First-principles calculations of phase transition, phonon spectra, thermodynamic properties and elasticity for PuO2 polymorphs

Abstract

The first principles are utilized to calculate the phase transition, phonon spectra, thermodynamic properties, and elasticity of PuO₂ under varying pressures. Under the compression condition, the anticipated pressure of phase transition from Fm $\bar{3}$ m to Pnma is 26.7 GPa, from Pnma to Cmcm is 112.9 GPa, and from Cmcm to I4/mmm phase is 588.0 GPa. As the pressure decreases, the phase transition pressure of anticipating from Fm $\bar{3}$ m phase to P4₂/mnm phase occur at −8.6 GPa. The phonon spectra of these six structures show that all phases are dynamically stable under the selected pressure. Meanwhile, the verification results of the mechanical stability criterion show that Fm $\bar{3}$ m, Pnma, Cmcm and I4/mmm of PuO₂ are mechanically stable, nevertheless P4₂/mnm is unstable in high pressure. The thermodynamic properties were then calculated to further research the patterns of variation in the coefficient of thermal expansion, bulk modulus, Gibbs free energy, and heat capacity with temperature, and these were compared with experimental data.