High Oxide-Ion Conductivity in a Hexagonal Perovskite-Related Oxide Ba 7 Ta 3.7 Mo 1.3 O 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 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.