Cationic charge distribution in mixed actinide oxides at An M4,5 edge by HR-XANES in mixed nanooxides

The structural properties of actinide (An) oxides are of particular interest for a number of applications including nuclear waste storage and disposal. In this work, cationic charge distribution in as-prepared and sintered nanocrystals [Ux,(Amy,Puz)]O2(+x) (y=1-20%, z=20%) were investigated. In order to evaluate the oxidation states of the actinide elements, An (An = U, Pu or Am) M4,5 edge high-energy resolution XANES (HR-XANES) and An L3 edge XANES were applied and the results from the two techniques were compared.
The mixed actinide oxides were synthesized by hydrothermal decomposition of mixed actinide oxalates. The crystal structure and morphology of the materials were characterized by powder XRD and TEM. The HR-XANES and XANES spectroscopic techniques were performed at the CAT-ACT-Beamline and INE-Beamline, respectively, 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. Multiplet effects were also included on top of the DFT to simulate the fine structures of the XANES spectra. ... mehr
An experimental verification that it is more difficult to oxidize Pu(IV) to higher oxidation states compared to U(IV) in actinide dioxide as it was previously suggested by thermodynamic calculations is presented. The spectroscopic results indicate that the U(V) stabilizes on the surface of the nanocrystals (10-13 nm size) 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) in as prepared and sintered nanocrystals. 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 the predominant Am(III). The interpretation of the experimental spectra is supported by calculations.

This project has received funding from the European Union’s Horizon 2020 research and innovation program, ERC Consolidator Grant 2020 “The ACTINIDE BOND” under grant agreement No. 847593.