Local structure in solid solutions of stabilised zirconia with actinide dioxides (UO2, NpO2)

The local structure of (Zr,Lu,U)O_2−x and (Zr,Y,Np)O_2−x solid solutions has been investigated by extended X-ray absorption fine structure (EXAFS). Samples were prepared by mixing reactive (Zr,Lu)O_2−x and (Zr,Y)O_2−x precursor materials with the actinide oxide powders, respectively. Sintering at 1600 °C in Ar/H₂ yields a fluorite structure with U(IV) and Np(IV). As typical for stabilised zirconia the metal-oxygen and metal-metal distances are characteristic for the different metal ions. The bond lengths increase with actinide concentration, whereas highest adaptation to the bulk stabilised zirconia structure was observed for UO and NpO bonds. The ZrO bond shows only a slight increase from 2.14 Å at 6 mol% actinide to 2.18 Å at infinite dilution in UO₂ and NpO₂. The short interatomic distance between Zr and the surrounding oxygen and metal atoms indicate a low relaxation of Zr with respect to the bulk structure, i.e. a strong Pauling behaviour.