Phonon mechanisms and transformation paths in Pu

Area of Science:

• Materials Science
• Solid-State Physics
• Plutonium Science

Background:

• The crystallographic mechanism of the delta to alpha-prime (face-centered cubic to monoclinic) transformation in plutonium (Pu) is a long-standing challenge.
• Orientation relationships between delta and alpha-prime phases restrict possible mechanisms, indicating a reconstructive transition.

Purpose of the Study:

• To elucidate the crystallographic mechanism of the delta to alpha-prime transformation in plutonium.
• To propose a sequence of displacive transitions that explains the reconstructive nature of the transformation.

Main Methods:

• Phonon dispersion analysis to predict instabilities in the delta phase.
• Formulation of a free energy model to describe atomic displacements during the transition.
• Theoretical modeling of reconstructive phase transitions.

Main Results:

• The delta to alpha-prime transformation is described as a sequence of three displacive transitions: fcc -> trigonal -> hexagonal -> monoclinic.
• Predicted instabilities along the Lambda and Sigma branches in the phonon dispersion of the delta phase.
• A free energy formulation captures atomic displacements across the transition.

Conclusions:

• The proposed mechanism provides a pathway for the reconstructive delta to alpha-prime transformation in plutonium.
• The delta to alpha-prime transition may represent a threshold for changes in orientation relationships across actinides.
• Correlations with electron itinerancy, magnetism, and volume changes are suggested for actinide phase transitions.