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Title: Structure and dynamics of molecular crystals
Author: Mackenzie, Gordon A.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1976
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Structural and dynamical properties of a number of molecular crystals have been investigated by coherent inelastic neutron scattering, Raman scattering and neutron powder diffraction. Measurements of the phonon dispersion relations in perdeuteronaphthalene have been assigned with the aid of a rigid molecule lattice dynamics calculation. The intermolecular forces were derived from semi-empirical potential functions which represented the interactions between non-bonded atoms. An attempt was made to improve the model by altering the potential function parameters to get a closer correspondence between observed and calculated frecuencies. The method of total profile refinement has been used in the analysis of neutron powder diffraction data from perfluorodiphenyl, octafluoronaphthalene and paradiiodo and para-dibromo-tetrafluorobenzene. It has been shown that it is possible, by means of reasonable constraints, to refine a molecular structure involving a large number of atoms. The Rietveld program for structure refinement was used for perfluorodiphenyl but the constraint facilities of this program have been found to be inadequate for the lower symmetry systems of interest. The development and use of a new refinement program, embodying more versatile constraint facilities is described. The Raman spectrum of perfluorodiphenyl has been interpreted in terms of a simple polarisability model. The low frequency totally symmetric spectra are explained by the coupling of a lattice vibration with a torsional mode of the molecule. The phase transition in octafluoronaphthalene has been investigated by Raman scattering and neutron powder diffraction. The weight of experimental evidence points to a unit cell doubling with no change in space group symmetry. Lattice dynamics calculations support this, and indicate that an instability in a zone boundary acoustic mode may be associated with the transition.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available