Scientists Determine Properties of Extremely Rare Element: Promethium

Promethium is extremely rare, with only about 0.5 kg naturally occurring in Earth’s crust at any given time.

Discovered in 1945 at Clinton Laboratories, promethium is a lanthanide element with symbol Pm and atomic number 61.

It is named after the mythological Titan who delivered fire to humans and whose name symbolizes human striving.

Some of its properties have remained elusive despite the rare earth element’s use in medical studies and long-lived nuclear batteries.

“The whole idea was to explore this very rare element to gain new knowledge,” 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.

Thus, they exposed the secrets of this element in a series of meticulous experiments.

“Because it has no stable isotopes, promethium was the last lanthanide to be discovered and has been the most difficult to study,” said Dr. Ilja Popovs, also from Oak Ridge National Laboratory.

“There are thousands of publications on lanthanides’ chemistry without promethium. That was a glaring gap for 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.

Then, using X-ray spectroscopy, they determined the properties of the complex, including the length of the promethium chemical bond with neighboring atoms — a first for science and a longstanding missing piece to the periodic table of elements.

Unlike other rare earth elements, only minute quantities 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 at a high enough purity to study its chemical properties.

Notably, they provided the first demonstration of a feature of lanthanide contraction in solution for the whole 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 numbers of these lanthanides increase, the radii of their ions decrease.

This contraction creates distinctive chemical and electronic properties because the same charge is limited to a shrinking space.

The authors got a clear promethium signal, which enabled them to better define the shape of the trend — across the series.

“It’s really astonishing from a scientific viewpoint. I was struck once we had all the data,” Dr. Ivanov said.

“The contraction of this chemical bond accelerates along this atomic series, but after promethium, it considerably slows down.”

“This is an important landmark in understanding the chemical bonding properties of these elements and their structural changes along the periodic table.”

“The achievement will, among other things, ease the difficult job of separating these valuable elements,” Dr. Jansone-Popova said.

“Our team has long worked on separations for the whole series of lanthanides, but promethium was the last puzzle piece. It was quite challenging.”

“You cannot utilize all these lanthanides as a mixture in modern advanced technologies, because first you need to separate them.”

“This is where the contraction becomes very important; it basically allows us to separate them, which is still quite a difficult task.”

“Anything that we would call a modern marvel of technology would include, in one shape or another, these rare earth elements,” Dr. Popovs said.

“We are adding the missing link.”

The team’s paper appears today in the journal Nature.

_____

D.M. Driscoll et al. Observation of a promethium complex in solution. Nature, published online May 22, 2024; doi: 10.1038/s41586-024-07267-6