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

In recent years yttrium doped ceria oxides (Ce1-xYxO2-x/2) have attracted increasing attention as electrolyte for Solid Oxide Fuel Cells because their high ionic conductivity can considerably reduce the operating temperature of such devices therefore limiting the overall manufacturing costs. At increasing doping concentration, the isothermal ionic conductivity reaches a maximum and then decreases. This behavior is attributed to the formation of defect clusters / domains, whose crystallographic structure is still under debate. A precise knowledge of the local structure of these materials is then needed and can be obtained through the combined real space and reciprocal space analysis of diffraction patterns. A complementary study can be performed due to the different X-rays and neutrons scattering lengths of the involved elements. In particular Neutrons allow for an accurate determination of oxygen related parameters, whose contribution to X-rays scattering is almost negligible compared to the cations. To this purpose both X-Ray Powder Diffraction (ESRF, ID31 beamline) and Neutron Powder Diffraction (ILL, D20 and D4 instruments) studies have been performed. The effect of doping on the average structure (i.e. on space group, mean atomic positions and atomic mean square displacements) was analyzed using Rietveld while Pair Distribution Function (PDF) Technique was also used to explore structural distortions as well as the spatial extent of disorder in the so called real space. The reciprocal space analysis shows that the atomic mean square displacements increase as a function of the doping concentration. At the same time the local structure obtained by the PDF analysis is not consistent with the mean crystallographic model and it is better described by considering complex defect clusters.