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

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.