Beyond the Periodic Table’s Edge: Scientists Unravel New Chemistry for Heavy Elements

Heavy ligands such as polyoxometalates are opening a new frontier in actinide chemistry.

Studying materials that are both radioactive and rare presents significant challenges. The elements that follow plutonium on the periodic table, known as the transplutonium elements, are both highly radioactive and scarce. As a result, their chemical properties remain poorly understood. To overcome these limitations, researchers often rely on non-radioactive lanthanide elements as surrogates, using them to infer the behavior of heavier actinides beyond uranium.

In this study, however, scientists developed a more efficient method for synthesizing transplutonium compounds. This advancement enabled direct and accurate comparisons between the chemistry of transplutonium actinides and their lanthanide surrogates. The findings reveal that transplutonium elements exhibit truly distinct chemical behavior, differences that cannot be reliably predicted based on lanthanide analogs alone.

A New Method for Safer, More Efficient Research

This research enables efficient experimental studies of actinides beyond plutonium through the novel synthesis of compounds using groups of atoms called polyoxometalate ligands. Using tiny samples of these compounds, scientists can then determine their structural, vibrational, and optical properties. This drastically cuts the cost and potential radiation exposure that comes with experimenting on these elements.

It also means more efficient use of research isotopes, needing less than 1% of the quantity required with traditional methods. Scientists have implemented the method to streamline the study of actinides americium and curium. They have also performed direct comparisons of lanthanide and transplutonium compounds. Only by direct study of the actinides instead of surrogates will scientists unravel their true chemical properties.

Uncovering Unique Chemistry in Transplutonium Elements

The experimental results in this study unequivocally show that transplutonium actinides exhibit their own unique chemistry—that is, that their chemistry is intrinsically different from lanthanide chemistry. These results show that even within the same coordination chemistry framework (provided by the polyoxometalate ligands), lanthanides and actinides exhibit fundamental chemical differences that cannot be explained by simple size-match arguments. For example, curium and americium yield crystal structures that could have not been predicted based on lanthanide chemistry.

This work will allow the creation of actinide-specific polyoxometalates compounds, which will unlock novel separation and isolation strategies. Polyoxometalate ligands magnify usually minuscule differences among actinides and lanthanides, and even between americium and curium. The structural and spectroscopic impact of the actinide elements within the polyoxometalate compounds can be seen even at long range, such as the bending and twisting of the overall structure as well as the arrangement of the actinide polyoxometalate complexes relative to each other.

Another previously unsuspected effect is that alkali metal counterions (i.e., sodium and cesium that charge-balance the compounds), which were previously considered “spectator ions,” have distinct chemical effects on actinide versus lanthanide polyoxometalate compounds. This opens the door to finding chemical systems for which actinides and lanthanides form very distinct compounds for future applications like detection, radionuclide capture, and f-element separation.

References:

“Characterization of the first Peacock–Weakley polyoxometalate containing a transplutonium element: curium bis-pentatungstate [Cm(W₅O₁₈)₂]⁹⁻” by Ian Colliard and Gauthier J.-P. Deblonde, 9 May 2024, Chemical Communications.
DOI: 10.1039/D4CC01381F

“Polyoxometalate Ligands Reveal Different Coordination Chemistries Among Lanthanides and Heavy Actinides” by Ian Colliard and Gauthier J.-P. Deblonde, 9 May 2024, JACS Au.
DOI: 10.1021/jacsau.4c00245

“From +I to +IV, Alkalis to Actinides: Capturing Cations across the Periodic Table with Keggin Polyoxometalate Ligands” by Ian Colliard and Gauthier J.-P. Deblonde, 22 August 2024, Inorganic Chemistry.
DOI: 10.1021/acs.inorgchem.4c02254

This material is based on work supported by the Department of Energy Office of Science, Office of Basic Energy Sciences, Heavy Element Chemistry program and conducted at Lawrence Livermore National Laboratory.

Never miss a breakthrough: Join the SciTechDaily newsletter.
Follow us on Google and Google News.