Scientists have revealed the essential properties of a mysterious radioactive substance for the first time promethium — nearly eight decades after the elusive rare earth element was discovered.
Promethium is one of the 15 lanthanide elements at the bottom of the periodic table. Also known as rare earths, these metals exhibit a number of useful properties, including strength magnetism and unusual optical properties that make them particularly important in modern electronic devices.
“They are used in lasers; are part of your smartphone screens. They are also used in very strong magnets in wind turbines and electric vehicles.” Ilya Popovsresearch and development fellow at Oak Ridge National Laboratory (ORNL) and co-author of a new study published in the journal Naturehe told Live Science.
“Rare and difficult to study”
Promethium itself, which was discovered by ORNL scientists in 1945, has several smaller applications in atomic batteries and cancer diagnostics. But scientists have very limited knowledge of the element’s chemistry, which precludes its wider use.
Studying the radioactive element has been a decades-long challenge, in part because of the difficulty of securing a suitable sample, a team member Alexander Ivanovalso a research and development scientist at ORNL, told Live Science.
“Promethium has no stable isotope – they are all radioactive, meaning they decay [into other elements] with time,” Ivanov said. “You get this element through a fission process, making it rare and difficult to study.’
ORNL is the sole US producer of promethium-147, an isotope of the element with a radioactive half-life of 2.6 years. Using a method developed last yearscientists separated this isotope from nuclear reactor waste streams, creating the purest possible sample for study.
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The team then combined this sample with a ligand – a molecule specifically designed to trap metal atoms – to form a stable complex in water. The coordination molecule, known as PyDGA, formed nine promethium-oxygen bonds, giving the researchers the first-ever opportunity to analyze the binding properties of the promethium complex.
However, the analysis itself was not a trivial matter.
“Because promethium is radioactive, once it decays, it turns into the neighboring element that it is samarium“So you will have a small amount of contamination in the form of samarium.
“The Last Piece of the Puzzle”
The team therefore used an extremely specialized, element-specific technique called synchrotron X-ray absorption spectroscopy. High energy photons generated by a particle accelerator bombarded the promethium complex to produce a picture of atomic positions and bond lengths. Subtle differences in metal-oxygen bond lengths then allowed the team to target the key promethium-oxygen bond without ruling out any contaminating samarium.
Crucially, this information allowed for the first time a comparison of the properties of promethium with other rare earth complexes.
“Promethium was the final piece of the puzzle between these elements,” Popovs said. The ligand provided a way to have a stable complex for all the lanthanides – the same element ratios and the same kind of geometry. This allowed the team to “study the basic physicochemical properties of these complexes throughout the series,” Popovs explained.
Lanthanides occur naturally as mixtures of elements, so understanding periodic trends such as bond lengths and complexation behavior 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.
“We hope that the basic insights we provide will inform other scientists on how to design better separation technologies and may possibly stimulate more interest in studying them for other applications,” said Popovs.