Protons in (Ga,Sc,In,Y)3+-doped BaFeO3 triple conductors — Site energies and migration barriers investigated by density functional theory calculations

Abstract

BaFeO_3-δ is a prototypical “triple-conducting” perovskite combining electronic, proton and oxygen vacancy conductivities. Here, the interaction energies of protons at different sites with Ga³⁺, Sc³⁺, In³⁺, and Y³⁺ dopants on the Fe site in BaFeO₃ are calculated using density functional theory (DFT). The effect of the dopants on the respective proton transfer barriers is also investigated. While for the smaller Ga³⁺ and Sc³⁺ dopants a slight trapping of protons in the first and second shell around the dopant is found, in the case of the strongly oversized In³⁺ and Y³⁺ the first shell exhibits a repulsive behaviour for protons (despite attractive electrostatic interaction). The calculated proton transfer barriers for different configurations depend sensitively on the local geometry. They follow the previously derived correlations with O-H bond lengths and O···O distances in BaFeO_3-δ, corroborating that these quantities are physically meaningful descriptors for proton transfer in perovskites. Overall, a very complex energy landscape is obtained, and the consequences for long-range proton transport are discussed only qualitatively. The combination of a proton-repulsive first shell and the tendency for increased proton barriers suggests that for BaFeO_3-δ, instead of the very oversized Y³⁺ smaller dopants should be considered.