The role of A-site ion on proton diffusion in perovskite oxides (ABO₃)

By performing detailed density functional theory (DFT) calculations, we investigate the effect of A ion vacancy on both hydroxide ion rotation and proton transfer in Y-doped BaZrO₃. We find that A ions reduce the barrier for proton diffusion in a perovskite oxide, demonstrating the significance of perovskite structures as proton conductors. Proton diffusion is understood as a two-step mechanism consisting of hydroxide ion rotation and proton transfer. In both steps, lattice deformations play an important role – specifically, the outward O-B-O bending and A ion motions are important for hydroxide ion rotation, and inward O-B-O bending facilitates proton transfer. By comparing the bond strength, we reveal that an A ion can reduce the bond strength of O and B ion, thereby reducing the energy barrier for local lattice deformation such as O-B-O bending motion, which facilitates both hydroxide ion rotation and proton transfer. Our analysis provides a detailed atomistic understanding of the role of A-site ion on proton diffusion in perovskite oxides. The study presented here not only indicates the advantage of perovskite oxides as proton conductors but also elucidates the origin of proton diffusion barrier which can pave the way for design of novel proton conductors.