A group of researchers recently made a fascinating discovery about how the rare element promethium forms chemical bonds in water.
Confirming a scientific theory is thrilling, but experiencing something entirely novel takes that excitement to another level. A team from the Oak Ridge National Laboratory (ORNL) of the U.S. Department of Energy (DOE) recently experienced this as they observed the formation of chemical bonds of promethium while it was in an aqueous environment. The study took place at the Beamline for Materials Measurement (BMM), a facility funded by the National Institute of Standards and Technology, located at the National Synchrotron Light Source II—a user facility of the DOE Office of Science at Brookhaven National Laboratory.
Though promethium is scarce, it has several intriguing uses, such as in the production of specialized glow-in-the-dark paint, radiation therapy, and durable atomic batteries for devices like pacemakers and spacecraft. However, its high instability leaves much about this radioactive element still unexplored. Gaining insights into its complex chemistry could lead to even more unique applications and further investigations.
Promethium is classified as a “lanthanide” or “rare-earth metal.” It is one of 15 elements located in the lower segment of the periodic table with atomic numbers ranging from 57 to 71. While these metals share similar visual and physical characteristics, they each possess distinct magnetic and electronic properties. These unique features may be influenced by a phenomenon known as “lanthanide contraction,” which suggests that the atomic and ionic sizes of these elements decrease even as their atomic numbers increase, similar to other groupings in the periodic table. Consequently, the atoms become smaller as one moves along this series. Prior to this research, scientists had not experimentally observed this trend among all lanthanides dissolved in a solution. The findings from this pioneering study were recently published in Nature.
Addressing Scarcity and Timing Challenges
At any moment, there are typically only around a pound of promethium available in its natural form on Earth. This element is radioactive and has an incredibly short half-life, contributing significantly to its scarcity. The longest half-life for a promethium isotope, promethium-145, is just 17.7 years. The researchers at ORNL managed to create a sample of promethium-147, which has a half-life of 2.6 years, utilizing waste produced during plutonium generation for space exploration. As soon as the sample is cut from its source, it begins to decay into the more stable element, samarium.
“At the beamline for the study, we had around 40 to 50% of the entire world’s supply of purified promethium,” Bruce Ravel, the lead beamline scientist at BMM and a co-author of the research, stated. “A couple of weeks later, the promethium was no longer viable, primarily because the water in the solution started evaporating. While the research was indeed captivating, the entire process of organizing it was equally interesting. A tremendous amount of logistical planning went into making this study possible, and everyone involved put in a lot of effort to get every aspect of this experiment executed swiftly and carefully.”
The journey of the sample began at ORNL, where scientists extracted material from the waste of the High Flux Isotope Reactor, isolating promethium from the rest of the by-product. The team then had to carefully package the sample and transport it from Tennessee to New York, undergo acceptance at NSLS-II, and finally execute experiments at the beamline—all of which consumed valuable time, inching away at the limited supply of promethium.
Experiencing a Series of Firsts
To analyze the chemical structure of promethium, the scientists first needed to stabilize it in water. They achieved this by using a water-soluble ligand named bispyrrolidine diglycolamide. Ligands are unique molecules that attach to metal atoms. Following this, the team transported the sample to BMM to carry out measurements using X-ray absorption spectroscopy (XAS), a well-known technique that examines the structure and properties of materials by shining X-ray light onto a sample and analyzing how the sample absorbs the X-rays. Different atoms absorb X-rays at distinct energy levels, which allows researchers to determine which elements are present and how they are organized within the material.
“To our knowledge, this was the first instance of anyone measuring promethium using XAS at any synchrotron,” Ravel commented. “We are the first to observe a spectrum like this, which, in itself, was incredibly exciting. I have been working with XAS for a long time and have never encountered something that no one else has ever documented.”
Within the solution, the promethium ion established bonds with nine surrounding oxygen atoms. After conducting analyses and measurements of the complex, the researchers were able to relate these findings to the broader series of lanthanides, confirming that it supported the theorized pattern of contraction.
This new understanding enabled the team to evaluate the entire lanthanide series, revealing its own interesting trends. Bond shortening was particularly rapid at the beginning of the series, but for the heavier lanthanides that followed promethium, the bond lengths decreased more gradually. Unraveling the chemical characteristics of promethium independently opens up new avenues for research while completing a previously incomplete understanding of the lanthanides.
“What we discovered aligned with our expectations, informed by prior science and knowledge of the lanthanide series,” Ravel shared. “However, we tackled a difficult measurement that had never been achieved before. Acquiring concrete knowledge, rather than making assumptions, is essential for quality science. Filling this gap in our understanding of lanthanides holds significant importance. After 30 years in science, I’ve never rushed into the street shouting, ‘Eureka!’ This was an achievement, but it wasn’t shocking. The most rewarding moments in science come not from someone shouting, ‘Eureka!’ but from someone pondering, ‘Huh, that’s peculiar.’
