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 PuO 2 . To properly describe the strong correlation in Pu 5f electrons, the local density approximation+U and the generalized gradient 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 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 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 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 (ThO 2 , UO 2 , and NpO 2 ) are also presented.

First-principles study of surface properties of PuO2: Effects of thickness and O-vacancy on surface stability and chemical activity

“…Fortunately, several approaches, including the LDA/GGA+U [13][14][15], the hybrid density functional of (Heyd, Scuseria, and Enzerhof) HSE [16], the self-interaction corrected local spin density SIC-LSD [17], and LDA combined dynamical mean-field theory DMFT [18], have been developed to correct these failures in calculations of actinide compounds. The effective modification of pure DFT by LDA/GGA+U formalisms has been confirmed widely in study of PuO 2 [19][20][21][22][23][24]. By tuning the effective Hubbard parameter in a reasonable range, the AFM Mott insulator feature was correctly calculated and the structural parameters as well as the electronic structure are well in accord with experiments.…”

“…Since the GGA+U and LDA+U give the similar description of the PDOS, here we only plot the GGA+U results. The orbital-resolved PDOS of the bulk PuO 2 at the ground state has been calculated and analyzed in detail by previous DFT+U [19,21,22,24] and hybrid DFT [9,16] studies, and those theoretical results of DOS are usually tested by comparing with the experimental photoelectron spectroscopy (PES) measurements [6,7].…”

“…The exchange and correlation effects are treated in both the LDA and the GGA [31], based on which the strong on-site Coulomb repulsion among the localized Pu 5f electrons is described by using the LDA/GGA+U formalisms formulated by Dudarev et al [13][14][15]. As concluded in some previous DFT studies [19,20,22,24], although the pure LDA and GGA fail to depict the electronic structure, especially the insulating nature and the occupied-state character of bulk AFM PuO 2 , the LDA/GGA+U approaches can capture the Mott insulating properties of the strongly correlated Pu 5f electrons adequately and well reproduce experimental ground-state parameters by tuning the effective Hubbard parameter U eff at ∼4 eV.…”

“…In this paper, the spherically averaged screened Coulomb energy U and the exchange energy J for the Pu 5f orbitals are set to be 4.75 and 0.75 eV respectively, which have been tested and applied in our previous studies of plutonium oxides [19,20,24]. For fluorite structure PuO 2 of AFM, our calculated equilibrium lattice parameter a 0 =5.466Å within GGA+U or a 0 =5.362Å within LDA+U is in good agreement with the experimental value of 5.398Å [5].…”

The environmental dependence of redox energetics of PuO2 and α-Pu2O3: A quantitative solution from

“…The effective modification of pure DFT by LDA/GGA+U formalisms has been confirmed widely in studies of PuO 2 [27][28][29][30][31][32][33][34][35], which have reported the AFM-insulator ground state of…”