Physical mechanism of δ-δ′-ε phase stability in plutonium

Abstract

Based on first-principle calculations, we have systematically explored the nature of the elastic stability and the δ-δ′-ε phase transitions in pure Pu at high temperature. It is found that, both the electron-phonon coupling and the spin fluctuation effects tend to decrease the tetragonal elastic constant (C′) of δ-Pu, accounting for its anomalous softening at high temperature. The lattice thermal expansion together with the electron-phonon coupling can stiffen C′ of ε-Pu, promoting its mechanical stability at high temperature. The δ-ε transition is calculated to take place around 750–800 K, and is dominated by the phonon vibration. The δ′ intermediate phase is realized around 750 K mainly because of the thermal spin fluctuation.

Phonon density of states for α-plutonium from density-functional theory

“…For the last twenty years, many papers have been published focusing on either of these two opposing views for plutonium’s electronic structure, see for example ref. 4–21 .…”

“…Particularly, the model explains the chemical 5f-electron bonding that governs the phase diagram, lattice constants, elastic constants, and phonons (δ plutonium). For ε plutonium the phonons and elastic constants have also been predicted 20,21 but judging their quality will await experimental studies. Theoretical lattice dynamics for the complex α phase has been avoided, however, because of technical and computational barriers that have made it too onerous to investigate.…”