The complete Raman spectra, taken at high resolution, are described for lead germinate, Pb 5Ge3011, from low temperatures to above the ferroelectric phase transition. The line-shapes of the soft mode spectra are analysed with models of varying complexity. The underdamped line-shape has a three-peaked response and attempts at analysing this within simple anharmonic soft mode theory lead to divergences in the strength, damping and frequency. An appropriate line-shape is provided if low frequency coupling to phonon density fluctuations is considered and the resulting soft mode parameters recover their expected behaviours. The lifetime of the phonon density fluctuations, however, is much smaller than is expected and there is evidence that there are large contributions from previously unconsidered low frequency relaxation mechanisms at structural phase transitions. The complete Raman spectra are described for two crystals of barium doped lead germanate, (Pb1_XBax)5Ge3011, from low temperatures to above the respective transitions. The results are compared to those of the pure material, particularly with respect to the pronounced non-linear effects of barium doping on the soft mode and the transition temperature. A derivation is presented, for the form of the wave-vector dependence of the piezoelectrically coupled dielectric-elastic response, close to the ferroelectric phase transition in KD2PO4. Calculations of the resulting scattering cross-section give intensity maps in reciprocal space and these predictions are compared to the results of a neutron scattering experiment performed to measure the response in the [001] zone. It is shown how the presence of coupling provides an alternative method of obtaining the ferroelectric mode structure factor, and the experimental data, at (220), are analyzed to obtain this. The results are discussed, of a preliminary neutron scattering experiment to determine if the central peak in SrTiO3 can be attributed to precursory transformed surface regions.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:652601 |
| Date | January 1978 |
| Creators | Hosea, Thomas Jeffrey |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/14101 |
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