A New Crystal Structure Family of Oxide-Ion Conductors NdBaInO4
Kotaro Fujii, Yuichi Esaki, Kazuki Omoto and, Masatomo Yashima (Tokyo Institute of Technology), Akinori Hoshikawa and Toru Ishigaki (Ibaraki University) and James Hester (ANSTO)
Neutron and synchrotron X-ray diffraction up to 1000 °C have led to the discovery of a new group of crystal structures (atomic arrangements) that have not been reported before (i.e. a new structure family), NdBaInO4. It was found that the crystal structure consists of (i) (Nd,Ba)InO3 perovskite and (ii) A-rare earth oxide Nd-O units, and that the oxide ions (O2-) can move two-dimensionally in the Nd-O unit. This discovery may open a new window for high-performance sensors, fuel cells, catalysts, electronic materials and so on.
The structure was determined from neutron diffraction data taken on our high-resolution powder diffractometer ECHIDNA (located at the Bragg Institute), the two Ibaraki-prefecture neutron powder diffractometers at J-PARC, and using X-ray diffraction data measured at the SPring-8 and Photon Factory synchrotrons located in Japan.
Oxide-ion conducting materials such as pure oxide-ion conductors and mixed oxide ion-electronic conductors have a wide variety of applications in fuel cells, oxygen-separation membranes and gas sensors. Since the oxide-ion conductivity is strongly dependent on the crystal structure, the discovery of the new oxide-ion conductor belonging to a new structure family may open a new window for further innovative developments in the applications of oxide-ion conductors.
In order to design a new layered perovskite-related structure, the Tokyo Institute of Technology research group has examined various chemical compositions of AA'BO4 where A and A' are larger cations and B is a smaller cation. After examining a number of chemical compositions, the group discovered a new structure family of oxide-ion conducting materials NdBaInO4. The Nd, Ba and In were chosen as cations, because (i) the different sizes of Nd and Ba can lead to the Ba/Nd cation ordering and (ii) the BaInO3 perovskite unit can form from the view point of the sizes of Ba and In cations. NdBaInO4 was prepared by solid-state reactions at 1400 °C using BaCO3, In2O3 and Nd2O3 powders.
The present NdBaInO4 sample is a single monoclinic phase with a new crystal structure as described below. Thus a new structure family of oxide-ion conducting material NdBaInO4 was discovered in this study.
The crystal structure of NdBaInO4 was investigated by neutron and synchrotron X-ray powder diffractometry and ab initio electronic calculations. The research group carried out the ab-initio crystal structure analysis using the X-ray powder diffraction data. Neutron diffraction enables the precise determination of positional parameters of oxygen atoms. The space group was found to be monoclinic P21/c. The validity of the crystal structure of NdBaInO4 was confirmed by Rietveld refinements of synchrotron X-ray powder diffraction data measured at (i) SPring-8 and at (ii) PF, by Rietveld analysis of neutron diffraction data measured at (iii) J-PARC and at (iv) ANSTO, (v) by bond-valence sums of Nd, Ba and In atoms, (vi) by the structural optimization based on the density functional theory calculations.
The refined crystal structure of NdBaInO4 consists of A-rare earth oxide A-O (Nd-O) and perovskite (A,A')BO3 (Nd2/8Ba6/8InO3) units, which indicates a new A/A' cation ordered perovskite-related layered structure. An outstanding and unique feature of this new structure is that the edge of the InO6 octahedron faces the A-O (Nd-O) unit. The oxide-ion diffusion path of NdBaInO4 was visualized using the bond-valence method (Fig. 2). The oxide ions can diffuse two-dimensionally in the A-O (Nd-O) unit.
Fujii, K; Esaki, Y; Omoto, K; Yashima, M; Hoshikawa, A; Ishigaki, T and Hester, JR , New Perovskite-Related Structure Family of Oxide-Ion Conducting Materials NdBaInO4, Chem. Mater. 26, 2488-2491 (2014)