Crystal structure and disorder in the oxide ion conductors Y1-xTaxO1.5+x (x=0.2 and 0.25)

Department of Materials Science and Engineering, Tokyo Institute of Technology
â—‹Takayuki Tsuji Masatomo Yashima

We report the results of a neutron powder diffraction study of the Y0.8Ta0.2O1.7 and Y0.75Ta0.25O1.75 compounds at room temperature. Y0.8Ta0.2O1.7 and Y0.75Ta0.25O1.75 materials were prepared by solid-state reactions. Neutron-diffraction data for both samples were collected at 299K on a multi-detector fixed wavelength powder diffractometer (HERMES) installed at the JRR-3M research reactor of the Japan Atomic Energy Association (JAEA), by the Institute for Materials Research, Tohoku University. A neutron beam with a 1.8207 angstrom unit wavelength was obtained by the (3 3 1) plane of a Ge monochromator. The diffraction data were analyzed by the Rietveld method, maximum entropy method (MEM) and MEM-based pattern fitting, in order to refine crystal parameters and to investigate the nuclear density distribution. The diffraction pattern of Y0.8Ta0.2O1.7 exhibited a background with a complicated profile shape due to diffuse scattering. Thus, we subtracted the background using a computer program PowderX before the Rietveld analysis. In the orthorhombic Y0.75Ta0.25O1.75compound, the Y and Ta atoms were ordered and the oxide ions were localized at around the stable position. On the contrary, in the defect fluorite-type cubic Y0.8Ta0.2O1.7 material, the Y and Ta atoms were disordered and the oxide ions exhibited a large spatial distribution. The large distribution of oxide ions is responsible for the high oxygen diffusion constant and conductivity at high temperatures.