Prediction of the phase stability and elastic property of ϵ-Pu at high temperature from theoretical study

The phase stability and elastic property of ϵ-Pu at 800 K are predicted through systematically theoretical calculations, by taking the temperature-dependent phonon smearing, spin fluctuation, phonon vibration, and volume expansion effects into account. Dominated by the phonon smearing at 800 K, ϵ-Pu is thermodynamically stabilized in the paramagnetic state with the absolute value of the local magnetic moment (μPu) about 3.8 μB, being smaller than the 0 K value (about 5.0 μB). The resulted thermal spin fluctuation, which prefers the δ−ϵ phase transition, greatly decreases the volume (V) and the shear elastic constants (C′ and C44) of the ϵ phase. However, the phonon smearing itself prefers the expansion of V and also the increase of C′ and C44. Besides the influence of the spin fluctuation and phonon smearing at 800 K, the equilibrium V of ϵ-Pu seems to be dominated by the phonon vibration, and the lattice thermal expansion also further increases C′ whereas decreases C44. The determined V as well as C′ and C44 of ϵ-Pu at 800 K is reasonable in comparison with the previous studies. This insight provides a good understanding of the physical mechanisms driving the thermodynamical stability of ϵ-Pu at high temperature.