Mechanism for the Enhancement of the Oxygen Diffusivity by Cation Substitution in La_2-xSr_xCuO₄

Ruddlesden-Popper oxides Ln₂MO₄ (Ln = La, Pr, Nd, Sm; M = Ni, Cu, Fe, Co, Mn) are one of the promising cathode materials for the intermediate temperature (500-750 °C) solid oxide fuel cell. The key property making them operate at relatively low temperatures is their higher oxygen diffusivity, but in general, it is a difficult task to balance it with the durability of the material. To establish guiding principles for systematic improvement, it is indispensable to understand the oxygen diffusion process at the atomic scale. For La_2-xSr_xCuO₄, we used density functional theory calculations to identify major diffusion paths and the crucial factors affecting the diffusion of oxygen vacancies. We found that out-of-plane equatorial-to-apical oxygen site hopping is the bottleneck of oxygen diffusion. Sr substitutional doping not only facilitates the formation of oxygen vacancies, i.e., the number of diffusion carriers, but also affects the diffusivity by locally lowering the formation energy. Two competing effects of Sr, weakly trapping the oxygen vacancies and alleviating the bottleneck of the above hopping, are quantified using our realistic random walk simulation, and the resulting diffusion coefficients reveal that the latter dominates at all doping concentrations, but the effect is saturated at x ∼ 0.3.