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 favoured, 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 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 favoured 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.

Keywords:

• plutonium
• phase transformation
• intermediate phase
• modified embedded atom method

Acknowledgements

We are grateful to Jim Smith for his inspiration and interest in our work on actinides and phase transformations. The authors acknowledge many discussions on the topic with Mike Baskes, Jason Lashley, Steve Valone, Chris Taylor, Terry Mitchell and Siegfried Hecker. This work has been supported by the LDRD project of the Seaborg Institute for Transactinium Science, Los Alamos National Laboratory.

Notes

Note

1. Similar measurements for the δ phase were not available at the time. These experiments were performed recently and are published in 17.