Cerium Glows Yellow: Chemists Discover How to Control Luminescence of Rare Earth Elements

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Researchers at HSE University and the Institute of Petrochemical Synthesis of the Russian Academy of Sciences have discovered a way to control both the colour and brightness of the glow emitted by rare earth elements. Their luminescence is generally predictable—for example, cerium typically emits light in the ultraviolet range. However, the scientists have demonstrated that this can be altered. They created a chemical environment in which a cerium ion began to emit a yellow glow. The findings could contribute to the development of new light sources, displays, and lasers. The study has been published in Optical Materials.

Rare earth elements are used in microelectronics, LEDs, and fluorescent materials because of their ability to emit light in precisely defined colours. This depends on how their electrons behave when absorbing and releasing energy.

When an atom absorbs energy—such as from light or an electric current—one of its electrons can be excited to a higher energy level. However, this excited state is unstable, and after a short time, the electron returns to its original level, releasing the excess energy as light. This process is known as luminescence.

Photo of the synthesised cerium complex and its molecular structure

Dmitrii Roitershtein

In rare earth elements, the glow results from electron transitions between 4f orbitals—regions around the atomic nucleus where electrons can reside. Typically, the energy of these transitions is fixed, meaning the colour of the glow remains constant: cerium emits invisible ultraviolet light, while terbium emits green. The 4f orbitals are situated deep within the atom and interact minimally with the surrounding environment. In contrast, the 5d orbitals are sensitive to external influences but generally do not contribute to the luminescence of lanthanides due to their excessively high energy. 

However, scientists from HSE University and the Institute of Petrochemical Synthesis of the Russian Academy of Sciences have demonstrated that the colour of the radiation can be altered by adjusting the chemical environment of the metals. They synthesised cerium, praseodymium, and terbium complexes using organic ligands—molecules that surround metal ions. These ligands shape the geometry of the complex and influence its properties. In all cases, three cyclopentadienyl anions were symmetrically arranged around the metal. These anions consist of regular pentagons of carbon atoms, to which large organic fragments are attached, providing the required structure for the complex. This environment generates a specific electrostatic field around the ion, which alters the energy of the 5d orbitals and, consequently, affects the luminescence spectrum.

Daniil Bardonov

'Previously, a change in the colour of the glow had been observed, but the underlying mechanism was not understood. Now, in collaboration with our physicist colleagues, we have been able to understand the mechanism behind this effect. We deliberately designed compounds with an electronic structure that is atypical for lanthanides. 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,' comments Daniil Bardonov, a master's student at the HSE Faculty of Chemistry.

In conventional compounds, cerium emits ultraviolet light with wavelengths between 300 and 400 nanometres. In the new complexes, its emission shifted to the red range, reaching up to 655 nanometres. This indicates that the energy gap between the 4f and 5d levels has decreased. A similar rearrangement of electronic levels was observed in the other lanthanides studied, also resulting in changes to their luminescence.

Dmitrii Roitershtein

'To understand how this process works, it’s important to first grasp the mechanism of energy transfer. 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,' explains Dmitrii Roitershtein, Academic Supervisor of the Chemistry of Molecular Systems and Materials Programme and co-author of the paper. 'However, in the new compounds, the process occurred differently: energy was transferred not directly to the 4f electrons, but via an intermediate 5d state.'

The researchers believe that being able 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. This could facilitate the creation of new and advanced light sources.

'We were able to demonstrate exactly how the environment of an atom influences its electronic transitions and lanthanide luminescence,' says Fyodor Chernenkiy, 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.'