Defect clustering in Ce1-xGdxO2-x/2 using the PDF Analysis

Aim of this work is to unveil the complex positional disorder induced by gadolinium doping and oxygen vacancies formation in Ce1-xGdxO2-x/2 (CGO) electrolytes for solid oxide fuel cells by means of the Pair Distribution Function (PDF) analysis. The whole range of Gd concentration xGd (0 < xGd < 1) of the CGO solid solution(s) was investigated through high resolution synchrotron radiation powder diffraction. The reciprocal space Rietveld analysis revealed in all the solid solutions the presence of positional disorder, which has been explicitly mapped into the real space. The average structural models, as obtained by Rietveld method, fit well the experimental PDF data only for a spatial range r > ~10 Å. The same models applied at lower r values fails to reproduce the experimental curves. A clear improvement of the fit quality in the 1.5 < r < ~6 Å range was obtained for all the CGO samples applying a biphasic model encompassing both a fluorite CeO2-like and a C-type Gd2O3-like phases. This provides evidence that extended defects at local scale exist in the CGO. Gd-rich and Ce-rich droplets coexist in the sub-nanometric range.