Researchers alter atomic structure of cerium to emit visible light.

In a discovery that challenges long-held assumptions about the behavior of rare earth elements, scientists have taught cerium – a metal best known for its invisible ultraviolet glow – to shine visibly yellow by reshaping the chemical environment around its atoms, opening a new frontier in how these elements can be coaxed into revealing capabilities hidden within their makeup.

Critical to nearly every facet of modern life, rare earth elements have become the elemental building blocks behind clean energy technologies, advanced defense systems, digital devices, and the powerful magnets and miniaturized components that enable everything from electric vehicles to satellites.

Though often referred to as a group, these 15 elements – known as the lanthanides, after the element lanthanum where the series begins – share a similar internal structure but express unique behaviors, particularly in how they interact with energy.

Among the most recognizable of those behaviors is luminescence – a phenomenon in which certain rare earths give off a distinctive glow, often so consistent and narrowly defined that it has become a reliable tool in technologies demanding optical precision.

Known for producing ultraviolet light, cerium has long been counted among the rare earths whose optical behavior remained stable regardless of surrounding chemistry, a constancy that made it useful in applications requiring predictable light response.

Cerium, the most abundant of the rare earth elements, has been incorporated into a wide range of industrial applications, spanning catalytic converters, glass polishing compounds, fuel additives, and solid oxide fuel cells; in light-based technologies, cerium compounds are used in materials that convert radiation or ultraviolet light into visible light, such as scintillators and phosphors, due to their reliable bright emission.

Now, that long-standing stability has been altered by researchers at HSE University and the Institute of Petrochemical Synthesis of the Russian Academy of Sciences, turning ultraviolet light into visible yellow light.

"Previously, a change in the color of the glow had been observed, but the underlying mechanism was not understood," said Daniil Bardonov, a master's student at the HSE Faculty of Chemistry. "Now, in collaboration with our physicist colleagues, we have been able to understand the mechanism behind this effect."

By surrounding cerium atoms with carefully designed molecules, researchers created a chemical environment that altered the behavior of electrons within the atom, shifting the energy levels responsible for light emission and enabling cerium to produce a yellow glow instead of its usual ultraviolet light.

"We deliberately designed compounds with an electronic structure that is atypical for lanthanides," said Bardonov. "Rather than focusing on a single example, we synthesised a series of compounds from cerium to terbium to observe how their properties change and to identify common patterns."

This shift in emitted light color also occurred in other rare earth metals, revealing a new way to control the colors these elements emit and opening possibilities for more efficient, customizable light-emitting materials.

"To understand how this process works, it's important to first grasp the mechanism of energy transfer," said Dmitrii Roitershtein, Academic Supervisor of the Chemistry of Molecular Systems and Materials Programme and co-author of the paper. "Typically, a ligand molecule absorbs ultraviolet light, enters an excited state, and then transfers this energy to the metal atom, causing it to emit light. However, in the new compounds, the process occurred differently: energy was transferred not directly to the 4f electrons, but via an intermediate 5d state."

In other words, rather than energy passing directly from the surrounding molecules to the deepest parts of the atom, it moves first through an intermediate energy level inside the atom, altering how the energy is released and allowing cerium to shine with visible yellow light instead of its usual invisible ultraviolet glow.

"We were able to demonstrate exactly how the environment of an atom influences its electronic transitions and lanthanide luminescence," said Fyodor Chernenkiy, a bachelor's student at the HSE Faculty of Chemistry. "We can now intentionally select the structure of compounds to control luminescence and produce materials with specific optical properties."

The researchers believe that the ability to predict the luminescence spectrum will make it possible to design materials with desired properties more efficiently by eliminating the need for time-consuming trial and error, which could ultimately facilitate the creation of new and advanced light sources.