Oxygen and Silicon Self-Diffusion in Quartz and Silica: The Contribution of First Principles Calculations

Despite its importance as a material in many domains, SiO2 is still a very badly known material from the point of view of materials science. Experimentally the silicon and oxygen diffusion has been determined in silica as well as in quartz, but several discrepancies arise between different authors. From a theoretical point of view the various possible atomic defects have mostly been studied in an electronic perspective, so even the simplest ones remained quite poorly known till recently, the silicon related ones remaining completely unknown. The great similarity between silica and quartz properties is in favour of a common model. The determination of the precise formation and migration energies of the various defects is then of paramount importance for the understanding of the kinetic properties of SiO2. We will present in this paper the results of a study of the formation and mobility properties of oxygen and silicon defects in the view of determining the self-diffusion mechanism(s). Our work relies on up to date ab-initio methods: total energy calculations in a DFT-LDA approach, using either plane wave or pseudo-atomic basis for the wave functions and pseudopotentials.We shall discuss the role of the various parameters controlling the kinetic behaviour: chemical potential of the species, nature of the main impurities, cristallinity, and preparation mode of the sample.