Bingyun Ao

First-principles study of nitrogen adsorption and dissociation on α-uranium (001) surface

The adsorption and dissociation of nitrogen on the α-uranium (001) surface have been studied with a first-principles density functional theory (DFT) approach. The effects of strong 5f electron–electron correlation and spin–orbit coupling on the adsorption of nitrogen on the uranium (001) surface are also discussed. Different coverages of nitrogen atoms and different initial configurations of nitrogen molecules are considered on the uranium surface. The structural parameters and electronic states of nitrogen on the uranium surface are obtained. The calculated results indicate that nitrogen atoms are energetically favorable at the hollow1 sites. The nitrogen molecules adsorbed horizontally on the long-bridge site are found to dissociate completely, and the corresponding adsorption energies are about −4 eV. The electron structure of the most preferred adsorption configuration is investigated, and it is found that the adsorbed nitrogen atoms only seize electrons from the top-most uranium layer. Based on ab initio atomistic thermodynamics, the surface phase diagram for nitrogen adsorption on the α-uranium (001) surface is obtained and the initial stages of nitridation for the uranium surface are discussed.

First-principles study on the interaction of nitrogen atom with α–uranium: From surface adsorption to bulk diffusion

Experimental studies of nitriding on uranium surfaces show that the modified layers provide considerable protection against air corrosion. The bimodal distribution of nitrogen is affected by both its implantation and diffusion, and the diffusion of nitrogen during implantation is also governed by vacancy trapping. In the present paper, nitrogen adsorption, absorption, diffusion, and vacancy trapping on the surface of and in the bulk of α–uranium are studied with a first-principles density functional theory approach and the climbing image nudged elastic band method. The calculated results indicate that, regardless of the nitrogen coverage, a nitrogen atom prefers to reside at the hollow1 site and octahedral (Oct) site on and below the surface, respectively. The lowest energy barriers for on-surface and penetration diffusion occur at a coverage of 1/2 monolayer. A nitrogen atom prefers to occupy the Oct site in bulk α–uranium. High energy barriers are observed during the diffusion between neighboring Oct sites. A vacancy can capture its nearby interstitial nitrogen atom with a low energy barrier, providing a significant attractive nitrogen-vacancy interaction at the trapping center site. This study provides a reference for understanding the nitriding process on uranium surfaces.

The Formation Energies and Binding Energies of Helium Vacancy Cluster: Comparative Study in Ni and Pd

Molecular dynamics calculations are performed to calculate the formation energy for helium in Ni-vacancy and Pd-vacancy clusters. The binding energies of helium and metal self-interstitial atoms (SIA) to the helium-vacancy cluster are also determined. The comparison of these energies indicates that helium to vacancy ratio (He/V) for helium in Pd is much higher.

Energetics of intrinsic point defects in aluminium via orbital-free density functional theory

Abstract The formation and migration energies for various point defects, including vacancies and self-interstitials, in aluminium are systematically reinvestigated using the supercell approximation in the framework of orbital-free density functional theory. In particular, the finite-size effects and the accuracy of various kinetic energy density functionals are examined. It is demonstrated that as the supercell size increases, the finite-size errors asymptotically decrease as . Notably, the formation energies of self-interstitials converge much more slowly than that of vacancies. With carefully chosen kinetic energy density functionals, the calculated results agree quite well with the available experimental data and those obtained through Kohn–Sham density functional theory, which has an exact kinetic term.

Density-functional study of plutonium monoxide monohydride

Abstract The structural, electronic, mechanical, optical, thermodynamic properties of plutonium monoxide monohydride (PuOH) are studied by density-functional calculations within the framework of LDA/GGA and LDA/GGA+U. From the total energy calculation, the lowest-energy crystal structure of PuOH is predicted to have space group F 4 ¯ 3 m (No. 216). Within the LDA+U framework, the calculated lattice parameter of F 4 ¯ 3 m -PuOH is in good agreement with the experimental value and the corresponding ground state is predicted to be an antiferromagnetic charge-transfer insulator. Furthermore, we investigate the bonding character of PuOH by analyzing the electron structure and find that there are a stronger Pu-O bond and a weaker Pu-H bond. The mechanical properties including the elastic constants, elastic moduli and Debyes temperature, and the optical properties including the reflectivity and absorption coefficient are also calculated. We then compute the phonon spectrum which verified the dynamical stability of F 4 ¯ 3 m -PuOH. Some thermodynamic quantities such as the specific heat are evaluated. Finally we calculate the formation energy of PuOH, and the reaction energies for the oxidation of PuOH and PuOH-coated Pu, which are in reasonable agreement with the experimental values.

First-principles studies on the charge density wave in uranium

The charge density wave (CDW) state of α-U (called -U) was studied through a first-principles total-energy minimization using the conjugate gradient algorithm. The optimized crystal structure of -U was found to have the space group Pbnm, which was proposed in the earlier Landau-type theory and is isostructural with the α-Np structure. In particular, the changes in the lattice parameters of Pbnm-U with respect to α-U are consistent with the experimental observations. In addition, the energetic stability of Pbnm-U with respect to α-U was confirmed by enthalpy calculations, and the value of the critical pressure in the pressure-induced quantum transition from Pbnm-U to α-U is in good agreement with the experimental result. Moreover, the phonon calculation verified the dynamical instability of α-U and the stability of Pbnm-U. Finally, the calculated electronic structures exhibit features of the CDW state.