Molecular dynamics studies of oxide ion transport in Sr-doped LaFeO3: role of cationic environments and cooperativity†

Classical molecular dynamics simulation is employed to study La_1−xSr_xFeO_3−x/2 at 1200 K over a range of dopant concentrations, x = 0.1 to 0.6. Sr ions tend to improve the energetics of the oxide ion sites, resulting in a higher fraction of vacancies in the vicinity of La. The oxide ion migration in the system involves predominantly intra-octahedral hops along the edges of the FeO₆ octahedra. This ion migration is controlled by a triangular bottleneck of cations, formed by two La/Sr ions and one Fe ion, appearing midway between two neighboring oxide ion sites. It is noted that these bottlenecks pose higher barriers as more Sr ions are introduced. This increase in the microscopic energy barriers for oxide ion migration in the system corroborates the observed slowdown of ion transport with dopant concentration. The study also elucidates a dynamic correlation between the mobile species and the cationic framework, wherein as an oxide ion approaches the bottleneck, the cations move apart, increasing the cross-sectional area of the bottleneck.