Analysis of Crystal Structure of BaCe0.80Y0.20O3-δ at Room Temperature with Various Diffraction Technique

Abstract

At present, operating temperature of solid oxide fuel cells (SOFCs) is as high as 800-1000 ° C due to requirement of sufficient oxide ion conductivity of electrolyte material. Under such high temperature operation, there remain problems of reliability, durability and limitation of materials. Development of electrolyte oxide which exhibits enough ionic conductivity at temperatures between 400-600 ºC should be required to solve the problems, resulting in development of so-called intermediate temperature SOFC (IT-SOFC). One of the probable methods to decrease the operation temperature is employing proton conducting oxide as electrolyte instead of oxide ion conducting oxide since lower energy is required for light proton transportation. Among proton conducting oxides, BaCe1−x Y x O3-δ shows the highest proton conductivity. Not only for practical application but also for elucidation of high proton conduction mechanism, information on accurate crystal structure of BaCe1−x Y x O3-δ is essential. So far, it has been reported that crystal structures of BaCe1-x Y x O3-δ are orthorhombic distorted perovskite and monoclinic distorted one for x=0.00-0.10 and x=0.15-0.20, respectively, at room temperature. The crystal structures have mainly been analyzed with Rietveld analyses of powder X-ray diffraction or neutron diffraction patterns. However, space groups employed for Rietveld analyses were not experimentally evidenced ones but assumed to be Pnma (No. 62) and I2/m (No. 12) for orthorhombic and monoclinic phase, respectively, in the most studies reported so far. In this study, we have clarified the space group of BaCe0.80Y0.20O3- δ by convergent beam electron diffraction. In addition, Rietveld analyses of powder X-ray diffraction and neutron diffraction patterns of BaCe0.80Y0.20O3- δ have been examined to elucidate minute crystal structure. BaCe0.80Y0.20O3-δ was prepared with Pechini method, with starting materials of BaCO3, CeO2 and Y2O3. BaCO3 was dissolved with citric acid. CeO2 and Y2O3 were dissolved with dilute HNO3 containing H2O2. After addition of ethylene glycol to mixed solution with stoichiometric cation composition, the solution was heated at 300 °C and burned to resin. The resin was calcined at 700 °C for 24 hour in air, followed by uniaxially pressing into pellets. The pellets were sintered at 1300~1400 °C for 10 hour in air. The obtained phase was revealed to be a single phase of monoclinic distorted perovskite structure with a=6.239 Å, b=8.740 Å, c=6.249 Å and β=91.06 º by X-ray diffraction measurement. Convergent beam electron diffraction (CBED) patterns were obtained at room temperature by using energy filtered TEM (JEOL Co., Ltd.: JEM-2010FEF) with operating voltage of 100 kV. X-ray diffraction pattern was obtained with RINT-2500 (Rigaku Co., Ltd.: CuKα, 50 kV, 250 mA). TOF neutron diffraction measurement was carried out using superHRPD at J-PARC. The Rietveld refinements for X-ray diffraction pattern and neutron diffraction one were carried out using RIETAN-FP [1] and Z-Rietveld [2], respectively. Figure attached shows zero-order Laue zone reflections of CBED pattern of BaCe0.80Y0.20O3-δ from [001] azimuth. The observation of the disc identified as 0k0 with odd k denied I-lattice. Space group of I2/m was also denied from A2-type dynamical extinction indicated by arrows, indicating existence of screw axis or glide plane. Combining various CBED patterns from various azimuths, space group of BaCe0.80Y0.20O3-δ was concluded to be P21/m (No. 11), which was a subgroup of Pnma (No. 62) of BaCeO3 [3]. It can be explained that substitution of Y and/or generation of oxide ion vacancy decreased crystal symmetry of BaCeO3. The both X-ray diffraction pattern and the neutron diffraction pattern could successfully be refined by Rietveld analysis with structure model of both I2/m and P21/m. This indicates that accurate space group cannot be clarified by only Rietveld analysis of powder diffraction patterns. The tilts of octahedrons of perovskite were represented as “a + b - c -“ and “a 0 b - c -“ for P21/m and I2/m, respectively [3]. It can be speculated that tilt angle according to a-axis in P21/m phase is so small that diffraction patterns of BaCe0.80Y0.20O3- δ can be refined by Rietveld analysis with space group of both I2/m and P21/m. [1] F. Izumi and K. Momma, Solid State Phenom., 130 (2007) 15. [2] R. O-Tomiyasu et. al. J. Appl. Cryst., 45 (2012) 299. [3] C. J. Howard and H. T. Stokes, Acta Cryst. B54 (1998) 782. Figure 1