Structural Relations in the Nd2O3-CeO2 Pseudo Binary System

Due to its similar crystal structure, properties and lack of radioactivity, CeO₂ is often used as a surrogate for UO₂ in research. Complex oxide phases in the CeO₂:Ln₂O₃ (Ln = Lanthanide element) system can be used to study the structural changes in UO₂ fuel as lanthanide fission products are incorporated into the fuel’s crystal lattice. The pseudo-binary phase diagram CeO₂–Nd₂O₃ has not been systematically studied. Indeed, the random defect fluorite model for the low Nd doping region (about 10 at.%) was extrapolated to the central part of the pseudo-binary phase diagram, though the lattice parameter dependence was not linear with the composition, suggesting changes in the stacking of the charge compensating oxygen vacancies. Chemical homogeneity in these systems requires high temperature annealing due to the low mobility of cations, which expose the system to kinetic demixing, surface segregation during quenching and out-of-equilibrium processes. In order to achieve chemical homogeneity and, at the same time to avoid out-of-equilibrium processes, various compositions of Ce_1-xNd_xO_2-x/2 were fabricated at low temperature as nanocrystalline powders using the freeze-drying method. The structures of these compounds were studied by x-ray diffraction and selected area electron diffraction. We observe a departure from the random oxygen-deficient fluorite structural model. A set of structures derived from the fluorite were used to model the effect of the vacancy order upon the diffracted intensities. While the fluorite structure remains present at moderate doping levels, a phase change occurs in the region 0.36 < x_Nd < 0.48, marked by the appearance of low intensity superstructure peaks in the diffraction patterns. The data is best described by a monoclinic RE₆O₁₁-type structure (SG P21/c), often referred to as “β phase” in LnO_2-x (Ln = Ce, Pr, Tb) binary compounds. These findings require a critical reassessment of the equilibrium phase diagram of this pseudo-binary system. This work was funded by EPSRC through the Next Generation Nuclear Centre for Doctoral Training (NGN-CDT) programme.