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Promethium's rare earth secrets unveiled

Metal Tech News - June 3, 2024

Oak Ridge National Lab scientists have made new promethium discoveries that may rewrite the rare earths section of chemistry textbooks.

A team of scientists at the U.S. Department of Energy's Oak Ridge National Laboratory has begun to unlock the secrets of promethium, the one rare earth that is as scarce as the name of this group of elements suggests.

The term rare earths refers to the group of 15 lanthanide elements found in their own row near the bottom of the periodic table. The 14 rare earth elements not called promethium boast special properties that have proven to be essential to many modern technologies.

Lasers used for eye surgery and cancer treatment, powerful magnets for electric vehicles and wind turbines, specialized glass for high-quality camera and telescope lenses, and portable X-ray machines that do not need electricity are just a few of the countless technologies that depend on rare earths.

Promethium's exclusion from the list of rare earths is not because it is not special – in some ways, it is the most exceptional of this unique group of elements. Instead, its omission is due to the fact that this radioactive element with no stable isotopes is so rare that only about one pound of promethium exists in Earth's crust at any one time.

Carlos Jones/ORNL, U.S. Dept. of Energy

The groundbreaking promethium research at Oak Ridge National Lab was led by (from left) Alex Ivanov, Santa Jansone-Popova, and Ilja Popovs.

Due to this extreme rarity, promethium was the last of the rare earths to be discovered and remains the least understood of the 15 lanthanide elements.

A team of 18 scientists at Oak Ridge National Lab (ORNL) have taken up the torch of Olympian fire stolen in Greek mythology by Titan Prometheus and given to humanity to ignite technology, knowledge, and civilization.

"There are thousands of publications on lanthanides' chemistry without promethium. That was a glaring gap for all of science," said Santa Jansone-Popova, one of the three ORNL scientists who co-led the study.

Protecting a promethium legacy

ORNL scientists felt they had a legacy to protect when it comes to filling in the glaring gaps in the scientific understanding of promethium. After all, this scarcest of the rare earth elements was first discovered in 1945 at Clinton Laboratories, which later became ORNL.

"The whole idea was to explore this very rare element to gain new knowledge," said Alex Ivanov, an ORNL scientist who co-led the research. "Once we realized it was discovered at this national lab and the place where we work, we felt an obligation to conduct this research to uphold the ORNL legacy."

Since that discovery eight decades ago, ORNL has been producing promethium isotopes in minute quantities. These isotopes include promethium-147, which is only produced at the national lab and was used for the study.

As the first step, the ORNL team produced promethium-147, which has a half-life of 2.62 years, in sufficient quantities and at a high enough purity to study its chemical properties.

Leveraging the skills of all 18 researchers working on the project, along with supercomputers and other scientific resources available at DOE national labs, the ORNL team was able to document the first observation of a promethium complex in solution.

To accomplish this, the ORNL scientists bound the promethium-147 with specialized organic molecules. This allowed the team members to use X-ray spectroscopy to determine 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.

Jansone-Popova said that up until this study, scientists could only postulate and theorize on many of promethium's properties.

"Now we can actually measure some of them," the ORNL researcher said.

Carlos Jones/ORNL, U.S. Dept. of Energy

Rewriting chemistry textbooks

The ORNL research not only began to unlock some of promethium's secrets, but it is beginning to provide greater insights into the entire suite of rare earth elements.

"Anything that we would call a modern marvel of technology would include, in one shape or another, these rare earth elements," said ORNL's Ilja Popovs, who co-led the research. "We are adding the missing link."

One of the things that make rare earths so special is a phenomenon known as lanthanide contraction.

In a nutshell, this means that all the rare earth elements are smaller than expected, when compared to the other elements on the periodic table. As the atomic numbers of these lanthanides increase from atomic numbers 57 to 71, the radii of their ions decrease.

Holding a charge within a shrinking space is a feature that provides rare earth elements with many of their distinctive properties.

The research work carried out by the ORNL scientists provided a clear lanthanide contraction signal for promethium, an element that serves as a key waypoint for this phenomenon. Lanthanide contraction accelerates over the first five rare earth elements, up to and including promethium, and then suddenly, the rate of elemental shrinkage slows.

"It's really astonishing from a scientific viewpoint. I was struck once we had all the data," said Ivanov. "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."

Jansone-Popova says unlocking promethium's secrets and better understanding the bonding properties of its fellow rare earths will, among other things, ease the notoriously difficult task of separating these valuable technology elements.

"You cannot utilize all these lanthanides as a mixture in modern advanced technologies, because first you need to separate them," she said. "This is where the contraction becomes very important; it basically allows us to separate them, which is still quite a difficult task."

The ORNL team says their landmark study into promethium, published in the journal Nature, marks a significant advance in rare earth research and may rewrite chemistry textbooks.

Besides Popovs, Ivanov and Jansone-Popova from ORNL's Chemical Sciences Division, the paper's co-authors include Darren Driscoll, Subhamay Pramanik, Jeffrey Einkauf, Santanu Roy and Thomas Dyke, also of ORNL's Chemical Sciences Division; Frankie White, Richard Mayes, Laetitia Delmau, Samantha Cary, April Miller and Sandra Davern of ORNL's Radioisotope Science and Technology Division; Matt Silveira and Shelley VanCleve of ORNL's Isotope Processing and Manufacturing Division; Dmytro Bykov of the National Center for Computational Sciences at ORNL; and Bruce Ravel of the National Institute of Standards and Technology.

Carlos Jones/ORNL, U.S. Dept. of Energy

Members of the promethium research team in front of ORNL's Radiochemical Engineering Development Center (from left) Santanu Roy, Thomas Dyke, Ilja Popovs, Richard Mayes, Darren Driscoll, Frankie White, Alex Ivanov, April Miller, Subhamay Pramanik, Santa Jansone-Popova, Sandra Davern, Matt Silveira, Shelley VanCleve, and Jeffrey Einkauf.

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Shane Lasley, Metal Tech News

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With more than 16 years of covering mining, Shane is renowned for his insights and and in-depth analysis of mining, mineral exploration and technology metals.


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