Discovery of a new structure family of oxide-ion conductors “SrYbInO4”

Scientists at Tokyo Institute of Technology (Professor Masatomo Yashima and colleagues, Tokyo Tech) and Australian Nuclear Science and Technology Organisation (Dr. James R. Hester, ANSTO) have discovered a new structure family of pure oxide-ion conductors SrYbInO₄. This new material is expected to lead to the development of solid oxide fuel cells, sensors, and oxygen separation membranes. This study would give a clue to solve energy and environmental problems in future.

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 shown in Fig. 1. Ionic radii of Sr²⁺ and (Yb³⁺, In³⁺) are larger than those of Ca²⁺ and Fe³⁺, respectively, thus the SrYbInO₄ is expected to have a lower activation energy for oxide-ion conductivity compared with CaFe₂O₄.

The structure field map shows 209 different compositions of M₂M′O₄ compounds, with the red hatched area representing the CaFe₂O₄-type structure field. The newly synthesized compound SrYbInO₄ is shown within the CaFe₂O₄-type structure field. ©American Chemical Society.

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 sample was a single orthorhombic phase with Yb/In occupational disordering at the B and C sites, and no vacancies at the cation and oxygen sites (Fig. 2). 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.

Yellow isosurface of the BVE at 0.8 eV suggests that the oxide ion migrates in the b direction along the edge/surface of light blue/purple octahedra BO₆ [B = Yb_0.574(2)In_0.426(2)] and purple octahedra CO₆ [C = In_0.574(2)Yb_0.426(2)] where the number in the parenthesis is the estimated standard deviation. ©American Chemical Society.

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.

Reference

Authors :	Ayaka Fujimoto1, Masatomo Yashima1,*, Kotaro Fujii1, James R. Hester2
Title of original paper :	New Oxide-Ion Conductor SrYbInO4 with Partially Cation-Disordered CaFe2O4-Type Structure
Journal :	The Journal of Physical Chemistry C
DOI :	10.1021/acs.jpcc.7b07911
Affiliations :	1Department of Chemistry and Materials Science, Graduate School of Science and Engineering and Department of Chemistry, School of Science, Tokyo Institute of Technology, Japan 2Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organisation (ANSTO), Australia

• Press release "New structure family of pure oxide-Ion conductors 'SrYbInO4' "
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• Researcher Profile | Tokyo Tech STAR Search - Masatomo Yashima
• Discovery of a new crystal structure family of oxide-ion conductors: NdBaInO4 | Tokyo Tech News
• Discovery of a New Crystal Structure Family of Oxide-Ion Conductors | Tokyo Tech News
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• Australian Nuclear Science and Technology Organisation(ANSTO)
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