Jordan, Winifred Michaela; (1990) Computer simulation and neutron diffraction studies of superionic fluorides. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Computer simulation and neutron diffraction are applied in the evaluation of defect structure, ion dynamics and transport mechanisms in three superionic conductors. The systems studied include the rare earth fluoride, LaF3, and mixed metal fluorides, RbBiF4 and CsPbF3. These materials have potential uses in a variety of technological applications. The defect structure and transport properties of mobile F- ions in, tysonite structured, LaF3 are analysed by extensive static and dynamic computer modelling studies. One consistent structural terminology is established across the literature. The static technique yields information on defect formation and migration for jumps between and within different F-sub-lattices. Anion-Frenkel pairs are identified as the intrinsic defects, with vacancies forming preferentially on one sub-lattice. The stability of neutral, bound Schottky quartets is also predicted. Calculated activation energies indicate preferred F- ion mobility in the horizontal a-b plane, although experiment points to motion in the vertical direction. Dopant studies reveal a dependency on the type of vacancy site and the radius of the dopant species, M2+, for the formation of nn or nnn configurational clusters. Calculated elastic constants are compatible with experimental values. Molecular dynamics is used to provide structural and dynamical information at elevated temperatures. The presence of Frenkel disorder and high levels of anion diffusion are confirmed. Anisotropic mobilities are calculated. In addition, graphical representations of F- ion trajectories provide the first detailed insight into ion transport in superionic LaF3. Neutron diffraction methods are used to explore the defect structures of RbBiF4 and CsPbF3. RbBip4 has the fluorite structure and is a good F- ion conductor at moderate temperatures. Refinement of powder diffraction data for the highly conducting γ-phase produces an improved R factor in comparison with previous studies. A high level of intrinsic disorder is observed with two possible interstitial sites identified. CsPbF3 is an ionically conducting fluoride perovskite at temperatures well below its melting point. Analysis of powder diffraction data, collected over several temperatures, reveals some interstitial features, but shows no evidence of high levels of interstitial disorder.

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