The study of oxide ion conductors has recently gained a great attention due to their use in electrolytes for solid oxide fuel cells (SOFC) and solid oxide electrolyser cells (SOEC), oxygen separation membranes, oxygen sensors and oxygen pumps. Currently, SOFCs require high temperatures (> 700 °C) to operate efficiently. The high operating temperature lead to poor durability of components, increased costs, slow start up times and issues with selecting compatible materials ^1-2. There is a significant amount of research being carried out into developing oxide ion conductors that can operate at intermediate temperatures (400-600 °C). Numerous crystal systems including fluorite and La₂Mo­₂O₉ (LAMOX) systems, silicon, and germanium apatites, Bi-based compounds (such as BIMEVOX and BICUVOX) and perovskite oxides have displayed fast oxide ionic conductivity. Substantial oxide ionic conductivity has recently been reported in cation-deficient hexagonal perovskite Ba₃M’M’’O_8.5 derivatives, with disordered hybrid 9R-palmierite average structures,^3-4 opens a new avenue to search novel oxide ionic conductors within this family. We have synthesised the hexagonal perovskite Ba₃VMoO_8.5 and elucidated its crystal structure. Rietveld refinement from neutron and X-ray diffraction data show that like Ba₃VWO_8.5, the cation vacancies are ordered on the M2 site leading to a structure consisting of palmierite-like layers of M1O_x polyhedra separated by vacant octahedral layers. In contrast to other members of the Ba₃M’M’’O_8.5 family, Ba₃VMoO_8.5 in not stoichiometric and both barium and extra oxygen vacancies are present. Ba₃VMoO_8.5 is unstable in air above 400 °C, which precludes measurement of the electrical properties. However, bond-valence site energy (BVSE) calculations strongly suggest that oxide ion conductivity would be present in Ba₃VMoO_8.5.

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References 

1. Jacobson, A. J., Materials for solid oxide fuel cells. Chemistry of Materials 2009, 22 (3), 660-674.
2. Adams, T. A.; Nease, J.; Tucker, D.; Barton, P. I., Energy conversion with solid oxide fuel cell systems: A review of concepts and outlooks for the short-and long-term. Industrial & Engineering Chemistry Research 2012, 52 (9), 3089-3111.
3. Fop, S.; Skakle, J. M.; McLaughlin, A. C.; Connor, P. A.; Irvine, J. T.; Smith, R. I.; Wildman, E. J., Oxide ion conductivity in the hexagonal perovskite derivative Ba3MoNbO8. 5. Journal of the American Chemical Society 2016, 138 (51), 16764-16769.
4. McCombie, K.; Wildman, E.; Fop, S.; Smith, R.; Skakle, J. M.; Mclaughlin, A., The crystal structure and electrical properties of the oxide ion conductor Ba 3 WNbO 8.5. Journal of Materials Chemistry A 2018, 6 (13), 5290-5295.