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

Abstract

Powder neutron diffraction experiments with Rietveld analyses were carried out on pure and Ta-substituted Zn 2 TiO 4 at room temperature to investigate the defect structure together with the oxide ion conduction mechanism. The refined structure of pure-Zn 2 TiO 4 agreed well with the previously reported inverse spinel-type structure with cubic symmetry. For Zn 2 x2 Ti 1 x Ta x O 4 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.

Defects and oxide ion transport properties in the substituted Zn2TiO4

“…3), assuming all the cation vacancies were formed in the octahedral sites. 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.…”

“…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.…”

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

“…This implies that tantalum substitution and cation vacancy formation were made at octahedral site as expressed by Zn[Zn1-x/2Ti1-xTax□x/2]O4, which was consistent with the previous TOF neutron diffraction study. 8) On the other hand for Zn2Ti1-2xTaxAlxO4, extra diffraction peaks attributed to ZnO phase were detected as shown in Fig. 5 (b).…”

“…In Zn2-x/2Ti1-xTaxO4 system, the obtained diffraction data could be analyzed based on the cubic structured single phase, whereas Rwp value was relatively poor in comparison with the previous study. 8) This might be due to smaller number of reflections to analyze in addition to difficulty of the cryostat set-up through which the incident and diffracted neutrons passed. Refinements were therefore carried out assum- …”

“…7) Our previous experiments using pulsed neutron diffraction showed that the cation vacancies form at the octahedral site of the substituted spinel-type structure. 8) Moreover, an interstitialcy diffusion mechanism was estimated where oxide ion would displace toward the cation vacancy to make the quasi-oxide ion vacancies, by evaluating the occupation factors of the tentatively located small amount of oxide ion interstitials. Nevertheless, it would be still hard to declare that only the cation vacancy must be essential for the oxide ion conduction, because this type of defect structure was accompanied by the introduction of tantalum ions and the effect of the substituted tantalum itself has been uncertain.…”