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Title: The synthesis and characterisation of oxide metal-insulator materials
Author: Reading, Jon
ISNI:       0000 0001 3510 4369
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2003
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Ca₂Os₂O₇ has been synthesised by the direct reaction of the component oxides in a sealed silica ampoule at ambient pressure. The structure of this system has been determined from analysis of neutron diffraction data. The material was found to adopt a stoichiometric, orthorhombic weberite structure type rather than the more usual cubic pyrochlore structure. The distortion of the OsO₆ octahedra and eightfold Ca-O co-ordinations decrease with increasing temperature, indicating that this material lies close to the weberite/pyrochlore phase boundary. The electronic behaviour of Ca₂Os₂O₇ has been measured by recording variable temperature conductivity measurements showing a metal-insulator transition across room temperature. The pyrochlores Pb₂Os₂O₇₋ₓ and Tl₂Os₂O₇₋ₓ have been synthesised and characterised using powder x-ray and neutron diffraction. Analysis of the data confirmed the presence of oxygen vacancies in the lattice, which was determined as cubic Fd-3m. There is a mixing of oxidation states on the 'A' cation site for both materials. The only known superconducting pyrochlore, Cd₂Re₂O₇, has been prepared at ambient pressure using a ¹¹⁴Cd isotope to enable power neutron diffraction data collection. The structure for Cd₂Re₂O₇ between 320 mK and room temperature has been investigated and a model has been proposed for the superconducting phase. The low temperature (< 200K) structure of Cd₂Re₂O₇ was modelled in the space group F-43m, where a marked distortion of the ReO octahedra was found to occur. Positional displacements in both the rhenium and oxygen sub-lattices contributed to the distortion, producing a trigonal environment for the rhenium atoms. A complete solid solution between metallic LaNiO₃ and insulating LaAlO₃ has been synthesised using a precursor citrate gel technique at ambient pressure. The results show a significant increase in resistivity for ≥ 60% aluminium concentration.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available