For the first time, researchers have experimentally examined the chemistry of the lanthanide element promethium. The investigation was carried out by Alex Ivanov and colleagues at Oak Ridge National Laboratory in the US – the same facility at which the element was first discovered almost 80 years ago.
Found on the sixth row of the periodic table, the lanthanide rare-earth metals possess an unusually diverse range of magnetic, optical and electrical properties, which are now exploited in many modern technologies. Yet despite their widespread use, researchers still know very little about the chemistry of promethium, a lanthanide with an atomic number of 61, which was first identified in 1945 by researchers on the Manhattan project.
“As the world slowly recovered from a devastating war, a group of national laboratory scientists from the closed town of Oak Ridge, Tennessee, isolated an unknown radioactive element,” Ivanov describes. “This last rare-earth lanthanide was subsequently named promethium, derived from the Greek mythology hero Prometheus, who stole fire from heaven for the use of mankind.”
Despite its relatively low atomic number compared with the other lanthanides, promethium’s chemical properties have remained elusive in the decades following its discovery. Part of the reason for this is that promethium is the only lanthanide with no stable isotopes. Only small quantities of synthetic promethium (mostly promethium-147 with a half-life of 2.62 years) are available, extracted from nuclear reactors, through tedious and energy-intensive purification processes.
Ultimately, this limited availability means that researchers are still in the dark about even the most basic aspects of promethium’s chemistry: including the distance between its atoms when bonded together, and the number of atoms a central promethium atom will bond to when forming a molecule or crystal lattice.
Ivanov’s team revisited this problem in their study, taking advantage of the latest advances in isotope separation technology. In a careful, multi-step process, they harvested atoms of promethium-147 from an aqueous solution of plutonium waste, and bonded them to a group of specially selected organic molecules. “By doing this, we could study how promethium interacts with other atoms in a solution environment, providing insights that were previously unknown,” Ivanov explains
Using synchrotron X-ray absorption spectroscopy to study these interactions, the researchers observed the very first appearance of a promethium-based chemical complex: a molecular structure whose central promethium atom is bonded to several neighbouring organic molecules.
Altogether, they observed nine promethium-binding oxygen atoms in the complex, which allowed them to probe several of the metal’s fundamental chemical properties for the first time. “We discovered how promethium bonds with oxygen atoms, measured the lengths of these bonds, and compared them to other lanthanides,” Ivanov describes.
Based on these results, the researchers then studied a complete set of comparable chemical complexes spanning all lanthanide elements. This enabled them to experimentally observe the phenomenon of “lanthanide contraction” across the whole lanthanide series for the first time.
Lanthanide contraction describes the decrease in the atomic radii of lanthanide elements as their atomic number increases, due to increasingly poor shielding from nuclear charge by inner-shell electrons. The effect causes the lanthanide–oxygen bond length to shrink. Ivanov’s team observed that this shortening accelerated early in the lanthanide series, before slowing down as the atomic number increased.
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The team’s discoveries have filled a glaring gap in our understanding of promethium’s chemistry. By building on their results, the researchers hope that future studies could pave the way for a wide range of important applications for the element.
“This new knowledge could improve the methods used to separate promethium and other lanthanides from one another, which is crucial for advancing sustainable energy systems,” Ivanov describes. “By understanding how promethium bonds in a solution, we can better explore its potential use in advanced technologies like pacemakers, spacecraft power sources and radiopharmaceuticals.”
The researchers report their findings in Nature.
