Fast ion conduction in cubic perovskite structures: An ion dynamics study of NaMgF₃

The origin of fast ion conduction in certain perovskites near their fusion points is not obvious, since they do not contain unoccupied equivalent cation or anion sites as do many other fast ion conductors. We report an ion dynamics simulation study of the perfect NaMgF₃ structure which yields the correct lattice energy to 1% but which, unlike the case of CaF₂, shows no diffusion of either cationic or anionic species, even under strong superheating. Since this indicates, for the laboratory material, a mechanism involving defects which form slowly on the computation time scale, or are excluded, we have investigated the consequences of introducing populations of defects into the simulated structure. Fluoride ion mobility has been observed in three cases, the most active one involving a novel cation-site exchange defect.