Oxide-Ion Occupational Disorder, Diffusion Path, and Conductivity in Hexagonal Perovskite Derivatives Ba3WNbO8.5 and Ba3MoNbO8.5

Hexagonal perovskite derivatives Ba3MNbO8.5 (M: W and Mo) are attracting much interest due to high oxide-ion conductivity and potential use for many applications. This work shows the electrical conductivities of Ba3WNbO8.5 (3.7 × 10–2 S cm–1) and Ba3MoNbO8.5 (8.8 × 10–2 S cm–1) at 900 °C and confirms higher activation energy for conductivity of Ba3WNbO8.5 than that of Ba3MoNbO8.5. Key factors governing the conductivity and activation energy are the ratio of tetrahedral O3 to octahedral O2 oxide ions and diffusion pathways in Ba3MNbO8.5. However, the O2/O3 disorders and oxide-ion diffusion paths are unresolved important issues in Ba3MNbO8.5. Here, Rietveld and maximum-entropy method (MEM) analyses of in situ neutron-diffraction data up to 800 °C were performed to obtain the crystal structure and neutron scattering length densities (NSLDs) of Ba3WNbO8.5. MEM NSLDs show two-dimensional oxide-ion migration through the octahedral O2 and tetrahedral O3 sites in the intrinsically oxygen-deficient layer. Numbers of the interstitial O3 and lattice O2 atoms n(O3) and n(O2) increase and decrease, respectively, with increasing temperature, which indicates that the O2/O3 disorder is more prominent at high temperatures. The O2/O3 disordering makes the minimum NSLD on the O2–O3 path higher, which enhances oxide-ion conductivity, leading to higher activation energies of Ba3WNbO8.5 compared with Ba3MoNbO8.5.