It is commonly assumed that solids which conduct electricity do so via the movement of charged electrons under an applied electric field, and electrically conducting liquids require the movement of charged ions. However, this is not always the case. Liquid mercury provides, perhaps, the best example of a liquid showing metallic (electronic) conduction, and there are a variety of solids whose conductivity is primarily due to ionic migration. Such solids may therefore show similar properties, and applications, as liquid electrolytes. An important application for solids which conduct via the migration of O2- ions is as electrolytes for high temperatures solid oxide fuel cells, SOFCs, where the electrolyte separates the active electrode materials, which may simply be O2 and H2. Other applications include O2- conducting membranes which can, for example, be utilised for the production of pure oxygen from impure sources, typically air, by passing a current through the membrane. Such devices could be used for large scale oxygen production or in portable devices, e.g. for medical purposes. For efficiency and technological stability, low temperature operation is a requirement, and the need for new materials which show high O2- conductivity at low temperatures provides the stimulus for this proposal. We have made an important observation that certain cations, when partially substituting Bi in bismuth oxide, produce what appear to be the best low temperature isotropic oxide ion conductors (i.e. the conductivity is independent of direction). It is now vital that we fully characterise these materials in order to:i optimise their properties;ii fully check (and possibly improve) their stability to long term usage at low temperatures;iii explore their potential for real applications;iv explore the possible extension of our observation to other systems.In particular, we need to explore the detailed structure of the materials we have already synthesised, especially the local structure around the ions substituted into the bismuth oxide framework, and the surface properties which are important for applications in real devices. We also need to have a better understanding of the mechanism applicable to the O2- migration in these materials, in order to rationalise the improved conductivity observed. This objective cannot be achieved experimentally, but requires the use of theoretical modelling. The proposal therefore will bring together four high quality research groups from different research centres, each of which will provide unique, internationally leading expertise in the areas necessary to achieve the objectives:Birmingham / synthesis, structure evaluation;Imperial College / surface properties and O2- ion diffusion parameters;Sheffield / conductivity measurements under variable atmospheric conditions;Bath / theoretical simulations to explore the mechanism of O2- migration and provide information on other possible methods to achieve similar, or better, properties.