First-ever measurement of promethium’s chemical bond fills gap in rare earth knowledge

Scientists have recently uncovered new properties of promethium, a rare earth element first discovered 80 years ago at Clinton Laboratories, now known as the Department of Energy's Oak Ridge National Laboratory (ORNL). This breakthrough opens a new pathway for exploring elements that are critical in modern technology, including in medicine and space travel.

Promethium, named after the mythological Titan who brought fire to humanity, was discovered in 1945 at ORNL. Despite its utilization in and nuclear batteries, many of its properties remained elusive. “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. The ORNL team's recent findings continue the laboratory's legacy of pioneering scientific research.

The research team, led by scientists at ORNL, prepared a chemical complex of promethium, allowing its characterization in solution for the first time. This significant achievement marks a notable advance in rare earth research, potentially rewriting chemistry textbooks. “Because it has no stable isotopes, promethium was the last lanthanide to be discovered and has been the most difficult to study,” said Ilja Popovs, a co-leader of the research at ORNL.

Most rare earth elements, including lanthanides, share similar but differ in size. Promethium, with an atomic number of 61, stood out as a significant gap in scientific literature. “There are thousands of publications on lanthanides' chemistry without promethium. That was a glaring gap for all of science,” said Santa Jansone-Popova, another co-leader of the study. The team's work has now allowed for actual measurement of some of promethium's properties.

The research leveraged unique resources and expertise at DOE national laboratories, including a research reactor, hot , and supercomputers. The ORNL scientists chelated radioactive promethium with special organic molecules called diglycolamide ligands and used X-ray to determine the properties of the complex. This included measuring the length of the promethium chemical bond with neighboring atoms—a first in scientific research.

Promethium is exceedingly rare, with only about a pound occurring naturally in the Earth's crust at any given time. Synthetic promethium is also produced in minute quantities due to its lack of stable isotopes. For this study, the ORNL team produced the isotope promethium-147, with a half-life of 2.62 years, in sufficient quantities and purity to study its chemical properties. ORNL remains the only U.S. producer of promethium-147.

The study also provided the first demonstration of lanthanide contraction in solution for the entire lanthanide series, including promethium. Lanthanide contraction refers to the where elements with atomic numbers between 57 and 71 become smaller than expected as their atomic numbers increase. “It's really astonishing from a scientific viewpoint. The contraction of this chemical bond accelerates along this atomic series, but after promethium, it considerably slows down,” said Ivanov. This discovery is pivotal in understanding the properties and structural changes of these elements.

The implications of this research are vast. Lanthanides and actinides have applications in various fields, from cancer diagnostics and treatment to technologies and deep . The ORNL team's achievement will also facilitate the separation of these valuable elements, a process that has been challenging without a complete understanding of promethium. “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,” explained Jansone-Popova.

The research project utilized several premier DOE facilities. At ORNL, promethium was synthesized at the High Flux Isotope Reactor and purified at the Radiochemical Development Center. The team performed X-ray absorption spectroscopy at the National Synchrotron Light Source II at DOE's Brookhaven National Laboratory and conducted quantum chemical calculations and molecular dynamics simulations using ORNL's Summit supercomputer.

This research marks a new era in the study of rare earth elements, with far-reaching implications for modern technology. “Anything that we would call a modern marvel of technology would include, in one shape or another, these rare earth elements,” said Popovs. The discovery and detailed study of promethium's properties fill a crucial gap in the periodic table, adding the missing link in our understanding of rare earth elements.

The study is published in the journal Nature.