Charge Distribution in U_1–xCe_xO_2+y Nanoparticles

Abstract

In view of safe management of the nuclear wastes, a sound knowledge of the atomic-scale properties of U 1−x M x O 2+y nanoparticles is essential. In particular, their cation valences and oxygen stoichiometries are of great interest as these properties drive their diffusion and migration behaviors into the environment. Here, we present an in-depth study of U 1−x Ce x O 2+y , over the full compositional domain, by combining X-ray diffraction and high-energy resolution fluorescence detection X-ray absorption near-edge structure. We show, on one hand, the coexistence of U IV , U V , and U VI and, on the other hand, that the fluorite structure is maintained despite this charge distribution.

Synthesis and characterization of homogeneous (U,Am)O2 and (U,Pu,Am)O2 nanopowders

“… 11 The results showed that the nanocrystalline solid solutions FCC structure formed up to a composition of U 0.5 Ce 0.5 O 2 , which indicates that this method might be applied for materials with an americium content up to 50%; above this concentration, the charge compensation needs further oxidation of uranium into hexavalent form. Note that nanocrystalline solid solutions with FCC structure can form over the full compositional range in the uranium–cerium system by using an alternative precursor 36 (hydrothermal treatment of mixed hydroxides).…”

“…This result tends to confirm that the oxidation of the nanocrystals occurred under storage conditions before the HR-XANES measurement, due to the very high sensitivity of such materials against oxidation, as it was observed previously. 36 …”

“…16,35 Note that the time from synthesis to measurement was 6 months for the nc-U 0.80 Am 0.20 O 2 . As previously described for (U,Ce)O 2 nanoparticles, 36 the oxidation of the material occurs under storage conditions due to the high sensitivity for oxidation of these materials due to their high specific surface area. Given the size of the nanocrystals (10 ± 2 nm) in nc-U 0.80 Am 0.20 O 2 , and the storage condition (nitrogen atmosphere with up to 1% O 2 ), it can be expected that this majority of U(V) in the nanocrystalline samples is not the result of the synthesis route, but the result of the oxidation of the material before the HR-XANES measurement.…”

“…11 The results showed that the nanocrystalline solid solutions FCC structure formed up to a composition of U 0.5 Ce 0.5 O 2 , which indicates that this method might be applied for materials with an americium content up to 50%; above this concentration, the charge compensation needs further oxidation of uranium into hexavalent form. Note that nanocrystalline solid solutions with FCC structure can form over the full compositional range in the uraniumcerium system by using an alternative precursor 36 (hydrothermal treatment of mixed hydroxides). 3.4 Cationic charge distribution sintered and nc-U 0.80 Am 0.20 O 2 HR-XANES spectroscopy data at the U M 4 edge for assynthesised nc-U 0.80 Am 0.20 O 2 revealed the presence of mixed U(IV) and U(V) oxidation states, with majority of the U atoms accommodating U(V) oxidation state.…”

“…This result tends to confirm that the oxidation of the nanocrystals occurred under storage conditions before the HR-XANES measurement, due to the very high sensitivity of such materials against oxidation, as it was observed previously. 36 Fig. 8 HR-XANES spectra acquired at U M 4 (a) and Am M 5 (b) edges for nc-U 0.80 Am 0.20 O 2 , nc-U 0.80 Am 0.20 O 2 after sintering and UO 2 , BiUO 4 , AmVO 4 , AmO 2 reference materials.…”