Phonon Mechanisms and Transformation Paths in Pu

A long-standing problem in Pu science is the crystallographic mechanism for the δ→α^' (fcc→monoclinic) transformation. Orientation relations between the two structures impose severe restrictions on the possible mechanisms and require the transition to be reconstructive, which we describe as a sequence of three displacive transitions: fcc→trigonal→hexagonal→monoclinic. We predict instabilities along the Λ and Σ branches in the phonon dispersion of the δ phase and formulate a free energy to describe the displacement of atoms across the transition. We suggest that the δ→α^' transition in Pu lies at the threshold of a change in character of the orientation relationship from lighter to heavier actinides, correlating with changes in electron itinerancy, magnetism, and volume.