Peculiar behavior of (U,Am)O2−δ compounds for high americium contents evidenced by XRD, XAS, and Raman spectroscopy

In U1 xAmxO2±δ compounds with low americium content (x ≤ 20 at.%) and O/M (oxygen to metal) ratios close to 2.0, trivalent Am is charge-balanced by an equivalent amount of pentavalent U while maintaining the fluorite structure of pure U+IVO2. Up to now, it is unknown, whether this observation holds also for higher americium contents. In this study, we combined therefore X-ray diffraction with Raman and X-ray absorption spectroscopies to investigate U0.5Am0.5O2±δ. Our results indicate that americium is again only present as Am+III, while U+V remains below the amount required for charge balance. Contrary to lower americium contents, this leads to an overall oxygen hypo-stoichiometry 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 (Am+III) and oxidized (U+V) cations. Significant deviations from the fluorite structure are, however, evidenced by both EXAFS and Raman spectroscopy in the oxygen sublattice, with the coexistence of vacancies and interstitials 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, their behavior being closer to that of trivalent-lanthanide-doped UO2 than to that of U1 xPuxO2±δ MOX fuels.