Atomistic studies of transformation pathways and energetics in plutonium

Abstract

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 δ → α ′ transformation in Pu that were recently proposed [1] 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 α ′ 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.

X-ray diffraction study of δ-stabilized plutonium alloys under pressure

“…a 0 transformation occurs via an intermediate hexago- nal structure [15][16]. However, the fact that the MEAM potential used in Gröger's calculations does not reproduce accurately the low symmetry structures of Pu (a-and b-phase) could bias the predictions.…”

“…a 0 -phase transformation (the prime denotes solute atoms trapped in the a lattice), which can be triggered by increasing pressure or lowering temperature has been studied extensively [1,[6][7][8][9][10][11][12][13][14][15][16], revealing often intermediate phases. Progress is being made in its characterization and description but given the variety of behaviours (resulting for instance to a many parameters dependence such as solute type, solute distribution, chemical impurities, kinetics, thermodynamic path.…”