On the nature of native defects in high OH-content silica glasses:: A first-principles study

First-principles simulations based on the density functional theory are used in order to generate silica glasses containing 3.84wt% water molecules employing different quench protocols. Using the Kohn-Sham density of states we find localized states in the band gap that can be associated with doubly occupied Si-O dangling bond (DB) which are negatively charged and are compensated by positively charged threefold coordinated oxygens. The position of these states above the O 2p valence band depends on the local environment of the dangling bonds, in particular on the presence of other defects in their neighborhood, and on the hydrogen bond length. These native defects, which could exist in optical fibers for instance, are compatible with the optical absorption and photoluminescence bands observed in amorphous silica and their dependence in the OH content. If present, these pre-existing defects would play a significant role as precursors in the laser-induced defect formation process. Copyright (c) EPLA, 2008.