New rare earth compounds offer window into exploring future catalytic and biomedical uses

A compound does not need many elements to be scientifically complicated.

Researchers at Arizona State University and international collaborators have created two previously unreported compounds made from just three elements: cerium, phosphorus and oxygen. They are known as cerium(III) oxyphosphates. 

The discovery could open new directions for studying cerium’s chemistry. 

Cerium-based materials are already used in catalysis, for example, catalytic converters in cars. 

Also cerium oxide nanoparticles have shown promise in experimental studies of protection against oxidative damage. Much of that work focuses on modifying or applying cerium oxide. The new research takes a different approach: changing the crystal framework surrounding cerium itself.

The team’s first discovery of the new cerium rare earth materials, reported in the Proceedings of the National Academy of Sciences, brought together a computational materials group led by Qijun Hong, assistant professor in the School for Engineering of Matter, Transport and Energy and an experimental group led by Alexandra Navrotsky, a Regents Professor in both the School of Molecular Sciences.

The new cerium oxyphosphates have intricate crystal structures that place cerium atoms in chemical environments unlike those found in more familiar cerium-based materials.

“We have studied cerium compounds for decades, yet they still surprise us," said Alexandra Navrotsky, Regents Professor in both the School of Molecular Sciences in The College of Liberal Arts and Sciences and the School for Engineering of Matter, Transport and Energy in the Ira A. Fulton Schools of Engineering.  

"Their exciting complexity arises from a myriad of energetically accessible structures and oxidation states. By laser melting monazite, we recently made two new oxyphosphates in a system containing only three elements. Simple composition does not mean simple structure or properties. We are also studying cerium oxide and phosphate nanoparticles for biomedical and environmental applications," said Navrotsky, who is also an affiliated professor with the School of Earth and Space Exploration and director of the Navrotsky Eyring Center for Materials of the Universe. 

"New science is exciting,” she said.

“Monazite contains cerium and other light lanthanides in the trivalent state," said Sergey Ushakov, a research associate professor in ASU’s School of Molecular Sciences. "It is a widespread accessory mineral, meaning it occurs in small amounts in many different kinds of rocks.” 

“Grains of monazite have even been found in tektites — natural glasses formed when asteroid impacts melt rocks on Earth. By laser-melting synthetic monazite in a reducing atmosphere, we synthesized two previously unreported cerium(III) oxyphosphates," he said.

Computer calculations pointed to one promising compound called Ce₃PO₇. Laser-melting experiments produced that compound along with a second new material, Ce₇P₃O₁₈.

"We shared our discovery of Ce₃PO₇ with geologists studying these impact materials, encouraging them to look for this new phase in nature,” Ushakov said.

“This discovery is especially meaningful to me because it grew out of relationships built over more than a decade,” Hong said.

“Alex (Navrotsky) played an important role in recruiting me to ASU, and I have been fortunate to collaborate closely with her and Sergey ever since. Alex and Sergey bring extraordinary depth in thermochemistry and experimental materials science, while my group contributes computational prediction." 

“Over the years, we have learned how to frame questions together that neither computation nor experiment could answer alone. In this case, our calculations pointed to a previously unreported cerium oxyphosphate, Ce₃PO₇. The importance of this work lies not only in the materials we discovered, but also in how decades of experimental knowledge and modern computation came together to open a new direction for understanding rare-earth oxyphosphates,” Hong said.

Two follow-up studies are now helping the team understand the compounds in greater detail. One develops a structural model for the family of materials containing seven rare-earth atoms for every three phosphorus atoms. The other measures the thermodynamic properties of Ce₃PO₇ and related 3:1 oxyphosphates, comparing the results with earlier calculations.

The oxyphosphate family itself is not new. Some members had been reported as early as the 1970s, but much of their crystal chemistry and energetics remained poorly understood. 

Researchers in Bonn, Germany, continued to investigate the materials. In 2018, Robert Glaum’s group at the University of Bonn determined the structure of Eu₃PO₇ using single-crystal X-ray diffraction, expanding the limited structural information available for the family.

“My interest in oxyphosphates goes back to my first paper with Alex, published in 2001, when I came across reports of these poorly characterized compounds,” Ushakov said.

“In 2020, Dustin Gilbert at the University of Tennessee, Knoxville, and I submitted a proposal to the National Science Foundation to study them, including their magnetic properties. The proposal was not funded. One concern raised in the reviews was whether we could establish their complex structures from powder diffraction without growing single crystals. Combining computation with experiment later helped us address that challenge.”

A new opportunity emerged in 2023, when oxyphosphate synthesis and thermodynamic research became part of a collaboration between the University of Virginia and ASU. Supported by the Department of Defense’s Multidisciplinary University Research Initiative, the project is led by Elizabeth J. “Beth” Opila of the University of Virginia and focuses on refractory materials — materials designed to withstand extreme heat.

Although the project’s broader goals were different, it provided support for investigating the same poorly understood compounds. The collaboration brought together synthesis, thermochemistry and computational prediction.

Hong’s team used quantum-mechanical calculations to compare the energies of possible rare-earth oxyphosphates with those of competing compounds. Experimental data from earlier orthophosphate studies, including 2001 thermochemistry work, helped the team evaluate the reliability of the calculations.

For Ce₃PO₇, the calculations indicated that the compound could form favorably from cerium(III) oxide and cerium orthophosphate. The calculations also showed why the reaction environment mattered. In the presence of oxygen, cerium dioxide and cerium orthophosphate were more favorable products.

The experimental team used a carbon dioxide laser to melt cerium orthophosphate, CePO₄, a material with the monazite crystal structure. When the experiment was carried out in a hydrogen–argon atmosphere, it produced a green-and-yellow sample. X-ray diffraction revealed the predicted Ce₃PO₇ as well as Ce₇P₃O₁₈.

These were the first reported syntheses of the two cerium compounds. The achievement was not simply a new method for producing familiar materials; it introduced previously unknown compounds.

Ce₇P₃O₁₈ belongs to what researchers call the 7:3 oxyphosphate family, named for the ratio of rare-earth atoms to phosphorus atoms. When the cerium compound was first reported, the atomic structure of this family was still unknown.

In a 2026 study in the Journal of the American Ceramic Society, the ASU–-Bonn collaboration combined computer modeling with powder diffraction data. Rather than trying to interpret the diffraction pattern alone, the researchers began with the known structure of a related lanthanum vanadate. They replaced vanadium with phosphorus in their calculations and optimized the resulting atomic arrangements.

The proposed models were then tested against experimental diffraction patterns.

One especially direct test used the diffraction pattern for Ce₇P₃O₁₈ that had been published with the original 2025 discovery. The researchers fitted the new model to that same pattern. The resulting unit-cell dimensions followed the trend observed in independently synthesized members of the broader family.

In this approach, computer calculations supplied the atomic positions, which were held fixed while researchers refined the dimensions of the crystal cell and other diffraction parameters. The combined method provided structural information without requiring a single crystal of every compound.

The researchers also developed another way to make Ce₃PO₇. Glaum’s laboratory prepared the compound through a conventional solid-state reaction, heating cerium metal, cerium dioxide and cerium orthophosphate together in an evacuated tube.

At ASU, researchers measured the compound’s thermodynamic properties as part of a broader study of oxyphosphates. Using calorimetry, they measured the heat released or absorbed when samples dissolved in a molten solvent. Combined with reference data, those measurements yielded formation enthalpies — the energy changes associated with forming the compounds from specified starting materials.

The experimental results agreed with calculations for the lighter rare-earth compounds, including the cerium material, while showing differences elsewhere in the series. The findings provide measurements that can be used to test and improve computational predictions.

The thermochemistry study therefore adds evidence about how the materials form and behave energetically. It is not a second discovery of Ce₃PO₇.

The next question is whether the unusual crystal structures produce useful differences in reactivity.

That will require direct testing. Potential applications in catalysis or biomedical research would depend on how active and stable the materials are. Biological applications would also require careful studies of compatibility and safety.

The fact that a compound contains cerium in the +3 oxidation state does not mean it will behave like cerium oxide nanoparticles — or that it will perform better. Those possibilities remain to be demonstrated.

For now, the new compounds provide researchers with something equally important: materials that can be studied.

A question first encountered during an early thermochemistry project has grown into an international collaboration involving prediction, synthesis, structural analysis and measurement. The resulting compounds show that even a system made from only three elements can contain complex arrangements of atoms and unexpected avenues for discovery.

The 2025 PNAS discovery, the 2026 study of the 7:3 structure and the just published ACS Omega thermochemistry study acknowledge support from U.S. Army Research Office award W911NF-23-2-0145.