Taito Murakami

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‐Ba₂ScAlO₅, α‐Ba₂Sc_0.83Al_1.17O₅, 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” β‐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 (≈10⁻³ S cm⁻¹ above 300 °C), comparable to representative proton conductors. In contrast, the structurally related oxides α‐Ba₂Sc_0.83Al_1.17O₅ and BaAl₂O₄ exhibit lower conductivity. Ab initio molecular dynamics simulations revealed that protons in β‐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.

Solid oxide‐ion conductors are crucial for enabling clean and efficient energy devices such as solid oxide fuel cells. Hexagonal perovskite‐related oxides have been placed at the forefront of high‐performance oxide‐ion conductors, with Ba₇Nb_4−xMo_1+xO_20+x/2 (x = 0−0.1) being an archetypal example. Herein, high oxide‐ion conductivity and stability under reducing conditions in Ba₇Ta_3.7Mo_1.3O_20.15 are reported by investigating the solid solutions Ba₇Ta_4–xMo_1+xO_20+x/2 (x = 0.2−0.7). Neutron diffraction indicates a large number of interstitial oxide ions in Ba₇Ta_3.7Mo_1.3O_20.15, leading to a high level of oxide‐ion conductivity (e.g., 1.08 × 10⁻³ S cm⁻¹ at 377 °C). The conductivity of Ba₇Ta_3.7Mo_1.3O_20.15 is higher than that of Ba₇Nb₄MoO₂₀ and conventional yttria‐stabilized zirconia. In contrast to Ba₇Nb_4−xMo_1+xO_20+x/2 (x = 0−0.1), the oxide‐ion conduction in Ba₇Ta_3.7Mo_1.3O_20.15 is dominant even in highly reducing atmospheres (e.g., oxygen partial pressure of 1.6 × 10⁻²⁴ atm at 909 °C). From structural analyses of the synchrotron X‐ray diffraction data for Ba₇Ta_3.7Mo_1.3O_20.15, contrasting X‐ray scattering powers of Ta⁵⁺ and Mo⁶⁺ allow identification of the preferential occupation of Mo⁶⁺ adjacent to the intrinsically oxygen‐deficient layers, as supported by DFT calculations. The high conductivity and chemical and electrical stability in Ba₇Ta_3.7Mo_1.3O_20.15 provide a strategy for the development of solid electrolytes based on hexagonal perovskite‐related oxides.