Modelling diffusion in large anion binary rock-salt compounds

Anion diffusion in binary rocksalt systems with large anions was studied using first principles methods. Representative materials for which experimental data was readily available, KCl and PbS from alkali halides and metal monochalcogenides, respectively, were considered. Several migration pathways, including single vacancies, vacancy pairs and interstitial pathways were investigated and their migration energies were determined. Activation energies were calculated for each pathway and compared with the experimental results. We found that Cl diffuses mostly due to vacancy pairs (Schottky dimers); and after accounting for relativistic phenomena, S diffuses through cation vacancies formed from dissociated Schottky pairs. Our theoretical results are in agreement with experimental reports. These results contrast with studies of diffusion of smaller anions that prefer to migrate via interstitial pathways. Moreover, we investigated the role of the base-interstitial site, a novel, stable, and energetically competitive interstitial site, in the diffusion of anions in binary rock salt materials. We found that the base-interstitial site is an intermediate hopping site for interstitial anion diffusion in KCl and PbS rock-salt structures. The anion base-interstitial is found to form a triatomic entity with the nearest lattice anions that affects the electronic structure relative to the tetrahedral (body) interstitial.