New magnetic model explains why delta-plutonium shrinks when heated

Most materials expand when heated. This happens because the rising temperature causes atoms to vibrate intensely, move further apart, and occupy more space. Delta-plutonium, however, breaks this ubiquitous rule—in conditions above room temperature, it unexpectedly shrinks instead of expanding, a phenomenon that has puzzled scientists for decades. 

In a new study published in Reports on Progress in Physics, researchers at the Lawrence Livermore National Laboratory (LLNL) in California have developed a detailed model that not only reproduces this counterintuitive thermal behavior but also explains the underlying physics. 

By calculating the material’s free energy—a measure of the useful energy available within a system—the model provides a deeper understanding of delta-plutonium’s unique and often unpredictable properties.

Magnetic effects unlock deeper understanding of plutonium’s properties

Among all elemental metals, plutonium stands out for the extraordinary complexity of its electronic structure, shaped by the interplay of relativity, magnetism, and crystal arrangement. 

For the first time, scientists have developed a free-energy model that incorporates the effects of magnetic fluctuations by accounting for how magnetic states shift and vary with temperature, providing a more comprehensive understanding of the subtle forces influencing plutonium’s distinctive behavior.

According to LLNL scientist and study author Per Söderlind, free energy is a fundamental factor in determining the state of a material, making it essential for the study of a particular matter. At the LLNM, a significant amount of work is dedicated to predicting how plutonium will act under various conditions. 

The accuracy of these predictions relies heavily on developing a deep theoretical understanding of the metal’s electronic structure and free energy, both key to explaining its unusual and often counterintuitive properties.

Dynamic magnetism model offers insights beyond plutonium

By incorporating temperature-dependent magnetic states, the model accurately reproduces the unusual experimental observation that delta-plutonium contracts at high temperatures. 

This innovative approach not only offers fresh insight into the metal’s complex thermal and electronic properties but could also be extended to other materials where dynamic magnetism plays a key role, such as iron and its alloys—which are important in geophysics. According to Söderlind, this unique inclusion of fluctuating magnetic states marks a significant advancement in modeling plutonium. 

Looking ahead, the researchers aim to expand their model to include the effects of microstructures, defects, and other imperfections that naturally occur in real-world plutonium samples. These factors can significantly influence the material’s behavior and properties, so incorporating them will help create an even more comprehensive understanding. 

By capturing these nuances, the team hopes to significantly enhance the accuracy of predictions regarding plutonium’s performance in real-world applications, including its stability under different environmental conditions. This deeper understanding could also lead to safer handling, better material design, and more effective use of the element in various technologies.

You can view the study here.

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