Rare earth element that glows violet has scientists excited about the future

When studying rare earth elements, noticing a violet glow in the lab used to be a sign for scientists that something intriguing lay on the other side of an impenetrable wall. That glow belonged to promethium, element 61, a shy neighbor on the periodic table that gives off faint light but avoids easy study.

Chemists had long known it existed, yet they couldn’t pin down how it acts in ordinary water, a gap that left textbooks with a dangling question mark.

The lanthanide family, stretching from lanthanum to lutetium, shows a steady shrink in ionic size – an effect known as the lanthanide contraction.

Every member had been measured except promethium. Without promethium’s data, the pattern looked like a half‑finished sketch.

Understanding rare earth elements

Rare earth elements (REEs) are a group of 17 metallic elements tucked away in the periodic table, and despite their name, they’re not actually rare – they’re just tricky to extract in usable form.

You’ll find them in everything from smartphones and electric cars to wind turbines and military gear. What makes them special is their magnetic, phosphorescent, and catalytic properties.

These elements power the modern world, often quietly, behind the scenes. Neodymium, for example, helps make the powerful magnets inside your earbuds, while europium gives LED lights their bright red glow.

But mining and refining rare earths isn’t exactly clean or easy. Most of the world’s supply comes from China, which has mastered the complex processes needed to isolate and purify these elements.

The challenge now is finding more sustainable, less toxic ways to source and recycle rare earths – without wrecking the environment or getting tangled in geopolitical knots.

Hidden neighbor of the periodic table

Promethium was first spotted in 1945 inside the wartime reactors of Clinton Laboratories, the forerunner of the Department of Energy’s Oak Ridge National Laboratory (ORNL).

Only about a pound of the element exists naturally in Earth’s crust at any moment, and none of its isotopes stay stable. That scarcity, coupled with radioactivity, explains the long wait for clear chemical measurements.

In practical terms, scientists must manufacture the element themselves. ORNL remains the sole U.S. source, producing tiny batches of promethium‑147, whose half‑life is 2.62 years.

“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.

Catching promethium in water

The team crafted a diglycolamide ligand – a claw‑like organic molecule – that latches onto promethium atoms in solution. That grip allowed them to dissolve the isotope safely in water and direct intense X‑ray beams at it.

Using ORNL’s High Flux Isotope Reactor, hot‑cell chemistry labs, and later the beamlines of Brookhaven’s National Synchrotron Light Source II, the researchers recorded the bond length between promethium and surrounding oxygen atoms.

This marks the first time anyone has seen the element’s hydrated radius rather than estimating it.

“Because it has no stable isotopes, promethium was the last lanthanide to be discovered and has been the most difficult to study,” said ORNL’s Ilja Popovs, who co‑led the research.

Contraction curves and promethium dots

By plotting the new bond length against those of the other 14 lanthanides, the scientists filled in the missing coordinate and could trace how quickly the ionic radius shrinks across the series. They found that the contraction speeds up until it reaches promethium, then slows in later elements.

Computer simulations on ORNL’s Summit supercomputer backed the measurements, allowing the group to confirm a subtle shift that had been predicted but never proved.

“There are thousands of publications on lanthanides’ chemistry without promethium. That was a glaring gap for all of science,” said ORNL’s Santa Jansone‑Popova, who also co‑led the study.

Tiny isotope, big jobs

Unlike the showier rare earth metals used in magnets or lasers, promethium’s chief value lies in its steady beta radiation.

Promethium‑147 has powered long‑lived nuclear batteries that once kept heart pacemakers ticking and that still illuminate spacecraft instruments.

Because its half‑life is short on a cosmic scale, engineers can design devices that fade safely over decades rather than millennia.

Understanding how the element interacts with solvents and extractants will help purify it more efficiently and open new applications, such as miniature power sources for deep‑sea sensors.

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

Skylines and stethoscopes

Rare earths underpin fluorescent lighting, wind‑turbine generators, electric‑vehicle motors, and the contrast agents that light up MRI scans.

Separating each lanthanide from its neighbors remains a stubborn cost driver, often involving miles of solvent‑extraction columns.

Precise knowledge of ionic size informs that separation, letting chemists tune ligands so they latch onto one element while ignoring another.

According to Jansone-Popova, “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; it basically allows us to separate them, which is still quite a difficult task.”

Promethium and future technology

These fascinating discoveries underscore how specialized facilities knit together to solve an atomic‑scale puzzle.

ORNL’s radiochemical engineers produced the isotope, Brookhaven’s synchrotron captured the spectra, and supercomputers crunched quantum calculations that once seemed out of reach.

Frontier, the lab’s new exascale machine, will take the next round of simulations from hours to minutes and may let researchers test how promethium bonds to advanced battery materials or medical chelators.

“Anything that we would call a modern marvel of technology would include, in one shape or another, these rare earth elements,” Popovs said. “We are adding the missing link.”

The violet glow that puzzled chemists eight decades ago now illuminates a complete picture of lanthanide behavior.

With promethium’s properties finally charted, the periodic table feels a little tidier – and the tools built from its elements stand to grow even smarter.

The full study was published in the journal Nature.

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