A new structure family of oxide-ion conductors 'SrYbInO4'

27-Oct-2017 - Japan

Because some A₂BO₄-based materials such as (Pr,La)₂(Ni,Cu,Ga)O_4+δ exhibit high oxide-ion conductivity, scientists at Tokyo Tech have been exploring new structure families of ABCO₄-based materials as BaRInO₄, where R represents a rare earth element. Here, A, B, and C are cations located at different crystallographic sites, and A, B, and C in ABCO₄ correspond to A, A, and B, respectively, in A₂BO₄.

Many researchers have investigated the optical, electrical, and magnetic properties of CaFe₂O₄-type materials, but the CaFe₂O₄-type pure oxide-ion conductors have not been reported yet. Therefore, Professor Masatomo Yashima and colleagues synthesized a new CaFe₂O₄-type material, strontium ytterbium indium oxide, SrYbInO₄ . They investigated its crystal structure from room temperature to 1273 K, its temperature and partial pressure dependence of electrical conductivity, and oxide-ion diffusion pathways. The occupancy factors are also carefully refined using not only conventional X-ray diffraction data but also time-of-flight (TOF) and angle-dispersive-type neutron and synchrotron X-ray diffraction data in order to obtain reliable results. They demonstrate a partial Yb/In occupational disorder in SrYbInO₄ through careful analyses of occupancy factors.

Prof. Yashima and colleagues have chosen the chemical composition SrYbInO₄, because it contains no transition-metal cation, which leads to less electronic conduction. Moreover, SrYbInO₄ was expected to have the CaFe₂O₄-type structure in the structure field map. Ionic radii of Sr²⁺ and (Yb³⁺, In³⁺) are larger than those of Ca²⁺ and Fe³⁺, thus the SrYbInO₄ is expected to have a lower activation energy for oxide-ion conductivity compared with CaFe2O4.

SrYbInO₄ was synthesized by a solid-state reaction. SrYbInO₄ was characterized through X-ray diffraction, chemical analysis, and thermogravimetric analysis. The band gap of SrYbInO₄ was also estimated using UV-vis reflectance spectra, which suggested that SrYbInO₄ is an electronic insulator. These results strongly suggested that SrYbInO₄ was a pure oxide-ion conductor.

Using neutron and synchrotron X-ray diffraction data and Rietveld method, Prof. Yashima and colleagues showed that SrYbInO₄ is a single orthorhombic phase with Yb/In occupational disordering at the B and C sites, and no vacancies at the cation and oxygen sites. Bond valence sums and DFT-based structural optimization indicated the validity of the refined crystal structure of SrYbInO₄. Therefore, the new material SrYbInO₄ is the first example of pure oxide-ion conductors with a CaFe₂O₄-type structure.

Further the temperature dependence of oxide-ion conductivity showed lower activation energy of SrYbInO₄ (1.76 eV) than that of CaFe₂O₄ (3.3 eV), which was supported also by the bond valence-based energy calculations. The lower activation energy is attributable to the larger bottleneck size for oxide-ion migration due to the larger ionic radii of Sr²⁺ and (Yb³⁺, In³⁺) than those of Ca²⁺ and Fe³⁺, respectively.

Prof. Yashima and colleagues claimed that the oxide ion conductivity of SrYbInO₄ could be improved by doping, changing the degree of cation ordering and disordering, and using larger A, B, and C cations in the ABCO₄-structure, which leads to further lowering the activation energy and higher oxide-ion conductivity. The findings of this study may open new pathways in the development of ABCO₄-based ion conductors.

Original publication

Ayaka Fujimoto, Masatomo Yashima, Kotaro Fujii, and James R. Hester; "New Oxide-Ion Conductor SrYbInO4 with Partially Cation-Disordered CaFe2O4-Type Structure"; J. Phys. Chem. C; 2017

https://www.chemeurope.com/en/news/165466/a-new-structure-family-of-oxide-ion-conductors-srybino4.html

Original publication

Ayaka Fujimoto, Masatomo Yashima, Kotaro Fujii, and James R. Hester; "New Oxide-Ion Conductor SrYbInO4 with Partially Cation-Disordered CaFe2O4-Type Structure"; J. Phys. Chem. C; 2017

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