Reilly, Lee Martin; (1999) Experimental and computational studies of bismuth molydates. Doctoral thesis (Ph.D.), University College London.

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Abstract

The work described in this thesis has two main strands. The first concerns the characterisation of the phase formation and transformation of the Bi2O3.MoO3 system. The second strand involves an investigation of the behaviour of the β Bi2Mo2O9 phase, using Interatomic Potential Modelling methods. The experimental work has utilised a large number of techniques such as Thermal Gravimetric Analysis, Infra Red Spectroscopy, and Combined XAS/XRD to study the phase changes. All of these methods used in conjunction bring a powerful insight into the physical processes that occur when a phase is formed from a precursor or transforms into a new structure. In the formation of y Bi2MoO6 (which has an Aurvillius structure), from a co-precipitate it is shown that the Bi2O22+ sheets form al 300°C, but the Mo only becomes octahedrally co-ordinated at 400°C. This observation is only possible with the use of in-situ Combined XAS/XRD, which probes both local and long range order in a system. The two-reported phase transformations are identified as occurring at ca. 608°C and ca. 670°C. The temperature that they occur at is independent of the atmosphere. The first phase change is not characterised by any obvious change in co-ordination around either the Mo or Bi ions. The second phase change is characterised by a change from octahedral to tetrahedral co-ordination around the Mo ion. Computational Redox and Dopant studies are presented for the β Bi2Mo2O9 phase, and their significance discussed.

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