Simultaneous Reduction of Proton Conductivity and Enhancement of Oxide-Ion Conductivity by Aliovalent Doping in Ba 7 Nb 4 MoO 20 .

Hexagonal perovskite-related oxides have garnered a great deal of research interest because of their high oxide-ion conductivity at intermediate temperatures, with Ba 7 Nb 4 MoO 20 being a notable example. However, concomitant proton conduction in Ba 7 Nb 4 MoO 20 may cause a decrease in power efficiency when used as the electrolyte in conventional solid oxide fuel cells. Here, through investigations of the transport and structural properties of Ba 7 Nb 4- x W x MoO 20+ x /2 ( x = 0-0.25), we show that the aliovalent substitution of Nb 5+ by W 6+ not only increases the oxide-ion conductivity but also dramatically lowers proton conductivity. The highest conductivity is achieved for x = 0.15 composition, with 2.2 × 10 -2 S cm -1 at 600 °C, 2.2 times higher than that of pristine Ba 7 Nb 4 MoO 20 . The proton transport number of Ba 7 Nb 3.85 W 0.15 MoO 20.075 is smaller compared with Ba 7 Nb 4 MoO 20 , Ba 7 Nb 3.9 Mo 1.1 O 20.05 , and Ba 7 Ta 3.7 Mo 1.3 O 20.15 . The structure analyses of neutron diffraction data of Ba 7 Nb 3.85 W 0.15 MoO 20.075 at 25 and 800 °C reveal that the aliovalent W 6+ doping introduces interstitial oxide ions in the intrinsically oxygen-deficient c' layers, thereby simultaneously increasing the carrier concentration for oxide-ion conduction and decreasing oxygen vacancies responsible for dissociative absorption of water. Neutron scattering length density distribution was examined using the maximum-entropy method and neutron diffraction data at 800 °C, which indicates the interstitialcy oxide-ion diffusion in the c' layers of Ba 7 Nb 3.85 W 0.15 MoO 20.075 . Ba 7 Nb 3.85 W 0.15 MoO 20.075 exhibits extremely high chemical and electrical stability in the wide oxygen partial pressure P (O 2 ) region [ex. 10 -23 ≤ P (O 2 ) ≤ 1 atm at 903 °C]. The present results offer a strategy for developing pure oxide-ion conducting hexagonal perovskite-related oxides for possible industrial applications.