New analysis pins down 5f electron count in plutonium

Researchers report what may be a major advance in the fundamental understanding of plutonium, a material crucial to national security and energy applications. Using a multi-pronged approach, they reevaluated the 5f electron occupancy in alpha (α) and delta (δ) phase plutonium. They set the 5f electron count in α-phase plutonium at 5.0 ± 0.1 and in δ-phase plutonium at 4.9 ± 0.2.

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Why this matters: The occupancy of the 5f electrons in plutonium has been a source of controversy, with previous estimates ranging from five to six electrons. Accurately determining the 5f electron count is essential for developing accurate theoretical models to describe the electronic, magnetic and thermodynamic properties of plutonium and other actinide materials.

What they did: Building on prior studies, researchers from Los Alamos National Laboratory’s Materials Science and Technology division and the University of Wisconsin-Oshkosh applied a novel thermodynamic evaluation of electronic configuration entropies to suggest a 5f occupancy of approximately 5.0. Their comprehensive reanalysis of past spectroscopy results, such as X-ray absorption spectroscopy, corrected issues in earlier electron energy loss (EELS) measurements. 

What’s next: The detailed analysis techniques employed, including the identification of pitfalls in certain data analysis methods, provide a roadmap for future studies of complex f-electron systems.

Funding: The entropic arguments are based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (OBES), Heavy Element Chemistry program. The Advanced Light Source is supported by the Director of the Office of Science, OBES at Lawrence Berkeley National Laboratory.

LA-UR-25-28707