Wilde, Peter Joseph; (1996) The defect chemistry of pyrochlore structured oxides. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

We first review the general phenomenon of fast-ion conductivity in solids and specifically the range of materials which adopt the fluorite and pyrochlore structures. We present a brief summary of the methods of computer simulation and give examples which illustrate its utility in interpretation of experimental observations. The pyrochlore structure and the mechanism postulated for ionic transport within it are then discussed. The technique of static lattice energy minimization is then described and results are presented of the calculations on the non-defective pyrochlore lattices Gd₂Zr₂O₇, Gd₂Ti₂O₇, Y₂Ti₂O₇. The investigation of oxide ion migration using point defect energy calculations is then presented. The results indicate that the creation of defects require high energies. The effect of cation disorder on the energies of Frenkel defects is described; the results suggest that such disorder may be essential for the observed conductivity. We then describe calculations of the solution energies of aliovalent cations in binary oxides and make estimates of the electronic band gap using the results of calculations which approximate the energetics of formation of electronic defects. The former indicate the possibility of extrinsic conductivity and the latter suggest that mixed conductivity may be possible in certain of these materials as observed experimentally. The results of molecular dynamics calculations on yttrium and gadolinium doped zirconia and Gd₂Zr₂O₇ are presented and discussed. These support the proposed mechanism for ionic migration and the observation that conductivity in the pyrochlore is closely linked to disorder. The model resulting from these and the static lattice calculations is described. The totality of the work is then summarized and conclusions are drawn as to how understanding of the processes involved in ionic conduction in pyrochlore structures oxides is advanced.

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