Vacancy-induced modifications to the electronic structure of barium lanthanum cobaltite: density functional theory calculations.

This work investigates the influence of oxygen and cobalt vacancies on the structural, electronic and magnetic properties of cubic barium lanthanum cobaltite (BLCO). A novel multi-objective evolutionary algorithm is applied in the generation of dedicated pseudopotentials which are later used in the density functional theory (DFT) calculations performed for the perfect and defective BLCO structures. The Hubbard + U correction is applied to La, Co and O atoms and the accuracy of the developed methodology is assessed through the cohesive energy and equation of state calculations, performed for the perfect BLCO. The results are found to be in agreement with the experimental observations and earlier all electron calculations. The validated methodology is applied in the investigation on defective BLCO, considering three types of defects: a single oxygen vacancy, transverse-oxygen (trans-O) vacancy pair, and complex oxygen-cobalt-oxygen vacancy, all in charged and neutral states. The obtained results reveal that all studied defects considerably affect the structural, electronic and magnetic properties of BLCO, which we comprehensively characterize. Among others, we show that in BLCO the redistribution of charge has a mixed character, with the introduction of defects resulting in both charge localization and delocalization. We attribute this to competing mechanisms originating from the half-metallicity of BLCO, its ferromagnetic ordering, and the tendency of Co atoms to undergo reduction. Our DFT + U calculations show only a mild reduction of the Co atoms near the defect, and this observation is contrary to the strong reduction predicted by defect chemistry models.