Designing Oxide Ion Conductors: Synthetic Routes and Structure-Property Relationships

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Oxide ion conductors are key components in a number of technologically important applications, including oxygen sensors and pumps, membranes for oxygen separation and solid oxide fuel cells. In the first case, they can be used in car exhaust systems to monitor oxygen partial pressures, helping maintain optimum engine performance in order to lower smoke emissions. In the latter case, they act as electrolytes transporting O2- to react with a fuel such as H2 in the direct, clean and highly efficient conversion of chemical to electrical energy. A better understanding of why oxide ion conductors show this unusual and highly exploitable behaviour, and a fuller insight into its origins and mechanisms, can lead to new and better materials for the applications described. This would have significant technological and environmental impact.Crystalline solids are usually thought of as materials with long-range three dimensional order of atoms, due to atoms or ions occupying specific fixed sites within a crystal lattice. The key requirement for oxide ion conductors is that they also have mobile atoms. This is the feature that we need to design into the materials. This proposal offers a programme of work which focuses on discovery and characterisation of new oxide ion conductors, with the aim of establishing composition-structure-property relationships in this class of materials. Three different synthetic strands will be pursued, which include a range of low-temperature and conventional solid state techniques. To a large extent, the choice of synthetic targets has been informed by the existing knowledge of crystal chemistry in oxide ion conducting materials, including some which originated from the PI's recent research. Specifically, the idea that complex extended solids containing coordination polyhedra of cations which can support variable coordination numbers and geometries essentially have a system which provides a conduction pathway for oxide ions will be explored. All compounds prepared will be characterised using a variety of techniques, including in-situ variable temperature powder X-ray and neutron diffraction, impedance spectroscopy, electron microscopy, solid state NMR and single crystal diffraction (where appropriate), as well as a number of auxiliary techniques. Some of the target compound classes are expected to adopt very complex crystal structures and these challenges will be tackled by application and further development of some of the most advanced and powerful methodologies for diffraction data analysis, such as simulated annealing and charge flipping. Complementary use of computational methods (DFT and MD), involving collaboration and exchanges of visits with Prof. Mark Johnson, Head of the Computational Group at Institut Laue Langevin in Grenoble, will be an important component of this project.