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

The local structure of (Zr,Lu,U)O2−x and (Zr,Y,Np)O2−x solid solutions has been investigated by extended X-ray absorption fine structure (EXAFS). Samples were prepared by mixing reactive (Zr,Lu)O2−x and (Zr,Y)O2−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.

Graphical abstractMetal–oxygen bond distances in (Zr,Lu,U)O2−x solid solutions with different oxygen vacancy concentrations (Lu/Zr=1 and Lu/Zr=0.5).Figure optionsDownload full-size imageDownload as PowerPoint slideResearch Highlights► EXAFS indicates high U and Np adaption to the bulk structure of stabilised zirconia. ► ZrO bond length is 2.18 Å at infinite Zr dilution in UO2 and NpO2. ► Low relaxation (strong Pauling behaviour) of Zr explains its low solubility in UO2.