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

Abstract 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 Ba 7 Nb 4− x Mo 1+ x O 20+ x /2 ( x = 0−0.1) being an archetypal example. Herein, high oxide‐ion conductivity and stability under reducing conditions in Ba 7 Ta 3.7 Mo 1.3 O 20.15 are reported by investigating the solid solutions Ba 7 Ta 4– x Mo 1+ x O 20+ x /2 ( x = 0.2−0.7). Neutron diffraction indicates a large number of interstitial oxide ions in Ba 7 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 Ba 7 Ta 3.7 Mo 1.3 O 20.15 is higher than that of Ba 7 Nb 4 MoO 20 and conventional yttria‐stabilized zirconia. In contrast to Ba 7 Nb 4− x Mo 1+ x O 20+ x /2 ( x = 0−0.1), the oxide‐ion conduction in Ba 7 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 Ba 7 Ta 3.7 Mo 1.3 O 20.15 , contrasting X‐ray scattering powers of Ta 5+ and Mo 6+ allow identification of the preferential occupation of Mo 6+ adjacent to the intrinsically oxygen‐deficient layers, as supported by DFT calculations. The high conductivity and chemical and electrical stability in Ba 7 Ta 3.7 Mo 1.3 O 20.15 provide a strategy for the development of solid electrolytes based on hexagonal perovskite‐related oxides.