Probing complex disorder in Ce1-xGdxO2-x/2 using the Pair Distribution Function Analysis

In this work the first Pair Distribution Function (PDF) study on Ce1-xGdxO2-x/2 (CGO) electrolytes for solid oxide fuel cells is presented, aiming to unveil the complex positional disorder induced by gadolinium doping and oxygen vacancies formation in these materials. The whole range of Gd concentration xGd (0 <= xGd <= 1) of the CGO solid solutions 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 the 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 system. Gd-rich and Ce-rich droplets coexist in the subnanometric range.