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Zintl-type materials are of great interest in the field of thermoelectrics as their structures lend themselves to independent optimization of properties. For this reason, three distinct series, that can all be described using a Zintl-type model, were prepared using standard solid state reactions. A detailed investigation of X1-xX’xNiSn (X = Ti, Zr, Hf) half-Heusler compositions was undertaken, results of which are presented in Chapters 3-5. These materials are well established as promising thermoelectrics but progress in increasing efficiencies has been hindered by a lack of understanding of the structure and irreproducibility of properties. The work presented herein therefore aimed to provide a detailed analysis of the structure of these compositions. This was achieved primarily by neutron powder diffraction which has not previously been used in the study of these materials. Complimentary electron microscopy analysis was also carried out to gain information on the structure over various length-scales and thermoelectric property measurements were performed. Significant multiphase behaviour was discovered in all compositions where x ≠ 0, 1. Our data indicates that the compositional variations occur over long length-scales and has no significant impact on the thermoelectric properties. In addition, 0-3% excess Ni, located on interstitial sites in the half-Heusler structure was found in all samples, regardless of the synthesis method used. This has not previously been acknowledged in the literature and is difficult to identify without neutron diffraction. Three TiNiMySn series (M = Ni, Co, Cu) were subsequently prepared. Up to 8% interstitial metal was successfully introduced to the TiNiSn structure and had a doping effect on the thermoelectric properties. The introduction of interstitial metals was therefore found to be a new route to controlling the electronic properties of these promising thermoelectric materials. TiNiX (X = Si, Ge) and RMnSbO (R = Nd, La) were considered as new materials to the field of thermoelectrics as described in Chapters 6 and 7, respectively. Both series showed some promise for thermoelectric applications, with a large Seebeck coefficient found in NdMnSbO and low resistivity values displayed by the TiNiX compositions. In addition, detailed X-ray and neutron powder diffraction experiments and measurements of their magnetic properties were undertaken. Large magnetoresistances were found in TiNiX and incommensurate magnetic ordering was uncovered in NdMnSbO.

This dissertation describes low-temperature electronic transport measurements on semiconductor structures of restricted dimensionality. The experiments fall into two sets. The first concerns anisotropic magnetotransport measurements and electron focusing in a varying external magnetic field. These are performed using MBE-grown high mobility two-dimensional electron gases formed on (311)B GaAs substrates. The second is a study of magnetic field induced insulator-quantum Hall liquid transitions performed on GaAs-AIGaAs heterostructures in which a number of InAs monolayers are inserted in the centre of a GaAs quantum well. The sample structures were characterised by STM, TEM, STEM, and AFM. Interest in electron transport on high-index GaAs surfaces is increasing, especially since the advent of patterned substrate regrowth. An anisotropic mobility in orthogonal directions seems to be universal for electron gases grown on (311)B-oriented GaAs substrates. The anisotropy depends on the two-dimensional electron gas carrier density, but mobilities are always higher in the [233] direction. The interface roughness scattering is a possible cause of the mobility anisotropy. The electron focusing results demonstrate that the effective mass and Fermi surface are isotropic even through the mobility is anisotropic. An explanation is proposed based on interface roughness scattering. In the second part, a magnetically induced direct transition from an insulating state at zero magnetic field to quantum Hall effect states with Hall resistance Pxy = h/2e2 and Pxy = h/e2 and back to an insulating state at higher field is observed. The phase boundaries are plotted as a function of disorder and magnetic field using two methods, firstly the temperature independent Pxx points and secondly the maxima in CJxx. This experimental phase diagram is related to the disorder induced collapse of spin splitting in the lowest Landau level obtained from activation energy studies.

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No description available.

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In-situ surface x-ray scattering measurements of the electrode/electrolyte interface have been made along with complementary electrochemical and ultra-high vacuum measurements. Using these techniques, two main systems 'have been studied; bimetallic surfaces which have shown enhanced activity for the oxygen reduction reaction (ORR) and single-crystal surfaces to probe the effect of temperature on the interfacial structure and surface reactions. The Pt3Ni(111) electrode surface has been found to be the most highly catalytically active surface ever detected for the ORR, 1/202 + 2H+ + 2e- = H20. , In-situ surface x-ray scattering measurements of this surface revealed that the Pt concentration profile oscillates over three atomic layers, whereby the oute'rmost atomic layer was found to be pure Pt, followed by a Ni-rich second layer and a slightly Pt-rich third layer, before reaching the bulk occupation value. It is the concentration profile which causes the surface' electronic structure of Pt3Ni(111) to be significantly different from the atomically identical surface of pure Pt(lll). The surface electronic structure of Pt3Ni(111) acts to optimise conditions for the adsorption of reactant species involved in the ORR. Density-functional theory calculations have indicated that 670-atom octahedral Pt3Ni nanoparticles, where each surface plane is [111] orientated, would be thermodynamically stable and have approximately the same Pt concentration profile as the Pt3Ni(111) single crystal, which, if synthesised, would prove to be an improved catalyst for practical application in fuel cells. The solution/substrate temperature plays an important role in determining the molecular adsorbate structure of CO (COad ) formed on the Pt(111) surface. At electrode potentials approaching hydrogen evolution, 0.05 V (versus the reversible hydrogen electrode), the (2x2) and vII9 CO-structures coexist at high temperature (rv45°C). This is due to a significant negative shift in the onset of OH adsorption and subsequent partial oxidation of COad through the LangmuirHinshelwood reaction (COad +OHad --+ CO2 +H+ +e-). The second investigation into the role of temperature in electrochemical systems has been made on the surface reconstruction of the AU(100) and Au(111) electrodes. Surface reconstruction involves the rearrangement of surface atoms and can be directly monitored by in-situ surface x-ray scattering measurements. The AU(100) reconstruction is greatly enhanced at high temperature, rv45°C, however, the Au(lll) reconstruction is unaffected by temperature changes. The apparent discrepancy is due to the initial step by which the respective reconstructions form. As observed in the CO/Pt(l11) system, the onset of OH adsorption is shifted negatively with increased temperature. The negative shift increases the nucleation sites of hex-strings, the initial step in forming the Au(100) reconstruction, which causes an enhancement of the reconstruction.