorphous materials.