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Studies on some fast ion conductors

We report diffraction experiments on some representative Fast Ion Conductors. The structures of anhydrous stoichiometric deuterium (DAl<sub>11</sub>O<sub>17</sub>) and silver (AgAl <sub>11</sub>O<sub>17</sub>) beta aluminas have each been determined at four temperatures in the range 4 . 2K - 773K using powder neutron diffraction (PND) data. In DAl<sub>11</sub>O<sub>17</sub>, a strong D-O(5) hydroxyl linkage is maintained at all four temperatures .For AgAl<sub>11</sub>O<sub>17</sub> 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, (NH<sub>4</sub>) <sub>1.25</sub>Al<sub>11</sub>- 0<sub>17.125</sub>, and the two derivatives (NH<sub>4</sub>) <sub>0.78</sub>H<sub>0.25</sub>Al<sub>11</sub>0<sub>17</sub> and 'HA1<sub>11</sub>- 0<sub>17</sub>.In the parent compound, the positions and orientations of the two independant (NH<sup>+</sup><sub>4</sub>) 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 (NH<sub>4</sub>) <sub>1.25</sub>Al<sub>11</sub>0<sub>17.125</sub> and we propose schemes by which stoichiometric beta aluminas are formed from this precursor and the related (H<sub>3</sub>0)<sub>1+x</sub>- Al<sub>11</sub>0<sub>17.+x/2</sub>. The crystal structures of I-Ag<sub>3</sub>PO<sub>4</sub>(25°C and 375°C) , II-Ag<sub>3</sub>PO<sub>4</sub>(650°C) ,II-Na<sub>3</sub>PO<sub>4</sub> (γ) (400°C) and the solid solutions Na<sub>3(1-x)</sub>Al<sub>x</sub>PO<sub>4</sub>are 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-Ag<sub>2</sub>SO<sub>4</sub> (25°C) reveals that it is isostructural with thenardite, V-Na <sub>2</sub>SO<sub>4</sub>.The high-temperature modifications a-Ag<sub>2</sub>SO<sub>4</sub> (490°C) and α-K<sub>2</sub>SO<sub>4</sub> (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.

Identiferoai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:279087
Date January 1980
CreatorsNewsam, J. M.
ContributorsCheetham, A. K.
PublisherUniversity of Oxford
Source SetsEthos UK
Detected LanguageEnglish
TypeElectronic Thesis or Dissertation
Sourcehttp://ora.ox.ac.uk/objects/uuid:75cf724a-cd3e-4bb2-af90-c378ab5c8196

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