Heavy Ligands Unravel New Chemistry for Heavy Elements

Heavy ligands, like polyoxometalates, open a new frontier in the chemistry of actinide elements.

Crystallographic and spectroscopic evidence now show that americium and curium yield a variety of polyoxometalate compounds that their lanthanide counterparts are unable to form. — Image courtesy of Eric B. Smith, Lawrence Livermore National Laboratory

The Science

Studying the properties of a material that is radioactive and rare is difficult. The elements that come after plutonium in the periodic table (called transplutonium elements) are both. The result is that scientists know relatively little about their chemical properties. Often researchers use non-radioactive lanthanide surrogates to extrapolate the properties of actinides beyond uranium. In this study, scientists instead streamlined the synthesis of transplutonium compounds. This allowed them to make accurate direct comparisons of the chemistry of lanthanides surrogates and actinides. The results show that transplutonium actinides are truly unique, with chemical properties that cannot be consistently extrapolated from studies using lanthanide surrogates.

The Impact

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.

Summary

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.

Contact

Gauthier Deblonde
Lawrence Livermore National Laboratory
[email protected]

Funding

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.