Defect structure of Ta- and Al- doped Zn2TiO4 showing oxide ion conduction via cation vacancy

To investigate the defect structure of Zn2-x/2Ti1-xTaxO4 and the subsequent oxide ion conduction, equimolar Al3+ with Ta5+ have been partly substituted instead of Ti4+ ions in Zn2TiO4, forming Zn2Ti1-2xTaxAlxO4 with no significant cation vacancy despite of tantalum substation. Inverse spinel-type structured solid solution was approximately formed up to x = 0.3 and the measured powder density agreed with the nominal model where all the cation sites ware almost occupied. A slight decrease in oxide ion conductivity was observed for Zn2Ti1-2xTaxAlxO4 with the dual substitution by x, while apparent increase for Zn2-x/2Ti1-xTaxO4, indicating that the cation vacancy is responsible for the oxide ion conduction in Zn2TiO4-based system. Powder neutron diffraction revealed the defect structure, which were then discussed in terms of the oxide ion conduction mechanism with the deduced isotropic temperature factors of oxide ions.