The distributions of structural parameters, formation energies and defect levels of neutral O vacancies in amorphous silica were calculated. All O sites in the amorphous structure were considered to be possible candidates for vacancy formation. The electronic structure of neutral O vacancy configurations at 75 selected sites were studied by using an embedded cluster method. The formation energies correlated with the Si-Si distance in relaxed vacancies, and with vacancy relaxation energies. Classical molecular dynamics calculations were carried out in order to test the possible effect of high-temperature annealing on the predictions of static calculations. It was found that it affected the high formation-energy neutral O-vacancy configurations. Using classical atomistic simulations, the structure and formation energies of neutral O vacancies in 220 different sites were then calculated. The optical absorption spectrum of neutral O vacancies was then calculated for the 23 low formation energy neutral O-vacancy configurations obtained using classical calculations. It was found that σ → σ* transitions determined the low energy tail of the optical absorption spectrum and were strongly affected by the Si-Si distance in the vacancies. Therefore the red part of the neutral O vacancy optical absorption spectrum should depend strongly on sample preparation and any further treatment which could create neutral O vacancies. The results demonstrated how a statistical approach based on the embedded cluster method could be effectively applied to studying the properties of defects in amorphous materials.

Correlation Between the Atomic Structure, Formation Energies and Optical Absorption of Neutral Oxygen Vacancies in Amorphous Silica. S.Mukhopadhyay, P.V.Sushko, A.M.Stoneham, A.L.Shluger: Physical Review B, 2005, 71[23], 235204 (9pp)