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Title: Structural studies of ferroelectrics by neutron diffraction
Author: Kennedy, Norman Stewart Joseph
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1977
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This thesis is concerned with structural studies of ferroelectric crystals, in particular the tetragonal paraelectric and orthorhombic ferroelectric phases of KH2P04, KD2PO4 and RbH2P01 and the cubic paraelectric phase of Cu3B7013'Cl and Ni3B7013I, using both neutron powder and single-crystal diffraction techniques. The methods of constrained least-squares refinement and significance testing are discussed since these techniques were used to determine the significance of certain structural features of interest in the single-crystal studies. Structural studies of tetragonal KH2PO4, KD2P04 and RbH2PO4 at atmospheric pressure revealed marked isotope and temperature effects of many of the structural features tested. On increasing pressure the P04 groups in tetragonal KH2P04 and KD2PO4 rotate such that the O-H,D 0 distance does not, within error, change. This may well explain the unexpectedly small pressure-dependence of n, the tunnelling frequency of a proton (deuteron) in the absence of any coupling, and Slo, the collective tunnelling mode frequency, reported by Peercy. In the orthorhombic phase the H ions order onto one of their two possible sites in the tetragonal phase. The displacements of the K, P and 0 ions are related to the choice of site of the H ion on the 0-0 bond. P-O(2) is significantly longer than P-O(l), the difference is smallest in KD2PO4. It is shown that it is not necessary to postulate disorder to explain the crystal structure of Ni3B7013I at 77°K. For Cu3B7013CR boracite it was not possible, within the resolution of the experiment to distinguish between the tetrahedral disorder or anharmonic (123 third cumulant) models for the distribution of the halogen ion. Increasing anharmonic effects were found as X I -> CL.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available