Surprise: At high pressures, curium can form unexpected covalent bonds, study finds

BUFFALO, N.Y. and TALLAHASSEE, Fla. — New research shows that curium — one of the heaviest known elements — can be manipulated to a larger degree than previously thought.

An international team published a study on July 15 in the journal Nature showing how curium — element 96 in the periodic table — responds to the application of high pressure created by squeezing a sample between two diamonds.

Led by a Florida State University Professor Thomas Albrecht-Schmitt and his collaborators at the University at Buffalo and RWTH Aachen University, scientists found that the behavior of curium’s outer electrons — which influence its ability to bond with other elements — can be altered by shortening the distance between curium and surrounding light atoms in a certain crystalline compound.

“This was not anticipated because the chemistry of curium makes it resistant to these types of changes,” said Albrecht-Schmitt, the Gregory R. Choppin Professor of Chemistry at Florida State University. “In short, it is quite inert.”

“The curium(3+) ion we studied has a half-filled outer electron shell that is very difficult to engage in chemical bonding,” says Jochen Autschbach, Larkin Professor of Chemistry in the University at Buffalo College of Arts and Sciences. “An integrated experimental and theoretical approach showed that the application of high pressure to a crystal containing curium(3+), along with sulfur-organic and ammonium ions, causes the outer shell of curium to participate in covalent chemical bonding with sulfur. This finding may help guide new ways of studying the mysterious behavior of chemically resistant actinide shells.”

The study was led by Albrecht-Schmitt; UB chemistry professors Autschbach and Eva Zurek; and Manfred Speldrich, a researcher at RWTH Aachen University in Germany.

Albrecht-Schmitt’s work is part of his lab’s overall mission to better understand the heavier, or actinide, elements at the bottom of the periodic table that still hold many mysteries for scientists. In 2016, he received $10 million from the U.S. Department of Energy to form the Center for Actinide Science and Technology to focus on accelerating scientific efforts to clean up nuclear waste.

Greater understanding of heavier elements could open the door to additional strategies to control chemical separation used in nuclear recycling and in designing resilient materials for long-term storage of radioactive elements, Albrecht-Schmitt said. The research team believes the results they achieved related to curium could translate to other heavy elements as well.

In the new study, Autschbach’s group at UB carried out calculations that helped to explain what happened during the high-pressure experiments, revealing details about how curium behaves chemically, when compounds containing the element are squeezed between diamonds, and how this influences the color of light emitted by curium. Zurek’s team laid the foundation for these computations by predicting the crystal structures of the compounds under high pressure.

“Under pressure, chemical compounds and materials can behave completely differently than they do at atmospheric conditions, making the discoveries in high-pressure research so exciting,” Zurek says.

“It’s an exciting experiment that showed that we have much greater control of the chemistry of these difficult to control elements than previously thought,” Albrecht-Schmitt said.

The research team plans to follow up on this work by designing similar experiments for heavier elements such as californium and einsteinium, where the effects of the pressure could be even greater than what they have found for curium.

This study was funded by the U.S. Department of Energy, with additional support from the U.S. National Science Foundation.