High Proton Conductivity in beta-Ba2ScAlO5 Enabled by Octahedral and Intrinsically Oxygen-Deficient Layers

Proton conductors are promising materials for clean energy, but most available materials exhibit sufficient conductivity only when chemically substituted to create oxygen vacancies, which often leads to difficulty in sample preparation and chemical instability. Recently, proton conductors based on hexagonal perovskite-related oxides have been attracting attention as they exhibit high proton conductivity even without the chemical substitutions. However, their conduction mechanism has been elusive so far. Herein, taking three types of oxides with different stacking patterns of oxygen-deficient layers (beta-Ba2ScAlO5, alpha-Ba2Sc0.83Al1.17O5, and BaAl2O4) as examples, the roles of close-packed double-octahedral layers and oxygen-deficient layers in proton conduction are shown. It is found that "undoped" beta-Ba2ScAlO5, which adopts a structure having alternating double-octahedral layer and double-tetrahedral layer with intrinsically oxygen-deficient hexagonal BaO (h') layer, shows high proton conductivity (approximate to 10(-3) S cm(-1) above 300 degrees C), comparable to representative proton conductors. In contrast, the structurally related oxides alpha-Ba2Sc0.83Al1.17O5 and BaAl2O4 exhibit lower conductivity. Ab initio molecular dynamics simulations revealed that protons in beta-Ba2ScAlO5 migrate through the double-octahedral layer, while the h ' layer plays the role of a "proton reservoir" that supplies proton carriers to the proton-conducting double-octahedral layers. The distinct roles of the two layers in proton conduction provide a strategy for developing high-performance proton conductors.