| Summary: | Powder neutron diffraction studies have been performed on the deuterated form of the proton conductor HU0<sub>2</sub>AsO<sub>4</sub>.4H<sub>2</sub>O both above and below the transition at 301K to a poorly conducting antiferro-electric phase. The observed structures have been rationalised in terms of the presence of two types of water dimer, H<sub>4</sub>O<sub>2</sub> and [H<sub>5</sub>O<sub>2</sub>], whose dynamic interchange above the transition, in a scheme of hydrogen disorder, provides a simple mechanism for the high hydrogen mobility. Further diffraction studies on cation substituted LiUO<sub.2</sub>AsO<sub>4</sub>.4D<sub>2</sub>O show a change to a structure which is completely ordered with adjacent water squares linked together via tetrahedral co-ordination about the lithium ion. Comparison of this structure with that of some similar cation substituted compounds allows the proposal of a mechanism for sodium mobility in NaUO<sub>2</sub>PO<sub>4</sub>.3H<sub>2</sub>O. The series of compounds Li<sub>2</sub>UCl<sub>6</sub>, Na<sub>2</sub>UCl<sub>6</sub> and K<sub>2</sub>UCl<sub>6</sub> have been investigated at room temperature by powder neutron diffraction, with the aim of assessing the importance of the light cation in the structures of such compounds. The decrease in polarising power of the cation through the series leads to structures in which the [UCl<sub>6</sub>]<sup>2-</sup> octahedron exhibits progressively greater o molecular character as the charge transfer from chloride to uranium increases. Measurements of the proton NMR spin-lattice relaxation time T<sub>1</sub> as a function of temperature have been performed on the ammonia intercalation compounds TiS<sub>2</sub>(NH<sub>3</sub>) for three different stoichiometries of TiS<sub>2</sub>, ranging from 0.1-2.0% excess titanium. There appears to be no influence of excess titanium on the mobility of the ammonia molecules in contrast to LiTiS<sub>2</sub> and NaTiS<sub>2</sub>, where there is a very marked decrease with increasing titanium content.
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