Spectroscopy analysis of mixed actinide oxides by an An M4,5 edge high energy resolution XANES (HR-XANES) in bulk- and nano-materials

Actinide (An) oxides are of interest for nuclear waste storage and disposal. In this work, valence in as-prepared and sintered nanocrystals [Ux,(Amy,Puz)]O2(+x) were investigated by An M4,5 edge high-energy resolution XANES (HR-XANES).
The mixed actinide oxides were synthesized by hydrothermal decomposition. The crystal structure and morphology were characterized by powder XRD and TEM. The spectroscopic techniques were performed at the CAT-ACT-Beamline at the KIT synchrotron light source, Karlsruhe, Germany. Theoretical calculations of the U M4 and Am M5 HR-XANES spectra were done using a density-functional theory (DFT) based model at the relativistic level of theory.
It is experimentally confirmed that it is more difficult to oxidize Pu(IV) to higher valence compared to U(IV) that was previously suggested by thermodynamic calculations. The spectroscopic study indicates that the U(V) stabilizes on the surface of the nanocrystals with cubic FCC structure and strongly reduces to U(IV) upon sintering at 1600-1650 °C. At the same time, Pu remains as Pu(IV). Unexpectedly, we found that minor contribution of Am (IV) is present in the nanocrystals for Am content down to 5%, likely as a result of surface oxidation of Am (III). ... mehrThe interpretation of the experimental spectra is supported by calculations.