A new form of uranium identified at the ESRF

Heavy elements or actinides, such as uranium and plutonium, have complex electron configurations, i.e. a variety of oxidation states that lead to unusual and diverse bonding behaviours. Studying these bonds and exotic oxidation states can shed light on the chemistry of heavy elements, which is crucial for nuclear energy and managing radioactive waste. In terms of applications, actinide bonding research could improve nuclear fuel design, improve radioactive waste management and could lead to new materials with unique properties.

Now researchers from Helmholtz-Zentrum-Dresden-Rossendorf and the Rossendorf Beamline at the ESRF (BM20) have identified a specific form of uranium, the U(III) oxidation state, and investigated how tightly uranium holds on to its electrons when it bonds with other elements such as fluorine and chlorine.

Clara Silva, first author of the paper, on the BM20 beamline.

For the last 15 years, the team had been studying uranium, which is less radioactive and more abundant than other actinides, and searching for its low valent form. “We tried to find it several times, with samples from different groups, but they always resembled U(IV)”, explains Kristina Kvashnina, head of the Rossendorf Beamline at the ESRF and corresponding author of the publication.

Keeping samples stable

One of the main challenges of low-valent uranium compounds is that they are less stable than other uranium-containing materials and require carefully controlled conditions during the transport and experiment. Kvashnina and her group joined forces with Florian Kraus, a professor at the Philipps-Universität Marburg (Germany), who advised them on how to prevent oxidation of the samples during transportation and the experiment itself.

To keep samples stable, the scientists sealed them under anoxic conditions to prevent the uranium systems from reacting with oxygen in the air, transported under LN2 (liquid nitrogen) conditions and carried out measurements under cryogenic conditions.

Unexpected results

The team used resonant inelastic X-ray scattering (RIXS) and high energy resolution fluorescence detection (HERFD) at the Uranium M4 edge to study the uranium on BM20.

“The results were unexpected because in general,  all U M4 HERFD data exhibit 1 single peak, whilst the U(III) systems showed two peaks in the experiment, so interpreting the data was a major challenge”, explains Clara Silva, PhD student at the Rossendorf beamline and first author of the publication. To overcome this, they applied cutting-edge electronic structure calculations to model the experimental data and extract detailed information about the electron transfer in the uranium systems and the nature of the actinide bonding. “We have now a deeper understanding of the uranium systems and actinides in general, which are the most complex elements in the periodic table”, she says.

The results also challenge existing theories and open up new avenues of research in the actinide field. The next step for the team is to analyse other elements in the actinide series and systematically study how the behaviour of electrons changes as we move towards the bottom of the periodic table.

Reference:

Silva, C.L., et al. On the origin of low-valent uranium oxidation state. Nat Commun 15, 6861 (2024). https://doi.org/10.1038/s41467-024-50924-7

Text by Montserrat Capellas Espuny