New research shows how chemical bonding distorts the 5f orbital in early actinides, evidencing computational ideas of covalency that have previously been difficult to prove. By analysing hexachlorides of tetravalent uranium, neptunium and plutonium with x-ray scattering, scientists were able to discern the inner and outer components of the 5f orbitals. It revealed that as you move along the series, the outer component of the 5f orbitals increases, while the inner component expands less due to increasing nuclear charge.
These observations are significant because the presence or absence of covalency in the 5f orbitals and how covalency changes across the actinides – if it exists at all – is contentious and has been for many decades. This is due, in part, to it being very difficult to probe these concepts experimentally because of the radioactive nature of the elements involved and the specialist analytical techniques required.
Thomas Dumas, who led the synthesis and analysis at the CEA Marcoule and the University of Montpellier in France, describes the study as an ‘experimental anchor’ for theoretical descriptions of 5f radial wavefunction behaviour. ‘The theoretical chemistry helps the experimental interpretation, while confirming the calculations are well designed and can produce a predictable result,’ he says.
Michael Baker, who initiated the project and led the extensive analytical work at the University of Manchester in the UK, says working with the team in France ‘just made sense’. Working with neptunium and plutonium at synchrotron facilities comes with significant challenges. ‘The Dumas team at CEA is uniquely equipped with the capabilities required to prepare and transport such highly radioactive materials,’ Baker says.
According to Baker, it’s exceedingly difficult to conduct synchrotron measurements on these samples without a specialist beamline designed for working with highly active samples. The Mars beamline at Synchrotron Soleil in France was built around this premise, making it ideally suited to the study.
RIXS analysis
The team used the beamline to probe the actinide complexes with resonant inelastic x-ray scattering (RIXS), a relatively young technique for studying actinide systems. ‘We are only now starting to get a really good understanding of how best to analyse RIXS,’ Baker notes.
After the experiments were performed in France the data was returned to Manchester where Baker and his team analysed it to understand exactly what was happening in the 5f orbitals of these complexes.
The team discovered the inner and outer components of 5f orbitals could be resolved separately while analysing the RIXS data – a ‘serendipitous’ result, Baker says, ‘probably the most exciting thing’.
Kristina Kvashnina, head of the Rossendorf beamline in Grenoble, France, and a specialist in actinide chemistry and RIXS, says the work is wonderful. ‘It’s not easy to study actinides, [but] it is extremely important to find out what the effect of covalency in those systems is, because we really don’t know much about them in general.’
Understanding actinide covalency could help with the problem of spent nuclear fuel. To recycle such waste, lanthanides and actinides must be separated. Once separated, some actinides could be fed back into the reactor.
‘We haven’t achieved this yet because we don’t really know how to separate them. We also don’t know how to separate different actinide materials because they’re very similar to each other,’ Kvashnina says. ‘So, if we discover we have covalent bonding, you can simply change the material, break the bond, and then you can add a supplementary species to the waste, which then attracts only one actinide. That would be fantastic.’
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