Ground state properties and high pressure behavior of plutonium dioxide: Systematic density functional calculations

TL;DR

This study uses density functional theory to systematically analyze the structural, electronic, and thermodynamic properties of PuO₂, including phase transitions under high pressure, with implications for nuclear waste storage.

Contribution

It provides a comprehensive first-principles investigation of PuO₂'s properties and phase transitions, optimizing the Hubbard U parameter for accurate modeling.

Findings

Optimal U parameter around 4 eV for best agreement with experiments

Prediction of metallic transition at approximately 133 GPa

Identification of an isostructural transition between 75-133 GPa

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 PuO$_{2}$. To properly describe the strong correlation in the Pu $5f$ electrons, the local density approximation$+U$ and the generalized gradient approximation$+U$ theoretical formalisms have been employed. We optimize the $U$ parameter in calculating the total energy, lattice parameters, and bulk modulus at the nonmagnetic, ferromagnetic, and antiferromagnetic configurations for both ground state fluorite structure and high pressure cotunnite structure. The best agreement with experiments is obtained by tuning the effective Hubbard parameter $U$ at around 4 eV within the LDA$+U$ approach. After carefully testing the validity of the ground state, we further investigate the bonding nature, elastic constants, various moduli, Debye temperature, hardness, ideal tensile strength, and phonon dispersion for fluorite PuO$_{2}$. Some thermodynamic properties, e.g., the Gibbs free energy, volume thermal expansion, and specific heat, are also calculated. As for cotunnite phase, besides the elastic constants, various moduli, and Debye temperature at 0 GPa, we have further presented our calculated electronic, structural, and magnetic properties for 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.