Relating the formation energies for oxygen vacancy defects to the structural properties of tungsten oxides
Tungsten is one of the primary materials of choice for several commercial fusion power plant designs, in particular for divertor targets and first wall. In maintenance conditions or during a loss of coolant accident, tungsten is expected to reach temperatures at which it readily volatilises as tungsten trioxide, potentially distributing radioactive material and posing a hazard to personnel. The oxidation of tungsten is reported to show an orientation dependence, however, the mechanism by which it occurs is not fully understood on an atomic level, providing a barrier to the understanding of tungsten smart alloys, which are being developed to reduce oxidation and mitigate the hazards. From the DFT simulations, we show how key features of the electronic structure of the tungsten-oxygen system change as the tungsten-oxygen ratio evolves. We then calculate the formation energies of oxygen vacancies and activation energies for their diffusion, allowing an assessment of their mobility in the different tungsten oxide phases. Our results provide a new level of understanding of the sub-stoichiometric Magnéli phases that are observed during the oxidation of tungsten.