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First-principles comparative study of UN and Zr corrosion

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Abstract

We studied surface corrosion effects on Zr and UN using first-principles density functional theory-based calculations. We focused on the energetics of Zr (1000), UN (100) and UN (110) surfaces, exposed to water and oxygen. Average distance between the terminating UN (100) surface and bulk increases due to the presence of additional oxygen content, as well as for the (110) surface. The average distance between the surface layer and bulk is greater in the (110) surface than the (100) surface after water adsorption. Oxygen concentration determines whether H2 or oxynitrde is formed on the (110) surface. Local density of states and partial charge density show the bonding between the UN surfaces and adsorbates. From an electronic energy of −2 eV to the Fermi level, the majority of electrons are found to be localized around U atoms. Electron localization function calculations further reveal the corrosion mechanism details.

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... To complement experimental studies, Density Functional Theory (DFT) has been a common method used to study UN surfaces exposed to H 2 O or O 2 at the atomic scale. [9][10][11][12][13][14] While experimental studies might utilize UN reactants of varying density, purity, and geometries, DFT allows for precise selection of the reactant. Though DFT calculations occur at 0 K, its precise reactant selection as well as further investigation of atomic corrosion initiation mechanisms capabilities make it a vital strategy for improving the understanding of UN corrosion. ...
... 13 Li et al. suggested that nonmagnetic treatment was appropriate for only total energy calculations. 11 Most prevalently, a few studies used FM treatment, 10,14,18,24 citing FM as the most energetically favorable structure. In experiments, Rafaja et al. studied the magnetic susceptibility of reactive sputtered UN thin films, suggesting FM for UN crystallites (averaging 17 nm). ...
... In our previous work, we found trends in electronic mapping to be converged for four monolayers, but binding energies varied between four and eight monolayer systems by up to approximately 0.5 eV. 14 Similarly, Bocharov et al. found defect energies to be converged for slabs with seven or more monolayers. 24 Bo et al. compared defect energies, surface energies, and bond lengths between supercells with surface areas corresponding to 2 × 2 and 3 × 3 unit cells and found the 2 × 2 surface area to be sufficient for corrosion studies. ...
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When there is a dipole moment in the repeat unit perpendicular to the surface in an ionic crystal, lattice sums in the electrostatic energy diverge and the calculated surface energy is infinite. The cause of this divergence is demonstrated and the surfaces of any ionic or partly ionic material are classified into three types. Type 1 is neutral with equal numbers of anions and cations on each plane and type 2 is charged but there is no dipole moment perpendicular to the surface because of the symmetrical stacking sequence. Both these surfaces should have modest surface energies and may be stable with only limited relaxations of the ions in the surface region. The type 3 surface is charged and has a dipole moment in the repeat unit perpendicular to the surface. This surface can only be stabilised by substantial reconstruction. These conclusions are important for the analysis of the surface structure of ionic crystals.
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We have used large-scale first-principles calculations based on density functional theory to investigate the structure, energetics, electronic, and magnetic structures of Fe(n)-doped C(60) monolayers supported by h-BN monolayer-covered Ni(111) surfaces. A systematic study of n-dependent physical properties has been performed (n=1-4,15). Binding energies on Fe atoms to the Fe(n-1)-C(60) complex have been calculated for n=1-4 after a thorough configuration search and structural optimization. The binding energy, electron charge transfer (from Fe(n) to C(60)), and magnetic moment all increase monotonically as functions of n. The electron charge transfer, ranging from approximately 1e(-) to 5e(-), is from the spin minority population. This leads to a situation in which the net spin of the C(60) molecule aligns with the spin minority and the magnetic moment in C(60) is opposite to the total magnetic moment of the system. For n=2, a competing antiferromagnetic state has been found. In this state, the net spin of the system as well as the C(60) is zero. Density of states and projected density of states analysis indicate that the system becomes metallic upon metal doping regardless its magnetic state. In addition, we have also performed calculations with the Hubbard U term (DFT+U) for two systems, n=4 and 15, to investigate possible gap opening near the Fermi surface.
Article
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
Article
LCAO and PW DFT calculations of the lattice constant, bulk modulus, cohesive energy, charge distribution, band structure, and DOS for UN single crystal are analyzed. It is demonstrated that a choice of the uranium atom relativistic effective core potentials considerably affects the band structure and magnetic structure at low temperatures. All calculations indicate mixed metallic-covalent chemical bonding in UN crystal with U5f states near the Fermi level. On the basis of the experience accumulated in UN bulk simulations, we compare the atomic and electronic structure as well as the formation energy for UN(001) surface calculated on slabs of different thickness using both DFT approaches.
  • Y Lu
  • B.-T Wang
  • R.-W Li
  • H Shi
  • P Zhang
Y. Lu, B.-T. Wang, R.-W. Li, H. Shi, P. Zhang, Structural, Electronic, and Thermodynamic Properties of UN: Systematic Density Functional Calculations, 2010, pp. 218e222, https://doi.org/10.1016/j.jnucmat.2010.08.026.