Molecular dynamics simulation of fast processes in non-stoichiometric ionic solids
Eugene S. Yakub
Problems existing in molecular dynamics simulation in fluorite-like ionic solids whose elemental composition may deviate from stoichiometry are discussed. Results of molecular dynamics simulations of fast diffusion processes in high-temperature non-stoichiometric uranium dioxide both in hyper- and hypostoichiometric domains performed within free polaron hopping approximation are presented. Different mechanisms of oxygen diffusion such as direct and indirect vacancy migration, interstitialcy migration, as well as some other mechanisms, observed in high-temperature UO2 ± x are described. An extended version of the diffusion model, which accounts for some specific mechanisms of oxygen migration proposed. Predicted diffusion coefficients in hypo- and hyper-stoichiometric uranium dioxide are compared with existing experimental and simulation data in a wide range of temperature and stoichiometry. We found that molecular dynamics simulation when combined with the partly-ionic model and polaron free hopping approximation is able to reproduce basic high-temperature properties of non-stoichiometric UO2 ± x solid, including lattice contraction, defects clustering as well as enhanced oxygen diffusion both in hyper- and hypo-stoichiometric domains.
Keywords: Computer simulation, non-stoichiometric solids, polaron hopping, diffusion mechanisms, visualization