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

Abstract

To investigate the defect structure of Zn2-x/2Ti1-xTaxO4 and the subsequent oxide ion conduction, equimolar Al 3+ with Ta 5+ have been partly substituted instead of Ti 4+ 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.

Defects and oxide ion transport properties in the substituted Zn2TiO4

“…5) These values were essentially in agreement with the products of · total and ionic transport numbers t ion obtained from EMFs of oxygen gas concentration cells, 1) which were also plotted by open circles. Moreover, consistent plotting was certainly possible in the aluminum-doped systems with a fixed tantalum content, Zn 1.925+y/2 Ti 0.85¹y Ta 0.15 Al y O 4 4) (open triangle symbols in Fig. 3) and no cation vacancy, Zn 2 Ti 1¹2x -Ta x Al x O 4 (rectangle symbols).…”

“…In the previous experiments on Zn 2¹x/2 Ti 1¹x -Ta x O 4 system, 9),10) the dielectric relaxation behavior was found to be strongly influenced by the change in crystal symmetry, i.e., frequency dependence of the imaginary part of the electric modulus appeared as double peaks when the structure varied from cubic to tetragonal with the tantalum substitution. On the contrary, Zn 2¹(x¹y)/2 Ti 1¹x¹y Ta x Al y O 4 system (x = 0.15) did not show such a phase change due to the introduction of cation vacancy, 4) which were then employed in the present relaxation measurement. Figure 4 showed the typical frequency dependence of the imaginary part of the electric modulus M¤¤ for (a) y = 0 and (b) y = 0.15 samples of Zn 2¹(x¹y)/2 Ti 1¹x¹y Ta x Al y O 4 (x = 0.15).…”

“…1)3) Since the conductivity enhancement was not observed in the equimolar amounts of Al 3+ and Ta 5+ co-doped system with no cation vacancy, Zn 2 Ti 1¹2x Ta x Al x O 4 , oxide ion conduction was surely induced by the resultant cation vacancy rather than the substitution of tantalum. 4) The previous structure analysis based on TOF neutron diffraction 5) suggested the oxide ion conduction would occur through the interstitialcy diffusion mechanism, 6) in which a regular oxide ion moves toward the octahedral cation vacancy leaving an oxide ion vacancy behind, allowing the neighboring oxide ions to migrate via newly formed oxide ion vacancies. However, pristine Zn 2 TiO 4 still exhibited a small degrees of oxide ion conduction, 2) which seemed to contradict the above simple interstitialcy diffusion model; the actual conduction mechanism has been supposed to be more complex.…”

Positron Annihilation in Off‐Stoichiometric and Ta‐Doped Zn2TiO4

“…Recent interest in Zn 2 TiO 4 as an electronic ceramic that displays ionic conduction has focused on the charge compensation mechanisms exhibited when the material contains excess Ti or is doped with pentavalent Ta ions . The stoichiometric material Zn 2 TiO 4 has an inverse spinel structure with a tetrahedral Zn ion and octahedral Zn and Ti ions.…”

“…R ECENT interest in Zn 2 TiO 4 as an electronic ceramic that displays ionic conduction has focused on the charge compensation mechanisms exhibited when the material contains excess Ti 1 or is doped with pentavalent Ta ions. 2,3 The stoichiometric material Zn 2 TiO 4 has an inverse spinel structure with a tetrahedral Zn ion and octahedral Zn and Ti ions. Kim et al 1 have shown that compositions of Zn 2 TiO 4 + xTiO 2 , when equilibrated in air at temperatures above 945°C formed a single-phase Zn 2 TiO 4 structure for x up to~0.33 and attributed the charge compensation mechanism to tetrahedral Zn vacancies.…”