Density functional theory investigation of native defects in SiO2:: Self-doping and contribution to ionic conductivity -: art. no. 174101

We present here a theoretical study of the effect of self-doping and the contribution of native defects to the ionic conductivity of alpha-quartz. A thorough first principles study of native defects in quartz, comprising silicon and oxygen defects in several charge states, is presented. On the basis of this comprehensive study of defect energetics, we can evaluate the equilibrium concentrations of defects at a given temperature, including the effect of doping of the material coming from its native defects. Moreover, in open conditions, oxygen partial pressure has an influence on the concentrations and self-doping of the material. Our results show that oxygen interstitial is the native defect of highest concentration at equilibrium and that charge compensation occurs between negative oxygen interstitials and holes in the valence band. Moreover, we find that in pure quartz neutral defects are dominant at low temperature and relatively high partial oxygen pressures, but for low enough oxygen pressures and sufficiently high temperatures, negative interstitials are expected to play a significant role in diffusion. The concentrations of the latter are low enough that their contribution to ionic conductivity is negligible, but their sensitivity to the Fermi level is high; thus, if for some reason (e.g., impurities) the Fermi level is raised, the intrinsic contribution to ionic conductivity will become important.