Variable Temperature Neutron Diffraction Study of the Oxide Ion Conductor Ba3VWO8.5

Ba₃VWO_8.5 is an oxide ion conductor with a bulk conductivity of 5.0 × 10^-5 S cm^-1 at 600 °C. Ba₃VWO_8.5 is anomalous to the other Ba₃M'M″O_8.5 (M' = Nb; M″ = Mo, W) oxide ionic conductors, as it exhibits cation order with vanadium and tungsten on the M1 site only. Here, we report a variable temperature neutron diffraction study of Ba₃VWO_8.5, which demonstrates that cation order is retained up to 800 °C. We show for the first time that the structural rearrangements reported for hexagonal perovskite derivatives Ba₃M'M″O_8.5 are dictated by water absorption. The significant water uptake in Ba₃M'M″O_8.5 (M' = Nb; M″ = Mo, W) arises due to the flexibility of the crystal structure, whereby a fraction of the transition metal cations move from the M1 site to the octahedral M2 site upon absorption of water. The results presented here demonstrate that the presence of 50% V⁵⁺ on the M1 site, which has a strong preference for tetrahedral geometry, is enough to disrupt the flexibility of the cation sublattice, resulting in the ordering of the cations exclusively on the M1 site and no significant water absorption.