High Proton Conductivity in Ba₅Er₂Al₂ZrO₁₃, a Hexagonal Perovskite-Related Oxide with Intrinsically Oxygen-Deficient Layers

For the development of proton-based electrolytes, high proton conductivity at intermediate temperatures (300–600 °C) is crucial, but the available materials have been confined to a limited number of the structure families, such as cubic perovskites. Herein, we report Ba₅Er₂Al₂ZrO₁₃, a hexagonal perovskite-related oxide, as a new class of proton conductors exhibiting higher conductivities than 10^–3 S cm^–1 between 300 and 1200 °C. The protons as charge carriers are found to exist in the inherently oxygen-deficient h′ layer of Ba₅Er₂Al₂ZrO₁₃, which are supported by Rietveld analysis of neutron-diffraction data, bond-valence-based energy calculations, and thermogravimetric analysis. Our discovery of a new structure family of proton conductors with the inherently oxygen-deficient h′ layer offers a strategy in designing superior proton conductors based on hexagonal perovskite-related oxides.