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First we shall consider the rare-earth titanate Yb2Ti2O7 which has been extensively studied within the past two decades but is still not very well understood. There has been much debate over the nature of the magnetic ground state of this system with studies reporting the development of long-range order below 250mK to the absence of long-range order below 250mK. One culprit suspected of generating these differences in behaviour of the ground state is the effect of crystal growth on sample quality. In order to address this discrepancy, measurements of the crystal-field levels of Yb2Ti2O7 were conducted on two powder samples of different composition since the exact ground state selection depends on size and anisotropy of the magnetic moment. The second focus of this thesis concerns the family of rock-salt ordered double perovskites of the form Ba2YXO6 in which X is either a 4d or 5d magnetic ion. The interplay between the strong spin orbit coupling expected for such heavy magnetic ions and frustration has been theoretically predicted to exhibit a variety of exotic phenomena. Neutron scattering is utilized to investigate the low temperature dynamics of the system Ba2YOsO6 to compare with that observed in the 4d analog, Ba2YRuO6. / Thesis / Master of Science (MSc)

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The method of isotope substitution in neutron spectroscopy is introduced to measure for the rst time the partial vibrational density of states of two network glass forming systems, namely GeSe2 at temperatures of 5, 20 and 292 K and GeO2 at a temperature of 10 K. This work included the development of a new data analysis procedure involving corrections for e.g. beam attenuation, multiple scattering and multiple phonon scattering. The measurements were made using the MARI and MERLIN spectrometers at the ISIS pulsed neutron source where measurements of the elastic lines were used to help deduce the mean squared atomic displacements and Debye-Waller factors. In the case of GeSe2, the latter were found as a function of temperature between 10 and 280 K. The results for GeSe2 glass at temperatures of 5, 20, and 292 K were found to be in good agreement, proving the ecacy of the data correction procedure. For both GeSe2 and GeO2, the results were interpreted with the aid of molecular dynamics simulations to identify the energies corresponding to rocking, bending and stretching motions. The method of in situ high pressure neutron diraction was developed using double toroid sintered diamond anvils in a Paris-Edinburgh press to measure, for the rst time, reliable diraction patterns for GeO2, SiO2 and B2O3 glasses at pressures up to 17.5 GPa. The total pair distribution functions were obtained, allowing the nearest neighbour Ge-O, Si-O or B-O coordination numbers and bond distances to be calculated. The glass networks collapse by two principal mechanisms. The rst mechanism, at lower pressures, involves a rearrangement of the structural motifs on an intermediate range length scale. The second mechanism, above thresholds in pressure of 5, 20 and 9 GPa for GeO2, SiO2 and B2O3, respectively, involves a change in the nature of the structural motifs.

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