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 $\text{Pu}5f$ electrons, the local-density approximation $\left(\text{LDA}\right)+U$ 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.

• Received 6 May 2010

DOI:https://doi.org/10.1103/PhysRevB.82.144110