Promethium is extremely rare; only about 0.5 kg is naturally present in the Earth’s crust at any given time.
Concept art shows the rare earth element promethium in a vial surrounded by an organic ligand. Photo credit: Jacqueline DeMink / Thomas Dyke / ORNL.
Discovered at Clinton Laboratories in 1945, promethium is a lanthanide element with the symbol Pm and atomic number 61.
It is named after the mythological Titan who brought fire to humans and whose name symbolizes human striving.
Some of its properties remain unknown despite the rare earth element’s use in medical studies and long-life nuclear batteries.
“The whole idea was to explore this very rare element to gain new insights,” said Dr. Alex Ivanov, a researcher at Oak Ridge National Laboratory.
Dr. Ivanov and his colleagues prepared a chemical complex of promethium, which enabled its characterization in solution for the first time.
In a series of careful experiments, they revealed the secrets of this element.
“Because it has no stable isotopes, promethium was the last lanthanide to be discovered and it was the most difficult to study,” said Dr. Ilja Popovs, also of Oak Ridge National Laboratory.
“There are thousands of publications on the chemistry of lanthanides without promethium. “That was a glaring gap in all of science,” said Dr. Santa Jansone-Popova, also from Oak Ridge National Laboratory.
“Scientists have to assume most of its properties. Now we can actually measure some of them.”
The researchers bound or chelated radioactive promethium with special organic molecules called diglycolamide ligands.
They then used X-ray spectroscopy to determine the complex’s properties, including the length of the promethium’s chemical bond with neighboring atoms – a first in science and a long-missing part of the periodic table of elements.
Unlike other rare earth elements, only small amounts of synthetic promethium are available because it has no stable isotopes.
For the study, the scientists produced the isotope promethium-147, with a half-life of 2.62 years, in sufficient quantities and with a sufficiently high purity to investigate its chemical properties.
In particular, they provided the first evidence of a lanthanide contraction property in solution for the entire lanthanide series, including promethium, atomic number 61.
Lanthanide contraction is a phenomenon in which elements with atomic numbers between 57 and 71 are smaller than expected.
As the atomic number of these lanthanides increases, the radius of their ions decreases.
This contraction creates different chemical and electronic properties because the same charge is confined to a shrinking space.
The authors received a clear Promethium signal that allowed them to better define the shape of the trend – across the entire series.
“From a scientific perspective, it’s really amazing. I was amazed when we had all the data,” said Dr. Ivanov.
“The contraction of this chemical bond accelerates along this row of atoms, but slows significantly after promethium.”
“This is an important milestone in understanding the chemical bonding properties of these elements and their structural changes along the periodic table.”
“This success will, among other things, ease the difficult task of separating these valuable elements,” said Dr. Jansone Popova.
“Our team worked for a long time to separate the entire lanthanide series, but promethium was the last piece of the puzzle. It was quite challenging.”
“In modern cutting-edge technologies, you cannot use all of these lanthanides as a mixture because you first have to separate them.”
“This is where contraction becomes very important; it basically allows us to separate them, which is still quite a difficult task.”
“Everything that we would call a modern engineering marvel contains these rare earth elements in one form or another,” said Dr. Popovs.
“We’re adding the missing link.”
The team’s article appears in the magazine today Nature.
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DM Driscoll et al. Observation of a promethium complex in solution. Naturepublished online on 22 May 2024; doi: 10.1038/s41586-024-07267-6