Heavy actinides - elements at the bottom of the periodic table, after plutonium - are radioactive, rare and chemically complex, making them notoriously difficult to study. Most studies conducted on these elements have traditionally been done one-compound-at-a-time or extrapolated from less toxic and non-radioactive surrogates, like lanthanides, that are safer to work with. As such, relatively little is known about the chemical properties of heavy actinides.
Researchers at Lawrence Livermore National Laboratory (LLNL) are breaking through the barriers of this field with a streamlined and efficient "serial approach" for the synthesis and analysis of heavy actinide compounds. A new study, published in the Journal of the American Chemical Society, reveals that the actinides americium and curium have truly unique chemical properties, departing from the historical consensus that some actinides and lanthanides exhibit the same chemistry.
According to the authors, LLNL scientists Ian Colliard and Gauthier Deblonde, this is likely the largest crystallographic and spectroscopic dataset ever released on americium and curium compounds.
"Our approach allows for the synthesis, crystallization and detailed structural and spectroscopic analysis of compounds containing americium and curium, two of the rarest and most challenging elements to study," said Colliard. "Thanks to the resources at Livermore, we can now do serial chemistry on elements like these - which was simply not possible before."
Funded by the Department of Energy's Office of Basic Energy Sciences, Heavy Element Chemistry program, Deblonde and Colliard synthesized and characterized coordination complexes of americium and curium, alongside their respective lanthanide analogs, neodymium and europium.
Here, a coordination complex refers to a type of chemical compound where americium or curium atoms are surrounded by polyoxometalates (POMs) ligands - dense, stable clusters of metal and oxygen atoms. These ligands wrap around the americium or curium atoms, creating a stable complex that can be studied in detail.
Creating a POM complex is the first step to discovering a new compound. For their study, the researchers investigated the luminescence properties of 25 curium-POM complexes in aqueous solutions, representing a significant experimental dataset for the field. Of these 25 complexes, seven curium compounds were successfully isolated as single crystals and structurally characterized, marking distinct contributions to the study of actinide coordination chemistry.
Compared to previous studies that required anywhere from 500 to 5,000 micrograms of a rare heavy element to produce a single compound, utilizing POMs allowed the researchers to drastically reduce the required amount to just 1-10 micrograms per reaction. In the present study, this innovation enabled the efficient synthesis of americium and curium, but it could also be applied to many other heavy elements in future research.
With the newly released dataset, about 45% of the curium compounds that have been structurally characterized to date are a result of research performed at LLNL.
"Now, we can get more data while using far fewer of the precious research isotopes produced by the Department of Energy. This means we can spot actual chemical trends across a series of compounds, not just extrapolate from single-compound experiments," said Deblonde. "We can also train students and the next generation of radiochemists on elements that have been traditionally off limits."
To study and characterize the compounds' structural, vibrational and optical properties, Colliard and Deblonde employed a combination of analytical techniques. Solid-state spectroscopy revealed rarely observed vibrational interactions between atoms in the curium complexes, indicating the potential for novel emissive (light-emitting) pathways. These pathways are essential for advancing understanding of luminescence phenomena and the properties of electrons that orbit heavy elements at close to the speed of light.
These findings challenge the assumption of actinide-lanthanide similarity, revealing that while they share some traits, actinides like americium and curium display unique chemical behaviors that cannot be fully predicted by studying lanthanides. The distinct chemical and physical properties of actinides were evident across the new series of compounds.
Next, the researchers are eager to apply their "serial approach" to other rare elements that are found in nuclear applications, further exploring the unique properties of elements at the edge of the periodic table.
-Shelby Conn
