High Oxide‐Ion Conductivity in a Hexagonal Perovskite‐Related Oxide Ba7Ta3.7Mo1.3O20.15 with Cation Site Preference and Interstitial Oxide Ions

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 Ba7Nb4−xMo1+xO20+x/2 (x = 0−0.1) being an archetypal example. Herein, high oxide‐ion conductivity and stability under reducing conditions in Ba7Ta3.7Mo1.3O20.15 are reported by investigating the solid solutions Ba7Ta4–xMo1+xO20+x/2 (x = 0.2−0.7). Neutron diffraction indicates a large number of interstitial oxide ions in Ba7Ta3.7Mo1.3O20.15, leading to a high level of oxide‐ion conductivity (e.g., 1.08 × 10−3 S cm−1 at 377 °C). The conductivity of Ba7Ta3.7Mo1.3O20.15 is higher than that of Ba7Nb4MoO20 and conventional yttria‐stabilized zirconia. In contrast to Ba7Nb4−xMo1+xO20+x/2 (x = 0−0.1), the oxide‐ion conduction in Ba7Ta3.7Mo1.3O20.15 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 Ba7Ta3.7Mo1.3O20.15, 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 Ba7Ta3.7Mo1.3O20.15 provide a strategy for the development of solid electrolytes based on hexagonal perovskite‐related oxides. High oxide‐ion conductivity (e.g., 1.08 × 10−3 S cm−1 at 377 °C) along with high chemical and electrical stability under reducing conditions in a hexagonal perovskite‐related oxide, Ba7Ta3.7Mo1.3O20.15, are reported. Furthermore, the structural analysis reveals both Mo6+ cation site preference adjacent to oxide‐ion conducting layers and a large amount of interstitial oxygen, leading to high conductivities.