New mineralogy of the outer solar system and the high-pressure behaviour of methane
Maynard-Casely, Helen E.
This thesis will introduce the study of methane as a mineral. Along with ammonia and water, methane is one of the main planetary-forming materials in the outer solar system. The topic of `new mineralogy of the outer solar system' is outlined and introduced, and previous studies in the area are discussed. This review identities a lack of highpressure structural knowledge on methane when compared to ammonia and water. The significance of this knowledge for the study of the planets Neptune and Uranus is discussed. The crystal structures of methane above 5.2 GPa were, prior to this thesis, unknown. To tackle this long-standing problem an integrated approach of high-pressure diffraction techniques had to be used. The dominance of hydrogen within the structures of methane necessitated the use of neutron diffraction. The difficulties and limitations of highpressure neutron powder diffraction are presented. It will be shown that the complexity of the subsequent structures required the use of single-crystal x-ray diffraction. Using a combination of x-ray and neutron diffraction the structures of methane phase A (5.2 - 10 GPa) and B (10 - 25 GPa) were solved. The structure of phase A, was shown to conform to an indexing from literature [Nakahata 99] of a rhombohedral unit cell with α ≈ 89.3° and a ≈ 8.6 Å. Powder data were insufficient to determine atomic positions for this phase, and a single-crystal xray diffraction study was undertaken. The process of growing samples for this study is described as well as data collection. As a result of these studies the carbon atoms were located within methane phase A, and the density of the structure confined. The heavy atom structure, of phase A, was refined against neutron powder diffraction data, enabling positions of hydrogen atoms to be found. Preliminary powder diffraction studies of methane phase B found that the structure did not conform to the unit cell described within the literature. The phase was instead assigned to a cubic unit cell with a ≈ 11.73 Å. Similarly to the studies of phase A, a single-crystal x-ray diffraction study was undertaken. This was complicated by the presence of a contaminant within the sample area. This contaminant was shown to have no effect on the structural results. From a single-crystal study the heavy atom structure of phase B was found. The thesis charts the attempt, but ultimate failure, to obtain neutron powder diffraction on this phase. Comparisons of phase B with the higher pressure phase HP (25 GPa +) led to the conclusion that there would still be some disorder within the hydrogen atoms of phase B. Other studies have been carried out on the methane phase diagram. A Raman spectroscopy study, in the literature, on the low-temperature and high-pressure region of the phase diagrams (20 K up to 30 GPa) had suggested the existence of 3 additional phases of methane. A low-temperature, high-pressure neutron diffraction experiment was undertaken to try and characterise these phases. It was found that the phase A structure persisted under all conditions (to 20 K and 5 GPa) throwing the original results into question. During the growth of single-crystals for the above studies on phase A and B, a high-temperature solid-solid phase transition was observed. This transition line was mapped out and the phase resulting from it characterised with high-temperature single-crystal x-ray diffraction.