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

To investigate the defect structure of Zn_2-x/2Ti _1-xTa_xO₄ and the subsequent oxide ion conduction, equimolar Al³⁺ with Ta⁵⁺ have been partly substituted instead of Ti⁴⁺ ions in Zn₂TiO₄, forming Zn₂Ti_1-2xTa_xAl_xO₄ 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 Zn₂Ti_1-2xTa_x:Al_xO ₄ with the dual substitution by x, while apparent increase for Zn_2-x/2Ti_1-xTa_xO₄, indicating that the cation vacancy is responsible for the oxide ion conduction in Zn ₂TiO₄-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.