Normally, materials expand when heated. Higher temperatures cause atoms to vibrate, bounce around and take up a larger volume. However, for one specific phase of plutonium - called delta-plutonium - the opposite inexplicably occurs: it shrinks above room temperature.
As part of its national security mission, Lawrence Livermore National Laboratory (LLNL) aims to predict the behavior of plutonium in all of its phases. Unraveling the mystery behind delta-plutonium's abnormal behavior at high temperatures is an important piece of the picture.
In a new study, published in Reports on Progress in Physics, researchers from LLNL demonstrate a model that can reproduce and explain delta-plutonium's thermal behavior and unusual properties. The model calculates the material's free energy, a quantity that reflects the amount of available or useful energy in a system.
"Free energy fundamentally dictates the state of a material, so it is foundational for understanding it," said LLNL scientist and author Per Söderlind. "An immense amount of effort at LLNL is dedicated to predicting the behavior of plutonium. The confidence in these predictions depends on a deep theoretical understanding of its electronic structure and free energy."
Plutonium's electronic structure is among the most complex of all elemental metals because its electrons are easily influenced by relativity, magnetism and crystal structure. The new free-energy model accounts for magnetic fluctuation effects for the first time.
"Our model is unique and novel because it includes magnetic states that are allowed to fluctuate and depend on temperature," said Söderlind.
Acknowledging those magnetic states in the theory causes it to match the odd experimental observations of contraction at high temperatures.
This methodology could be extended to other materials where dynamic magnetism plays a role, such as iron and its alloys, which are important in geophysics. Going forward, the authors plan to address the impacts of microstructures, defects and imperfections that are present in real-world material.
