Modelling of oxygen diffusion in perovskite-type neodymium strontium ferrite Nd_0.3Sr_0.7FeO_3-δ by application of molecular dynamics

The diffusion of oxygen in Nd0.3Sr0.7FeO3-δ (NSF37) with δ = 0.35 (space group: Pm3‾m) was modelled in the temperature range from 2000 to 873 K by means of molecular dynamics (MD). The Nd and Sr atoms on the A-site sublattice as well as the oxygen vacancies on the oxygen sublattice were distributed randomly. The MD simulations yielded the trajectories as well as mean square displacements of oxygen atoms, resulting in vacancy self-diffusion coefficients which showed a non-Arrhenius type temperature dependence. The activation energy increased from 0.9 to 1.2 eV with decreasing temperature. The radial pair distribution functions, computed for Nd – O and Sr – O correlations, indicated the formation of defect interactions (preferential residence of vacancies in the vicinity of Sr) which might give rise to the variation of the activation energy. Moreover, the effect of symmetrical Σ13 (320) grain boundaries on the oxygen transport in NSF37 was investigated in detail. In particular, the effective oxygen self-diffusion coefficient (comprising both bulk and grain boundary regions) was determined as a function of temperature ranging from 1673 to 873 K. When the interfaces were equilibrated properly at 1673 K, leading to amorphous interfacial regions, the grain boundaries were clearly blocking for the oxygen transport. If, however, the equilibration at 1673 K was omitted (largely preventing the amorphization of the grain boundaries), fast grain boundary diffusion could be observed at low temperatures. At 873 K the grain boundary diffusion coefficient exceeded the bulk diffusivity by a factor of 2–3.