Scientists at Tokyo Institute of Technology and the Australian Nuclear Science and Technology Organisation (ANSTO) have discovered a new oxide-ion conductor SrYbInO4 with a CaFe2O4-type structure. SrYbInO4 is the first example of a CaFe2O4-type pure oxide-ion conductors in which the oxide-ion conduction is dominant.
This new material, described in a paper in the ACS Journal of Physical Chemistry C may lead to the development of new solid oxide fuel cells, batteries, sensors, and oxygen separation membranes.
Ceramic oxide-ion (O2−) conductors are crystalline inorganic materials exhibiting O2− conduction and have attracted considerable attention owing to their many applications in solid oxide fuel cells (SOFCs), batteries, catalysts, gas sensors and oxygen separation membranes. Since the oxide-ion conduction paths in the crystal structure are governed by the cation network, high oxide-ion conduction occurs in limited crystal structure families such as fluorite-, perovskite- and K2NiF4-type structures. Therefore, the discovery of a new structure family of oxide-ion conductors is important for innovative developments in materials chemistry.
Since some of A2BO4-based materials such as (Pr,La)2(Ni,Cu,Ga)O4+δ are known to exhibit high oxide-ion conductivity, we have explored new structure families of ABCO4-based oxide-ion conductors in the form of BaRInO4 (R: Rare earths). 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. Through the exploration of many compositions, we discovered a new structure family of oxide-ion conductors BaNdInO4, which led to a rich chemistry of related materials as doped BaNdInO4 and BaNdScO4. In the present work, we have explored new materials SrRInO4 (R: rare earths) as a new structure family of ABCO4 oxide-ion conductors and report a new material SrYbInO4. As shown later, A, B, and C in ABCO4 are Sr, Yb0.574(2)In0.426(2) and In0.574(2)Yb0.426(2), respectively, in SrYbInO4.—Fujimoto et al.
Many researchers have investigated the optical, electrical, and magnetic properties of CaFe2O4-type materials, which include possible oxide-ion conduction and good thermal and chemical stability. However, CaFe2O4-type pure oxide-ion conductors have not been reported yet.
Tokyo Tech Professor Masatomo Yashima and his colleagues synthesized the new CaFe2O4-type material, strontium ytterbium indium oxide, SrYbInO4. Prof. Yashima and colleagues chose SrYbInO4 because it contains no transition-metal cation, which leads to less electronic conduction.
The 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.
They investigated its crystal structure from room temperature to 1273 K (1,000 ˚C), its temperature and partial pressure dependence of electrical conductivity, and oxide-ion diffusion pathways.
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. 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.
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.
Ayaka Fujimoto, Masatomo Yashima, Kotaro Fujii, and James R. Hester (2017) “New Oxide-Ion Conductor SrYbInO4 with Partially Cation-Disordered CaFe2O4-Type Structure” The Journal of Physical Chemistry C 121 (39), 21272-21280 doi: 10.1021/acs.jpcc.7b07911