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

BaFeO3-delta 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 & sdot;& sdot;& sdot;O distances in BaFeO3-delta, 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 BaFeO3-delta, instead of the very oversized Y3+ smaller dopants should be considered.