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 Ba₇ Nb_4- _x Mo₁₊ _x O₂₀₊ _x _/2 (x = 0-0.1) being an archetypal example. Herein, high oxide-ion conductivity and stability under reducing conditions in Ba₇ Ta_3.7 Mo_1.3 O_20.15 are reported by investigating the solid solutions Ba₇ Ta_4- _x Mo₁₊ _x O₂₀₊ _x _/2 (x = 0.2-0.7). Neutron diffraction indicates a large number of interstitial oxide ions in Ba₇ Ta_3.7 Mo_1.3 O_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 Ba₇ Ta_3.7 Mo_1.3 O_20.15 is higher than that of Ba₇ Nb₄ MoO₂₀ and conventional yttria-stabilized zirconia. In contrast to Ba₇ Nb_4- _x Mo₁₊ _x O₂₀₊ _x _/2 (x = 0-0.1), the oxide-ion conduction in Ba₇ Ta_3.7 Mo_1.3 O_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 Ba₇ Ta_3.7 Mo_1.3 O_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 Ba₇ Ta_3.7 Mo_1.3 O_20.15 provide a strategy for the development of solid electrolytes based on hexagonal perovskite-related oxides.

Toshiya Shibata

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