Role of disorder in incorporation energies of oxygen atoms in amorphous silica

Abstract

We investigate the role of static disorder on defect energetics on examples of interstitial oxygen atoms in amorphous $(a)\ensuremath{-}{\mathrm{SiO}}_{2}.$ We generate representative amorphous structures using molecular dynamics with empirical potentials and refine them using the periodic plane-wave density-functional method (DFT). We calculate the DFT distribution of incorporation energies for 96 peroxy-linkage (PL) configurations in a periodic model of $a\ensuremath{-}{\mathrm{SiO}}_{2}.$ The calculations show a big site-to-site variation of incorporation energies. We partition the oxygen atom incorporation energy into contributions from a small local cluster around the defect and from the rest of the amorphous network. The striking result is that the incorporation of a defect can create as well as release the strain energy in the embedding network. The variation of the PL incorporation energy is dominated by the contribution from the surrounding amorphous network, with the distortion of the local geometry of the defect contributing only about one third of the total variation. The two contributions are statistically independent. Our results provide an analysis of the distribution of defect incorporation energies in $a\ensuremath{-}{\mathrm{SiO}}_{2}$ and emphasize the importance of disorder and statistical approaches, which cannot be achieved in crystalline and cluster models of amorphous structure. Additionally, since the defect energies can be so strongly dependent on the longer-range strain fields, amorphous samples prepared differently and hence having different distributions of strain may perform differently in applications.