| Popis: |
An in-depth analysis of neutron powder-diffraction data from a sample of ice VII is presented. This study involved an attempt to extract information about the detailed nature of atomic disorder present in the water molecule, information only accessible via neutron techniques. The resulting difficulties lead to an exploration of the fundamental limits of powder-diffraction. This investigation highlighted a need to extend the maximum accessible pressure for single-crystal techniques that, for structural neutron-diffraction, is currently limited to 25kbars. The majority of the work constituting this thesis was devoted to increasing this limit. In order to realise this advance, two different techniques were developed in parallel. The first of these centred on the use of the Paris-Edinburgh (PE) cell to compress single-crystal samples. The new instrumentation, experimental methodologies and software developed by the author included modifications to both the cell and diffractometer and are presented here. These successfully facilitated a significant increase in accessible pressure up-to at least 70kbars. This new capability was used in a comparative powder and single-crystal study of the structure of potassium di-hydrogen phosphate (KDP). The results of this study challenged the previously published structure. They also facilitated a detailed comparison of the relative capabilities of powder and single-crystal measurements confirming the significant advantages of the latter in the measurement of low d-spacing intensities. The second technique to be developed by the author used a new large-volume diamond-anvil cell (DAC). This device provides optical access to the sample and thus the possibility of the in-situ growth of high-pressure phases. This property combined with large apertures for the diffracted beam gives the DAC a significant advantage over the PE cell. The techniques developed to perform neutron diffraction with the cell are presented as are the techniques of crystal growth. It was found that the DAC was currently limited in maximum pressure by the size of available anvils. However, this was sufficient to enable a investigation of D2-D2O clathrate, a compound only formed under high pressure. The results of this first neutron study are discussed in comparison with the previous published structure determined by x-ray diffraction, which was unable to resolve the hydrogen atoms. |
