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Title: Crystal-structure studies and techniques at high pressure
Author: Allan, David Robert
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1993
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This thesis is concerned with the application of high-pressure diffraction techniques - involving both x-rays and neutrons, single crystal and powder samples - to the study of crystal-structures at high pressure and to the development of such techniques for the determination of crystal-structures with greater accuracy and more reliability. The current state-of-the-art high-pressure diffraction techniques are reviewed and applied to the study of a number of interesting materials. The crystal structure of KTiOPO4(KTP) has been determined as a function of pressure as its phase transition at 5.5 GPa is approached, using single-crystal x-ray diffraction techniques. The principal changes with pressure are in the co-ordination environments of the potassium atoms and in the linking angles of the TiO6-PO4 structural framework. In general the distortions of the TiO6 octahedra, which have been linked to the second harmonic generating properties of KTP, are retained although there are some small changes in the Ti-O bond lengths. The structure of the high-pressure phase of KTP has also been determined, using single-crystal x-ray diffraction, and it appears that, although the phase transition is strongly first-order and involves a 4.2% reduction of the unit-cell volume, the Pna21 space-group symmetry of the low-pressure phase is retained. The main structural changes at the phase transition involve tilting of the PO4 tetrahedra relative to the TiO6 octahedra, while both units remain relatively unchanged in size and shape. In particular, the TiO6 octahedra remain highly distorted and retain their anomalously short Ti-OT bonds. The potassium atoms are also observed to undergo relatively large movements of about 0.5 AA along the polar c-axis at the transition. The crystal structure of KH2PO4 (KDP) has been determined at 2.9 GPa, a pressure just below that of its high-pressure ambient-temperature phase transition, using single-crystal neutron diffraction. It appears that the oxygen-oxygen distance, 2R, undergoes a uniform compression to 2.9 GPa which is moderated by the rotation of the essentially rigid PO4 tetrahedra around their =4-axes.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available