The crystal structure and electrical properties of the oxide ion conductor Ba3WNbO8.5Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ta08989a

The structural and electrical properties of the hexagonal perovskite derivative Ba 3 WNbO 8.5 have been investigated. Ba 3 WNbO 8.5 crystallises in a hybrid of the 9R hexagonal perovskite and palmierite structure as recently reported for the novel oxide ion conductor Ba 3 MoNbO 8.5 . Ba 3 WNbO 8.5 is also an oxide ion conductor and appears to exhibit oxide ionic conduction with negligible electronic conductivity over a wider pO 2 range than Ba 3 MoNbO 8.5 . A neutron diffraction study has revealed that at 20 °C the average structure of Ba 3 WNbO 8.5 contains just 13% of W(1)/Nb(1)O 4 tetrahedra within the average structure of Ba 3 WNbO 8.5 in comparison to 50% of Mo(1)/Nb(1)O 4 tetrahedra in Ba 3 MoNbO 8.5 . The presence of (M/Nb)O 4 tetrahedra with non-bridging apical oxygen atoms is an important prerequisite for the ionic conduction observed in the Ba 3 MNbO 8.5 system (M = W, Mo). The strong reduction in the ratio of (M/Nb)O 4 tetrahedra to (M/Nb)O 6 octahedra upon replacement of W 6+ for Mo 6+ results in a reduction in the ionic conductivity by an order of magnitude at 450 °C. The bulk conductivities converge upon heating so that at 600 °C the bulk conductivity of Ba 3 WNbO 8.5 , σ b = 0.0017 S cm −1 , is comparable to that of Ba 3 MoNbO 8.5 ( σ b = 0.0022 S cm −1 ). The results demonstrate that other members of the Ba 3 MM′O 8.5 family can support oxide ion conductivity and further studies of hexagonal perovskite derivatives are warranted. The crystal structure of the novel oxide ion conductor Ba 3 WNbO 8.5 .