  One of the most challenging problems in understanding the structural phase
transformations in Pu is to determine the energetically favored, continuous
atomic pathways from one crystal symmetry to another. This problem involves
enumerating candidate pathways and studying their energetics to garner insight
into instabilities and energy barriers. The purpose of this work is to
investigate the energetics of two transformation pathways for the delta to
alpha' transformation in Pu that were recently proposed [Lookman et al., Phys.
Rev. Lett. 100:145504, 2008] on the basis of symmetry. These pathways require
the presence of either an intermediate hexagonal closed-packed (hcp) structure
or a simple hexagonal (sh) structure. A subgroup of the parent fcc and the
intermediate hexagonal structure, which has trigonal symmetry, facilitates the
transformation to the intermediate hcp or sh structure. Phonons then break the
translational symmetry from the intermediate hcp or sh structure to the final
monoclinic symmetry of the alpha' structure. We perform simulations using the
modified embedded atom method (MEAM) for Pu to investigate these candidate
pathways. Our main conclusion is that the path via hcp is energetically favored
and the volume change for both pathways essentially occurs in the second step
of the transformation, i.e. from the intermediate sh or hcp to the monoclinic
structure. Our work also highlights the deficiency of the current
state-of-the-art MEAM potential in capturing the anisotropy associated with the
lower symmetry monoclinic structure.
