Structural study of the ferroelectric materials PbNb_2O_6 and PbTa_2O_6

This thesis describes a structural study of the ferroelectric materials PbNb_2O_6 and PbTa_2O_6. This has allowed links between crystalline structure and physical properties to be deduced. Single-crystal samples have been grown via spontaneous nucleation from the melt or by using a flux method. Polycrystalline powders have also been prepared using solid state sintering techniques. Single-crystal X-ray diffraction experiments using synchrotron radiation and an area detector, combined with high resolutions powder neutron diffraction have allowed a detailed structural study to be completed. At room temperature these materials possess an orthorhombic superstructure which can be described as a distortion of the tetragonal tungsten bronze structure. Occupational and displacement disorder of the Pb atoms has been found to be present in both compounds. Planes of diffuse scattering in reciprocal space have been observed during the single crystal diffraction experiments and short-range ordering models are presented to explain this observation are discussed. Both compounds are shown to possess sizeable second harmonic generating efficiencies. The origins of the second-harmonic generation and ferroelectric effects in both materials are also examined. In order to study the high-temperature crystal structure and phase-transitions of these materials, a gas-flow furnace has been designed and built. PbNb_2O_6 undergoes a transition from orthorhombic to tetragonal symmetry. The apparatus has allowed the structure of the paraelectric phases of these materials to be determined. All of the X-ray derived structural refinements are complemented with thermal neutron diffraction studies using polycrystalline samples. The gas furnace has also been successfully used by a number of groups running experiments at the Daresbury synchrotron, notably The Royal Institution of Great Britain, London, researching zeolites. Duplicate apparatus is at present being built for exclusive use on the single-crystal station and the equipment is also being looked at commercially by Oxford Cryosystems. (author)SIGLEAvailable from British Library Document Supply Centre-DSC:D205961 / BLDSC - British Library Document Supply CentreGBUnited Kingdo