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![A photo of the group of researchers](https://cdn.mos.cms.futurecdn.net/KEKevERVE9s6PWyVNzxtga-320-80.jpg)
Then, the team combined this sample with a ligand — a molecule specially designed to trap metal atoms — to form a stable complex in water. The coordinating molecule, known as PyDGA, formed nine promethium-oxygen bonds, giving researchers the first-ever opportunity to analyze the bonding properties of a promethium complex.
However, the analysis itself was no trivial matter.
“Because promethium is radioactive, once it’s decaying, it’s getting transmuted into the adjacent element, which is samarium,” Ivanov said. “So you will have a tiny amount of contamination in the form of samarium.”
‘The last puzle piece’
The team therefore used an extremely specialized, element-specific technique called synchrotron-based X-ray absorption spectroscopy. High-energy photons generated by a particle accelerator bombarded the promethium complex to build a picture of the positions of atoms and the lengths of bonds. Subtle differences in the metal-oxygen bond lengths then allowed the team to focus on the key promethium-oxygen bond, discounting any contaminating samarium.
Crucially, this information enabled a comparison of promethium’s properties with other rare earth complexes for the first time.
“Promethium was the last puzzle piece among those elements,” Popovs said. The ligand provided a way to have a stable complex for all of the lanthanides — the same element ratios and the same kind of geometry. That allowed the team to “study the fundamental physical chemical properties of these complexes across the whole series,” Popovs explained.
Lanthanides are naturally found as mixtures of elements, so understanding periodic trends such as bond lengths and complex-forming behaviors helps scientists develop new and more efficient methods to separate these valuable metals.
Now, the ORNL team is studying promethium in water to build a clearer picture of the coordination environment and chemical behavior of this unusual element.
“Hopefully, the fundamental insights that we’re providing will inform other scientists how to design better separation technologies and can perhaps spur more interest in studying it for other applications,” Popovs said.