Fast-ion conduction and local environments in BIMEVOX

The energy landscape of the fast-ion conductor Bi₄V₂O₁₁ is studied using density functional theory. There are a large number of energy minima, dominated by low-lying thermally accessible configurations in which there are equal numbers of oxygen vacancies in the vanadium-oxygen layers, a range of vanadium coordinations and a large variation in Bi-O and V-O distances. By dividing local minima in the energy landscape into sets of configurations we then examine diffusion in each different layer using ab initio molecular dynamics. These simulations show that the diffusion mechanism mainly takes place in the <110> directions in the vanadium layers, involving cooperative motion of the oxide ions between the O(2) and O(3) sites in these layers, but not O(1) in the Bi-O layers, in agreement with experiment. O(1) vacancies in the Bi-O layers are readily filled by migration of oxygens from the V-O layers. The calculated ionic conductivity is in reasonable agreement with experiment. We compare ion conduction in δ-Bi₄V₂O₁₁ with that in δ-Bi₂O₃.