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Title: Structure determination using electron diffraction and microscopy
Author: Haydon, S. K.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2001
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This thesis presents a program of work initiated to implement, assess and test a difference potential methodology of structure refinement using quantitative electron diffraction. The results of a parallel synthetic and structural study of the pentagonal tunnel (PT) structure, tetragonal lead tungsten bronze (lead-TTB), and the unexplored quaternary Pb-Nb-W-O system are also presented. Quaternary Pb-Nb-W-O phases are synthesised via fully oxidised and reduced reaction mechanisms. The phases observed in both experiment types are similar and based on superstructures of basic TTB, formed by the ordering of pentagonal columns (PCs) within the host framework. The formation of a fully oxidised quaternary analogue of lead-TTB is discussed. Reduced phase preparations prove it is possible to incorporate lead into PC structures whilst maintaining the parent framework PC distribution. In other experiments an alternative PT/PC ordering scheme is found. A model for this phase and its hidden supercell is presented and discussed. A new TTB related phase is also observed possessing a tetragonal cell repeat five times the (210)TTB plane spacing. A possible model for this phase is proposed. Methods of extracting experimental intensities for the difference potential calculation from an exposure series of diffraction patterns are discussed. A software package (EDIM) for performing the intensity extraction automatically is presented. Tests performed to investigate the characteristics of the difference potential are described, and the results of a limited refinement of lead-TTB using EDIM generated experimental electron diffracted intensities are presented. These calculations confirm that the electron difference potential is a viable means of determining light atom positions in structures containing heavy atom components.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available