The only rare earth element whose chemistry was largely unknown is now less of a mystery. Although promethium itself is unlikely to become as important to society as its neighbors on the periodic table, the information revealed about it could help improve our approach to the rest.
The category known as “rare earth elements” has caused some unfortunate confusion recently as many of these elements are becoming much more important. Neodymium, for example, helps realize the strongest permanent magnets, making it a valuable commodity for the clean energy revolution. Designation as a rare earth element has encouraged the idea that there may not be enough of it to meet our needs, but in fact it is almost as common as copper in the earth’s crust and much more so than lead.
Not so promethium, which is among the rarest elements on Earth, even behind some of the transuranics, which were once thought not to exist until we made them. As a result, promethium was only discovered in 1945 and its chemical composition was thus deficient.
Now Oak Ridge National Laboratory (ORNL), where promethium was first discovered, has tried to change that. “The whole idea was to explore this very rare element to gain new knowledge,” Dr. Alex Ivanov said in a statement. “Once we realized that it was discovered at this national laboratory and at the place where we work, we felt an obligation to do this research to maintain ORNL’s legacy.” The ORNL team is the first to characterize promethium in solution.
The team purified the promethium-147 produced as a waste product in the production of plutonium.
Fittingly, given the mythical figure it was named after, the team attached their promethium to organic molecules called diglycolamide ligands and used X-ray spectroscopy to measure the bond lengths between the promethium and its neighbors. The team then did the same with the 3+ ions of all the other lanthanides for comparison.
In this way, they were able to confirm that, like its neighbors, the promethium atom experiences a phenomenon known as “lanthanide contraction,” which packs the electrons closer to the nucleus than in most other atoms. This is one of the reasons why mercury is the only liquid element at room temperature.
With all the other lanthanides that have undergone this contraction, it was no surprise that promethium does as well, but the extent of contraction is not the same for each element. “The contraction of this chemical bond accelerates along this atomic row, but after promethium it slows down significantly,” Ivanov said. Consequently, without measuring the contraction, we did not know whether promethium would be more like neodymium on the left or samarium on the right.
However, it is much rarer than the other lanthanides because none of its isotopes has a half-life longer than 17.7 years. Billions of years after the atoms that make up most of Earth were formed in a supernova, nothing is left. The only reason there are small natural amounts on Earth at all is because of rare events, such as neodymium-146 being hit by cosmic rays. Every other lanthanide has at least one stable isotope.
“Because it has no stable isotopes, promethium was the last lanthanide discovered and the most difficult to study,” said Dr. Ilja Popovs.
The chemistry of the other 14 lanthanides is well known—in fact, they are frustratingly similar, making them difficult to separate when mined from the same ore. The ORNL team thinks their work will help with that. Understanding the details of the contraction for each element will make it easier to identify ways to separate them to meet growing demand.
“There are thousands of publications on lanthanide chemistry without promethium. This was a glaring gap for all of science,” said Dr. Santa Jansone-Popova. “Scientists must take on most of its properties. Now we can actually measure some of them.”
The study is freely accessible in nature.