First-principles study of phase stability, electronic and mechanical properties of plutonium sub-oxides

Abstract

The formation energies (ΔH_f) of fluorite PuO₂, α-Pu₂O₃ and sub-oxides PuO_2−x (0.0 < x < 0.5) are determined from atomic scale simulations based on density functional theory (DFT) employing the generalised gradient approximation (GGA) corrected with an effective Hubbard parameter (U_eff). The variation of structural and electronic properties of PuO₂ and α-Pu₂O₃ is determined while ramping up U_eff from 0 eV to 5 eV (U_eff-ramping method) to treat the presence of metastable magnetic states and to determine the most suitable U_eff value matching the experiments. The GGA+U calculated lattice parameter variation as a function of stoichiometry (a(x)) for PuO_2−x shows a positive volume of relaxation and an almost linear variation presented by the relation a(x) = a₀ − 0.522738x, where a₀ is the equilibrium lattice parameter of PuO₂. The GGA+U calculated ΔH_f values of PuO_2−x lie above the tie line connecting the ΔH_f of PuO₂ and Pu₂O₃, and with decreasing O/Pu ratio, the stability of the sub-oxides increases. The crystal and electronic structure analysis of the oxygen vacancy in PuO₂ shows outward anisotropic relaxation of four Pu atoms around the vacancy site. The electronic charges within the Wigner–Seitz sphere around these Pu atoms show an overall gain of only (0.12–0.22)e per Pu atom, signifying an incomplete localization of charges. Finally, the GGA+U calculated single crystal elastic constant values decrease continuously with decreasing O/Pu ratio from 2.0 to 1.5. The rate of decrease of the average C₁₁ is almost 11–15 times higher compared to the rate of decrease of C₁₂ and C₄₄.