Ba-deficient and K-doped Ba3MoNbO8.5: Hydration properties, electrical conduction, and stability

Ba 3 MoNbO 8.5 is a recently emerging hexagonal perovskite-derivative oxide with high oxide ion conductivity. Mo and Nb-sites were commonly doped with acceptor dopants to regulate the transport properties. In this work, Ba-site was introduced with cation deficiency and doped with K. The tolerance of Ba-deficiency ( x ) in Ba 3‒ x MoNbO 8.5‒ x is within 0.03–0.06, and the solubility of K ( x ) in Ba 3‒ x K x MoNbO 8.5‒ x /2 is higher than 0.3. Although forming Ba-deficiency of x = 0.03 does not influence the conductivity significantly, doping K to x = 0.15 and 0.3 decreases the bulk conductivity obviously. Furthermore, even though these samples can be hydrated, H 2 O/D 2 O isotope effect was not observed, indicating that there is no or negligibly small proton conduction, and hydration even decreases the bulk conductivity. Interestingly, doping K reduces the difference in conductivity between the measurements in the dry and wet atmospheres. By annealing in a wet atmosphere for 90 h, the Ba 2.7 K 0.3 MoNbO 8.35 composition is unstable at 400°C, but shows good chemical stability at 600°C.