High Oxide-Ion Conductivity in a Hexagonal Perovskite-Related Oxide Ba₇Ta_3.7Mo_1.3O_20.15 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 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.