Neutron scattering studies of hydrogenated molecular materials
This thesis employs three neutron scattering techniques; neutron diffraction, inelastic neutron scattering, and quasielastic neutron scattering; to probe the properties of hydrogenous systems. These results are complemented by other techniques in order to provide useful information on the three systems to be studied. These systems are chosen for their relevance to industrial situations and the key influence of hydrogen bonding upon their properties. Neutron scattering has a key advantage over X-ray scattering, in that the cross-section of hydrogen is much larger for neutron scattering, allowing for hydrogenous systems and hydrogen bonding phenomena to be directly probed. Neutrons are also better suited to inelastic scattering measurements, as their lower energy allows for energy transfer between the probe and the sample to be measured with higher resolution. The first family of systems studied in this thesis are the methylammonium lead halide (MAPbX3) compounds, where X = Cl or Br. This family of compounds is of interest due to their high performance as solar panel materials, and relative ease of manufacture. The studies carried out use inelastic neutron scattering, quasielastic neutron scattering, and Raman spectroscopy to probe the molecular excitations in these compounds as a function of temperature. It is found that there is a step change in the molecular dynamics in these compounds as they transition out of the orthorhombic phase, with the short-lived tetragonal phase acting as a region of transitional dynamics. In the case of MAPbBr3 this change in molecular dynamics is corroborated with a change in photovoltaic properties at these temperatures. Secondly, the phase diagram with pressure of NH4F is studied using neutron diffraction and computational studies. NH4F is analogous to ice in structure at ambient pressure, and the known phases of NH4F are all analogous to those observed in ice. As gas inclusion compounds are easily formed with ice, this makes NH4F a candidate gas storage material. An apparent tetragonal distortion is observed in the cubic phase of NH4F commencing at approximately 6GPa and increasing with pressure. However, as this distortion is not reproduced in computational studies, further experiments are carried out to establish that this effect is instead due to deviatoric stress. The tetragonal distortions reported in the literature at high pressures and low temperatures are therefore inferred to also be due to strain effects. Finally, the mixing behaviour of CH4 in H2O with pressure is investigated using quasielastic neutron scattering. This system is relevant to natural gas extraction processes. Selective deuteration is used to isolate the dynamics of the H2O molecules. It is found, at pressures below mixing, the diffusion coefficient of the water is reduced, whereas above the mixing pressure it is in agreement with that of pure water. This disparity is explained through the increase in viscosity of the water due to the presence of bubbles before mixing, and the reduced proportion of bubbles as the mixing pressure is approached.
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