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Title: Synthetic, structural and electrochemical studies of superstructured rare-earth niobates
Author: Bayliss, Ryan David
ISNI:       0000 0004 2732 1496
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2013
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Materials with inherent oxygen excess are of great interest for high-temperature electrochemical applications, such as solid oxide fuel cells (SOFCs) and oxygen permeation membranes due to their rapid ion conductivity. Cerium niobate has received a lot of attention recently due to its rapid oxide ion conductivity in the intermediate temperature range (500-650 °C). Cerium niobate has several interesting characteristics, such as the range of excess oxygen structures that stabilise in the monoclinic crystal system and the phase transition from monoclinic to tetragonal crystal structures associated with a change in ionic conductivity. During this work the first single crystal study of CeNbO4 has been performed and as a result of the higher quality of data available with single crystal diffraction a correction to the space group reported in literature has been suggested. Furthermore, single crystals have been thermally treated in order to synthesis the excess oxygen modulated structures. CeNbO4.25 has been successfully synthesised and the crystal structure has been solved with a large superstructure of 75 independent atomic positions. Structural analysis of the complex anion framework suggests a potential anisotropic oxygen ion conduction pathway through the material. XANES work has been reported on the cerium niobate structures looking at in-situ measurements as a function of temperature. This work has allowed identification of a new phase with an approximately stoichiometry of CeNbO4.16. Bond valence calculations have been performed to study the effect of additional oxygen content on overall crystal strain. Finally, in an effort to synthesise a novel electrolyte using cerium niobate as a model structure, an analogous material, LaNb0.84W0.16O4.08 has been synthesised in an attempt to reproduce the ionic conductivity observed whilst eliminating the electronic component associated with the mixed valence of the cerium cation. LaNb0.84W0.16O4.08 has been successfully synthesised and some initial structural and electrochemical characterisation have been performed with very promising results. Total conductivity has shown the material to outperform YSZ at 1000 °C and in addition the material has a large ionic domain with negligible electronic conductivity at 10-22 atm p(O2). Initial diffusivity measurements and fuel cell testing results correlate well with electrochemical measurements via AC impedance. Furthermore, the thermal expansion coefficient of the material is suitable with typical electrode materials meaning this is a promising new material for high oxide ion transport.
Supervisor: Skinner, Stephen Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral