Stability and optical properties of plutonium monoxide from first-principle calculation

Exploring thermodynamic stability of plutonium oxycarbide using a machine-learning scheme

Plutonium oxycarbide plays a crucial role in the fabrication of a carbide fuel and the corrosion of plutonium. In this work, a machine-learning (ML) scheme is used to predict the thermodynamic stability of plutonium oxycarbide PuOxC1-x. The training data are generated within the framework of density-functional theory (DFT) and its Hubbard correction. Four ML schemes combined with three structural descriptors are considered and their performance is compared. The optimal ML model for the DFT data set yields remarkably small average errors of approximately 3 meV per atom for mixing energy and 0.003 Å for the lattice parameter, indicating its high prediction accuracy. Utilizing the ML model, we predict the convex hull of PuOxC1-x as well as several ordered atomic structures for a specific value of x. The enhanced energetic stability observed in these ordered structures could probably be attributed to the strong hybridization between Pu 5f/6d and C/O 2p, namely robust Pu-C and Pu-O bonds.

Thorium and Rare Earth Monoxides and Related Phases

Thorium was a part of energy infrastructure in the 19th century due to the refractory and electronic properties of its dioxide. It will be a part of future energy infrastructure as the most abundant energy reserve based on nuclear fission. This paper discusses the solid-state chemistry of the monoxides and related rocksalt phases of thorium and the rare earths, both at atmospheric and at high pressure. The existence of solid thorium monoxide was first suggested more than 100 years ago; however, it was never obtained in bulk and has been studied mostly theoretically. Monoxides of lanthanides from Eu to Ho are ferromagnetic semiconductors sought for spintronics and were studied in thin films. La to Sm metallic monoxides were synthesized in bulk at pressures below 5 GPa. Recently, ThO formation in thin films has been reported and the stability of bulk ThO at high pressure was theoretically predicted based on first principles computations at 0 K. New ab initio computations were performed accounting for temperature effects up to 1000 K using lattice dynamics in the quasi-harmonic approximation. New computational results confirm the stabilization of pure ThO above 30 GPa and suggest the possibility of high-pressure synthesis of (Th,Nd)O at 1000 K and 5 GPa.

Thermodynamical stability of substoichiometric plutonium monocarbide from first-principles calculations

Plutonium monocarbide, which contains a considerable amount of vacancies in the carbon sublattice, has never been synthesized in a stoichiometric form. The intriguing substoichiometric behavior of plutonium monocarbide is investigated here using first-principles calculations combined with the special quasirandom structure. It is found that the NaCl-type substoichiometric plutonium monocarbide is stable for PuC_0.741-0.923, which is in good agreement with the experiment. From the electronic structure calculations and chemical bond analyses, the stabilization of PuC_1-x in this range is attributed to strengthened Pu-C bonds opposite the carbon vacancies.

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

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.

Can water continuously oxidize the PuO molecule? Mechanisms, topological analysis and rate constant calculations

A detailed description of the PuO continuously oxidized by water in the gas-phase.