High pressure structural investigations of solids using x-ray and synchrotron radiation.

X-ray diffraction has been used to investigate the structure and structural behaviour of several inorganic solids under variable pressure and temperature conditions. A brief description of the diamond anvil cell (d.a.c.) and its applications to the high- pressure investigations is given. A spectroscopic system built for pressure calibration is discussed, plus an account of powder diffraction data analysis including a detailed description of a software package engineered for the specific reduction of powder data obtained under variable pressure. Potassium nitrate (KNO3) has seven polymorphs in the pressure range 0.0-4.0 GPa. This material has been studied with energy dispersive powder diffraction (EDXRD) to 9.3 GPa. The structure of the non-ambient phase IV refined by neutron diffraction at 0.36 GPa has been confirmed, the compressibility of the high-pressure phase measured and found to be anisotropic with axial compression ratios a:b:c = 1.0:0.64:0.50. The valence induced structural transition at 0.8 GPa in EuPd2Si2 has been studied using EDXRD and uni-axial compressibilities calculated. A basic explanation of the electronic theory which accompanies the subtle shortening of the a lattice parameter in the tetragonal asymmetric unit is also given. The structures of two compounds CH3HgX (X = I, Cl) have been investigated using single crystal and angle-dispersive powder techniques respectively. The crystal structure of methyl mercuric iodide is reported. CH3HgCl has been observed above and below the temperature induced I/II phase boundary at 162.5 K. A hypothesis detailing the iso-structural nature of CH3HgCl low temperature phase II and the ambient phase I of the analogous methyl mercuric bromide is also tested.