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.
Lookman
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Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
Saxena
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Albers
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