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Lanthanum is one of the most common so-called "donor-dopants" for BaTiO 3. Combined synthesis, phase diagram and electrical studies of La-doped BaTiO3 ceramics were undertaken to determine the doping mechanisms(s) and to understand the unusual resistance behaviour of air-heated ceramics. Charge compensation was found to be maintained via the creation of Ti vacancies, resulting in the solid solution Ba1-xLax Ti1-x/4O3 with x &le; 0.25 at 1350[Special character omitted]C. No evidence for the existence of the commonly cited La-"donor-doping" or barium vacancy mechanisms was observed. ac Impedance spectroscopy on samples heated in O 2 at 1350[Special character omitted]C indicated the presence of insulating bulk and grain boundary components. The tetragonal cubic and orthorhombic tetragonal phase transition temperatures, Tc and To/t, respectively, decreased dramatically with increasing x, the former approximately linearly at a rate of dTc/dx ca. 0.24 [Special character omitted]C/x. The value of 'max at T c increased with x for 0 &le; x &le; 0.06 and reached a value > 25,000 at ca. -9 [Special character omitted]C. For x &ge; 0.08, Tc and To/t coalesce and, with increasing x, ' max decreased and "relaxor"-type, frequency-dependent ' behaviour was observed. The semiconductivity observed in air-heated samples was attributed to oxygen loss. Combined impedance and modulus spectroscopy of samples heated in air showed all samples to be electrically inhomogenous, however, all contained a semiconducting bulk component with an associated resistance < ca. 20O. The total dc sample resistance was dependent on the level of re-oxidation during cooling and was dominated by insulating thin layer regions. Two oxidised thin layer effects were identified. At low La concentrations, e.g. x = 0.03, re-oxidation occurred at the grain boundaries and on individual grain surfaces whereas the grain interiors remained semiconducting. At higher La concentrations, e.g. x = 0.20, re-oxidation occurred preferentially at pellet surfaces whereas the pellet interior remained semiconducting. The observed room temperature resistance anomaly for air-heated La-doped ceramics is not linked to La-"donor-doping" but is an artefact of sample processing which results from oxygen loss during heating to high temperatures, > 1200 [Special character omitted]C.

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 KTiOPO<SUB>4</SUB>(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 TiO<SUB>6</SUB>-PO<SUB>4</SUB> structural framework. In general the distortions of the TiO<SUB>6</SUB> 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 Pna2<SUB>1</SUB> space-group symmetry of the low-pressure phase is retained. The main structural changes at the phase transition involve tilting of the PO<SUB>4</SUB> tetrahedra relative to the TiO<SUB>6</SUB> octahedra, while both units remain relatively unchanged in size and shape. In particular, the TiO<SUB>6</SUB> 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 KH<SUB>2</SUB>PO<SUB>4</SUB> (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 PO<SUB>4</SUB> tetrahedra around their =4-axes.

For several decades the formation of different kinds of superstructures in solids has been a topical issue in condensed matter physics. The superstructures (or spatially modulated structures) may be of a different nature: magnetic patterns like spin-density waves, inhomogeneous charge distributions in charge-ordered compounds, dipolar and quadrupolar ordering in ferroelectrics or ferroelastics, regular lattice distortions and related orbital structures, stripe-like arrangements of dopants in alloys, etc. The phase diagrams of such compounds can be rather complicated involving a large number of phases with non-trivial types of ordering. Fortunately, all this wealth of seemingly unrelated phenomena can be often described by rather simple models with a due account taken of a competitive character of the most important interactions. In this thesis I will investigate the Ising model in 2-D with nearest and next-nearest neighbour interactions using several methods including exact diagonalisation of small clusters, transfer matrix technique and Monte Carlo simulation of large lattices.

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The experimental investigation of the thermal and electrical conductivities of semiconductors and the way in which these properties depend upon temperature is of importance in attempting to contribute to greater understanding of transport processes in such solids. This dissertation is an account of the determination of the thermal conductivity, electrical conductivity, and the Halh coefficient of the semiconductors indiumantimonide, gallium arsenide and indium phosphide. The results are used to discuss the predominant transport processes occurring in each material. The experimental method is based on the relation between the potential and the temperature in a conductor through which a current flows.

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No description available.