Neutron Diffraction Study on the Defect Structure of Ta-Substituted Zn2TiO4 Oxide Ion Conductors

Powder neutron diffraction experiments with Rietveld analyses were carried out on pure and Ta-substituted Zn2TiO4 at room temperature to investigate the defect structure together with the oxide ion conduction mechanism. The refined structure of pure-Zn2TiO4 agreed well with the previously reported inverse spinel-type structure with cubic symmetry. For Zn2-x/2Ti1-xTaxO4 solid solutions of cubic phase, the tantalum substitution as well as the vacancy formation was made at the octahedral cation site, while the tetrahedral cation site was occupied by zinc ions without any significant deficiency. Essentially the similar results were attained in the tetragonal phase obtained by long-term annealing. By evaluating the occupation factors of tentatively introduced oxide ion interstitials at various positions, oxide ion conduction was supposed to be made with the interstitialcy diffusion mechanism, where displacement of a regular oxide ion toward the cation vacancy enabled the migration of other oxide ions through the oxide ion vacancy at which the displaced oxide ion had been initially.