Phonon mechanisms and transformation paths in Pu

A long-standing problem in Pu science is the crystallographic mechanism for the delta ->alpha' (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 Lambda and Sigma branches in the phonon dispersion of the delta phase and formulate a free energy to describe the displacement of atoms across the transition. We suggest that the delta ->alpha' 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.