A team of scientists led by the U.S. Department of Energy’s Oak Ridge National Laboratory (ORNL) recently observed how the rare element promethium forms chemical bonds in an aqueous solution. The research was conducted using the Beamline for Materials Measurement (BMM) at the National Synchrotron Light Source II, a DOE Office of Science user facility at Brookhaven National Laboratory.
Promethium, despite its rarity and high instability, has applications in specialized glow-in-the-dark paint, radiation therapy, and long-lasting atomic batteries for pacemakers and spacecraft. Understanding its complex chemistry could lead to new uses and further studies.
Promethium is a lanthanide or rare-earth metal, one of 15 elements with atomic numbers 57 through 71. These metals have unique magnetic and electronic properties due to "lanthanide contraction," where atomic and ionic radii decrease despite increasing atomic numbers. This phenomenon had not been experimentally observed in all lanthanides in solution until now. The findings were published in Nature.
At any given time, only about a pound of naturally occurring promethium exists on Earth due to its short half-life. ORNL created a sample of promethium-147 with a half-life of 2.6 years from nuclear reactor waste. “We had around 40 or 50% of the entire stock of purified promethium on the planet at the beamline to study,” said Bruce Ravel, lead beamline scientist at BMM and co-author of this research.
The sample started at ORNL, where scientists extracted material from the High Flux Isotope Reactor waste stream and separated the promethium. Safely packaging and transporting it from Tennessee to New York took significant logistical planning.
To study promethium's chemical structure, scientists stabilized it in water using bispyrrolidine diglycolamide ligands before measuring it with X-ray absorption spectroscopy (XAS) at BMM. “To the best of our knowledge, this was the first time that anyone measured promethium with XAS,” remarked Ravel.
In solution, the promethium ion formed bonds with nine neighboring oxygen atoms. The results confirmed that promethium fits into the theorized pattern of lanthanide contraction.
“This was a situation where what we measured was in line with our expectations,” said Ravel. “Having actual knowledge rather than inference is how one does good science.”
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