A long-standing problem in Pu science is the crystallographic mechanism for the $\delta \to {\alpha }^{\prime }$ ($\mathrm{fcc}\to \mathrm{\text{monoclinic}}$) 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: $\mathrm{fcc}\to \mathrm{\text{trigonal}}\to \mathrm{\text{hexagonal}}\to \mathrm{\text{monoclinic}}$. 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 $\delta \to {\alpha }^{\prime }$ 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.

• Received 21 October 2007

DOI:https://doi.org/10.1103/PhysRevLett.100.145504