High Proton Conductivity in β‐Ba₂ScAlO₅ Enabled by Octahedral and Intrinsically Oxygen‐Deficient Layers

<jats:title>Abstract</jats:title><jats:p>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 (β‐Ba<jats:sub>2</jats:sub>ScAlO<jats:sub>5</jats:sub>, α‐Ba<jats:sub>2</jats:sub>Sc<jats:sub>0.83</jats:sub>Al<jats:sub>1.17</jats:sub>O<jats:sub>5</jats:sub>, and BaAl<jats:sub>2</jats:sub>O<jats:sub>4</jats:sub>) as examples, the roles of close‐packed double‐octahedral layers and oxygen‐deficient layers in proton conduction are shown. It is found that “undoped” β‐Ba<jats:sub>2</jats:sub>ScAlO<jats:sub>5</jats:sub>, which adopts a structure having alternating double‐octahedral layer and double‐tetrahedral layer with intrinsically oxygen‐deficient hexagonal BaO (h<jats:italic>'</jats:italic>) layer, shows high proton conductivity (≈10<jats:sup>−3</jats:sup> S cm<jats:sup>−1</jats:sup> above 300 °C), comparable to representative proton conductors. In contrast, the structurally related oxides α‐Ba<jats:sub>2</jats:sub>Sc<jats:sub>0.83</jats:sub>Al<jats:sub>1.17</jats:sub>O<jats:sub>5</jats:sub> and BaAl<jats:sub>2</jats:sub>O<jats:sub>4</jats:sub> exhibit lower conductivity. Ab initio molecular dynamics simulations revealed that protons in β‐Ba<jats:sub>2</jats:sub>ScAlO<jats:sub>5</jats:sub> migrate through the double‐octahedral layer, while the h<jats:italic>′</jats:italic> 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.</jats:p>