Francisco J. Espinosa-Faller

Complex behavior in quaternary zirconias for inert matrix fuel: what do these materials look like at the nanometer scale?

Conventional crystallographic analysis of cubic stabilised zirconias (CSZ) over a wide range of stabilisers and stoichiometries has shown O atom displacements along lattice vectors and cation disorder with irreproducible characteristics. However, because diffraction is only sensitive to the average long-range order and biased towards periodic and symmetric distortions, it is essential to supplement it with probes of short-range order that may reveal additional, element-specific details about local ordering. We have performed X-ray Absorption Fine Structure (XAFS) spectroscopy measurements on a series of Er-Y-(U-Ce)-Zr oxides to determine the local environment around each of the cations. Results from these experiments demonstrate that not only are there significant element- and composition-dependent distortions away from the crystallographic structure but also interactions between specific pairs of the metal ions indicative of cooperative behaviour. This suggests that some of the metal ions are not randomly, isomorphically substituted into the lattice but instead have tendencies to form nanometer-scale clusters and networks. In this respect, substantial differences are seen in the influence of Ce and U on the Y-centered distortions, consistent with different roles for these two elements in inert matrix fuel (IMF) matrices. This type of nanometer-scale heterogeneity is typically coupled to the phase stability and other properties of other materials and may therefore be important to these zirconias in their IMF application.