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High-pressure techniques have become increasingly important in the synthesis of ceramic and metallic solids allowing the discovery of new materials with interesting properties. In this research dense solid oxides have been synthesised at high pressures, and structural investigations have been conducted using x-ray and neutron diffraction. The perovskite LaPdO3 has been synthesised at pressures of 6{10GPa. Neutron diffraction studies have been carried out from 7{260K to investigate any structural distortions, particularly related to the possibility of charge order at low temperatures. No reduction in symmetry associated with charge ordering has been observed; the material appears to remain metallic with only one unique Pd site down to 7K. LaPdO3 adopts the GdFeO3-type Pbnm structure. The PdO6 octahedra exhibit a tetragonal distortion throughout the temperature range with a shortening of the apical Pd{O bonds of 2:5% relative to the equatorial bonds. Attempts to prepare analogues of the perovskite containing smaller rare earths have resulted in multi-phase samples, and further RPdO3 perovskites remain inaccessible although there is evidence for a small amount of the perovskite phase in the products of synthesis attempts with neodymium. Three new oxypnictide superconductors, RFeAsO1 xFx (R = Tb, Dy and Ho) have been synthesised at 7{12GPa. The materials are isostructural with other recently discovered iron arsenide superconductors and have Tc's of 52:8 K, 48:5K and 36:2K respectively, demonstrating a downturn in Tc in the series for smaller R. Systematic studies on TbFeAsO0.9F0.1 and HoFeAsO0.9F0.1 show negative values of dTc=dV in contrast to those reported for early R containing materials. Low-temperature neutron diffraction measurements on both materials, and synchrotron studies on HoFeAsO0.9F0.1 reveal no tetragonal to orthorhombic transitions as observed in early R-containing materials with lower doping levels. Magnetic reflections are evident but they are shown to be from R2O3 and RAs impurities with TN's of 5:5K for Tb2O3, 6:5K for HoAs and 1:7K < TN < 4K for Ho2O3. The implications of these results for superconductivity in the iron arsenides are discussed.

The physics of the series Pr_yY_1-yBa₂Cu₃O_6&plus;x, and ability of Pr to suppress superconductivity, has been a subject of frequent discussions in the literature for more than a decade. This thesis describes a polarised neutron diffraction (PND) experiment performed on PrBa₂Cu₃O_6.24 designed to find out something about the electron structure. This experiment pushed the limits of what can be done using the PND technique. The problem is one of a limited number of measured Fourier components that need to be inverted to form a real space image. To accomplish this inversion the maximum entropy technique has been employed. In some cases, the maximum entropy technique has the ability to increase the resolution of ‘inverted’ data immensely, but this ability is found to depend critically on the choice of constants used in the method. To investigate this a Bayesian robustness analysis of the maximum entropy method is carried out, resulting in an improvement of the maximum entropy technique for analysing PND data. Some results for nickel in the literature have been re-analysed and a comparison is made with different maximum entropy algorithms. Equipped with an improved data analysis technique and carefully measured PND data for PrBa₂Cu₃O_6.24 a number of new interesting features are observed, putting constraints on existing theoretical models of Pr_yY_1-yBa₂Cu₃O_6&plus;x and leaving room for more questions to be answered.

Thesis advisor: Daniel A. Kirschner / The myelin sheath of the central nervous system contains a network of interlamellar tight junctions known as the radial component. Ablation of claudin-11, a tight junction protein, results in the absence of the radial component and compromises the passive electrical properties of the myelin sheath. Although tight junctions are known to regulate paracellular diffusion, this barrier function has not been directly demonstrated for the radial component, and some evidence suggests that the radial component may also, or instead, mediate adhesion between myelin membranes. To investigate the physical properties of claudin-11 tight junctions, we first compared fresh, unfixed Claudin 11-null and control nerves using X-ray diffraction. In Claudin 11-null tissue, we detected no changes in myelin structure, stability, or membrane interactions, which argues against the notion that myelin tight junctions exhibit significant adhesive properties. To examine myelin permeability in the absence of the radial component, we measured the kinetics of osmotic compaction and recovery in knockout and control myelin. We found that myelin lacking claudin-11 responded more rapidly to osmotic stress, indicating an increase in permeability to water and small osmolytes. To further test this hypothesis, we explored the possibility of measuring the diffusion of water through myelin using neutron diffraction, a technique that had been pioneered in myelin decades ago but was largely unused because of previous limitations in neutron technology. After establishing that present-day neutron instruments were capable of measuring diffusion in myelin, we applied this technique to samples from mice lacking claudin-11. Consistent with our X-ray diffraction studies, we found that H2O-D2O exchange was more rapid in Claudin 11-null mice compared to controls. Thus, our data indicate that the radial component serves primarily as a diffusion barrier and elucidate the mechanism by which tight junctions govern myelin function. / Thesis (PhD) — Boston College, 2016. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Biology.