Investigation of V5+ Substitution on the Crystal Structure and Oxide Ion Conductivity of Ba7Nb4MoO20

The effect of isovalent V5+ substitution for Nb5+ on the crystal structure and oxide ion conductivity of Ba7Nb4-xVxMoO20 (x = 0.00, 0.05, 0.15) has been investigated. Despite no change in nominal oxide ion carrier concentration, a significantly reduced bulk oxide ion conductivity is observed below ∼ 400 °C upon increasing x, suggesting enhanced local oxide ion trapping. This may arise from the formation of defect associates between V5+ centres and interstitial oxide ions, creating energetically favourable trapping sites that reduce the population of mobile charge carriers available for long-range diffusion. This behaviour contrasts with that of Ba3Nb1-xVxMoO8.5​, where moderate V5+ substitution significantly improves oxide ion transport. This suggests that oxide ion migration in Ba7Nb4MoO20 is more sensitive to local structural perturbations than the hexagonal perovskite derivative Ba3NbMoO8.5. At higher temperatures, thermal activation overcomes trapping effects, while lattice expansion and increased tetrahedral distortion promote oxide ion diffusion so that the conductivities of the substituted and unsubstituted compositions converge near 650 °C. These results highlight the importance of local structural chemistry in controlling oxide ion transport in hexagonal perovskite derivative oxide ion conductors.