Studies on some fast ion conductors
We report diffraction experiments on some representative Fast Ion Conductors. The structures of anhydrous stoichiometric deuterium (DAl11O17) and silver (AgAl 11O17) beta aluminas have each been determined at four temperatures in the range 4 . 2K - 773K using powder neutron diffraction (PND) data. In DAl11O17, a strong D-O(5) hydroxyl linkage is maintained at all four temperatures .For AgAl11O17 at 4.2K,we have observed, for the first time in metal beta aluminas, a completely ordered cation distribution. The material, which was prepared by a new route, is accurately stoichiometric. At 298K,the silver atoms have become somewhat delocalised and by 773K they are organised in a highly diffuse manner. Single crystal neutron diffraction was used to elucidate the proton distributions in ammonium beta alumina, (NH4) 1.25Al11- 017.125, and the two derivatives (NH4) 0.78H0.25Al11017 and 'HA111- 017.In the parent compound, the positions and orientations of the two independant (NH+4) species are such that all protons are involved in favourable hydrogen-bonding schemes. In formation of the first derivative, one of these sites becomes completely depopulated. Our results clarify the thermal behaviour of (NH4) 1.25Al11017.125 and we propose schemes by which stoichiometric beta aluminas are formed from this precursor and the related (H30)1+x- Al11017.+x/2. The crystal structures of I-Ag3PO4(25°C and 375°C) , II-Ag3PO4(650°C) ,II-Na3PO4 (γ) (400°C) and the solid solutions Na3(1-x)AlxPO4are described. The high-temperature forms are considered in relation to the fluorite structure and mechanisms for the ionic conduction of these phases are proposed. A single crystal X-ray diffraction study of β-Ag2SO4 (25°C) reveals that it is isostructural with thenardite, V-Na 2SO4.The high-temperature modifications a-Ag2SO4 (490°C) and α-K2SO4 (620°C) have been characterised using PND data. A PND experiment on silver (I) fluoride confirms that it adopts the rocksalt structure and we find no firm evidence for a significant population of defects.