Abstract

Fast ion conductors are by convention three dimensional materials that offer isotropic conduction, and in the case of oxide ion conductors, the conduction mechanism is typically via lattice oxygen vacancies. Much research over the last 25-30 years has been devoted to the optimisation and identification of new materials that will be of use in high temperature ion conducting devices such as solid oxide fuel cells, sensors and batteries, with the focus on oxide ion conductors being the perovskite (ABO3) and fluorite (AO2) structure types. Despite extensive research activity it is apparent that step changes in ion conduction are unlikely to be found in these materials, and that new approaches to materials engineering and development are required to improve on the performance of existing devices. One such approach is to consider interstitial ion conduction in anisotropic materials. In recent years fast oxide ion conduction has been demonstrated in layered perovskites such as the A2BO4 +d and AA’B2O5+d systems1-3, both of which display performance competitive with La1-xSrxCoO3-d, the material acknowledged to have the highest level of oxide ion conduction at elevated temperatures, and have introduced the concept of fast ion conducting anisotropic materials. Using these concepts, we have investigated structurally complex materials with nominally excess oxygen contents and have demonstrated competitive oxide ion conductivity. In CeNbO4+d, for example, conduction in a superstructured, monoclinic, oxygen excess, modulated phase has been demonstrated which leads to questions regarding the conduction mechanism in these phases4, 5. Given the structural complexity of the materials this provides an intriguing challenge for crystal chemists, materials modellers and spectroscopists. Here we will discuss recent progress in characterising the structure of the CeNbO4 and CeNbO4.25 materials, linking these observations with redox chemistry and will discuss a suggestion as to the oxide ion conduction pathway which is proposed to be via helical CeO polyhedral chains.

References 1. Tarancon, A.; Morata, A.; Dezanneau, G.; Skinner, S. J.; Kilner, J. A.; Estrade, S.; Hernandez-Ramirez, F.; Peiro, F.; Morante, J. R.,. J. Power Sources 2007, 174, 255. 2. Tarancon, A.; Burriel, M.; Santiso, J.; Skinner, S. J.; Kilner, J. A., J. Mater. Chem. 2010, 20, 3799. 3. Sayers, R.; De Souza, R. A.; Kilner, J. A.; Skinner, S. J., Solid State Ionics 2010, 181, (8-10), 386 4. Packer, R. J.; Tsipis, E. V.; Munnings, C. N.; Kharton, V. V.; Skinner, S. J.; Frade, J. R., Solid State Ionics 2006, 177, 2059. 5. Packer, R. J.; Skinner, S. J., Adv. Mater. 2010, 22, 1613.