Ba‐deficient and K‐doped Ba ₃ MoNbO _8.5 : Hydration properties, electrical conduction, and stability

Abstract 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.