Atomistic studies of transformation pathways and energetics in plutonium

TL;DR

This study investigates the energetics of two proposed transformation pathways in plutonium, finding the hcp pathway energetically favored and highlighting limitations of current simulation potentials in capturing structural anisotropy.

Contribution

It provides the first detailed energetic comparison of two transformation pathways in Pu using MEAM simulations, identifying the hcp pathway as more favorable.

Findings

The hcp pathway is energetically favored over the sh pathway.

Volume change occurs mainly during the second step of transformation.

Current MEAM potentials have limitations in modeling anisotropic structures.

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 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.