Unveiling the local structure of doubly doped CeO2: a synchrotron X-ray pair distribution function study

Rare earth-doped ceria systems form one of the most thoroughly studied materials classes to be employed as solid electrolytes in solid oxide cells. In order to unveil the structural details ruling the ionic conductivity properties, the (Ce1-xREx)O2-x/2 series with RE = (Nd0.63Dy0.37), (Nd0.74Tm0.26), and (Sm0.67Gd0.33), and x ranging between 0.05 and 0.60, has been synthesized and characterized by means of synchrotron X-ray powder diffraction and pair distribution function analyses. By coupling the mentioned techniques, the whole spectrum of structural order and disorder characterizing these compounds at different length scales was revealed and discussed in detail. With reference to the average scale, for x >= 0.40 or 0.50, depending on the system, the Fmm crystal structure of the CeO2-based solid solution is accompanied by defect aggregates with the Ia structure growing within the ceria matrix. The local structure (r < 10 & Aring;), in contrast, even at the lowest doping degree can be described as a disordered fluctuation between the average structures characterizing the Fmm and Ia phases, which explains the reduction in ionic conductivity observed even within the stability range of the CeO2-based solid solution.