Local disorder in yttrium doped ceria (Ce1-xYxO2-x/2) probed by joint X-Ray and Neutron Powder Diffraction

In recent years yttrium doped ceria oxides (Ce1 12xYxO2 12x/2) have drawn increasing attention as electrolyte for Solid Oxide Fuel Cells because their high ionic conductivity can considerably reduce the operating temperature of such devices, hence limiting the overall manufacturing costs. At increasing doping concentration, the isothermal ionic conductivity reaches a maximum and then decreases [1]. This behavior is attributed to the formation of defect clusters / domains, whose crystallographic structure is still under debate [2]. A precise knowledge of the local structure of these materials is thus needed and can be obtained through the combined real space and reciprocal space analysis of diffraction patterns. A comprehensive diffraction study can be performed due to the different X-ray and neutrons scattering lengths of the involved elements. In particular, neutrons allow determining accurately the oxygen-related parameters, whose contribution to X-Ray scattering is almost negligible when compared to that of cations. To this purpose both X-Ray Powder Diffraction (ESRF, ID31 beamline) and Neutron Powder Diffraction (ILL, D20 and D4 instruments) studies were performed. The effect of doping on the average structure is investigated using the conventional Rietveld analysis, while the Pair Distribution Function (PDF) technique is used to explore the structural distortions and the spatial extent of disorder as well. The reciprocal space analysis shows that the atomic mean square displacements increase as a function of the doping concentration. At the same time, the first coordination shells observed in the real space are not consistent with the mean crystallographic structure and are better described by a biphasic model, i.e. describing the chemical environment of the cations in their pure oxides. Figur