Ground-state properties and high-pressure behavior of plutonium dioxide: Density functional theory calculations

Abstract

Plutonium dioxide is of high technological importance in nuclear fuel cycle and is particularly crucial in long-term storage of Pu-based radioactive waste. Using first-principles density-functional theory, in this paper we systematically study the structural, electronic, mechanical, thermodynamic properties, and pressure-induced structural transition of ${\text{PuO}}_{2}$. To properly describe the strong correlation in $\text{Pu}\text{ }5f$ electrons, the local-density approximation $(\text{LDA})+U$ and the generalized gradient $\text{approximation}+U$ theoretical formalisms have been employed. We optimize $U$ parameter in calculating the total energy, lattice parameters, and bulk modulus at nonmagnetic, ferromagnetic, and antiferromagnetic configurations for both ground-state fluorite structure and high-pressure cotunnite structure. Best agreement with experiments is obtained by tuning the effective Hubbard parameter $U$ at around 4 eV within $\text{LDA}+U$ approach. After carefully testing the validity of the ground-state calculation, we further investigate the bonding nature, elastic constants, various moduli, Debye temperature, hardness, ideal tensile strength, and phonon dispersion for fluorite ${\text{PuO}}_{2}$. Some thermodynamic properties, e.g., Gibbs free energy, volume thermal expansion, and specific heat are also calculated. As for cotunnite phase, besides elastic constants, various moduli, and Debye temperature at 0 GPa, we have further presented our calculated electronic, structural, and magnetic properties for ${\text{PuO}}_{2}$ under pressure up to 280 GPa. A metallic transition at around 133 GPa and an isostructural transition in pressure range of 75--133 GPa are predicted. Additionally, as an illustration on the valency trend and subsequent effect on the mechanical properties, the calculated results for other actinide metal dioxides (${\text{ThO}}_{2}$, ${\text{UO}}_{2}$, and ${\text{NpO}}_{2}$) are also presented.