Peculiar Behavior of (U,Am)O2−δ Compounds for High Americium Contents Evidenced by XRD, XAS, and Raman Spectroscopy

• Published in: Inorganic chemistry 2015-10, Vol.54 (20), p.9749-9760
• Main Authors: Lebreton, Florent, Horlait, Denis, Caraballo, Richard, Martin, Philippe M, Scheinost, Andreas C, Rossberg, Andre, Jégou, Christophe, Delahaye, Thibaud
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.5b01357

In U1–x Am x O2±δ compounds with low americium content (x ≤ 20 atom %) and oxygen-to-metal (O/M) ratios close to 2.0, AmIII+ cations are charge-balanced by an equivalent amount of UV+ cations while the fluorite structure of pure UIV+O2 is maintained. Up to now, it is unknown whether this observation also holds for higher americium contents. In this study, we combined X-ray diffraction with Raman and X-ray absorption spectroscopies to investigate a U0.5Am0.5O2±δ compound. Our results indicate that americium is again only present as AmIII+, while UV+ remains below the amount required for charge balance. Unlike lower americium contents, this leads to an overall oxygen hypostoichiometry with an average O/M ratio of 1.92(2). The cationic sublattice is only slightly affected by the coexistence of large amounts of reduced (AmIII+) and oxidized (UV+) cations, whereas significant deviations from the fluorite structure are evidenced by both extended X-ray absorption fine structure and Raman spectroscopies in the oxygen sublattice, with the observation of both vacancies and interstitials, the latter being apparently consistent with the insertion of U6O12 cuboctahedral-type clusters (as observed in the U4O9 or U3O7 phases). These results thus highlight the specificities of uranium–americium mixed oxides, which behave more like trivalent lanthanide-doped UO2 than U1–x Pu x O2±δ MOX fuels.