Surface atomic relaxation and magnetism on hydrogen-adsorbed Fe(110) surfaces from first principles

Abstract

We have computed adsorption energies, vibrational frequencies, surface relaxation and buckling for hydrogen adsorbed on a body-centred-cubic Fe(110) surface as a function of the degree of H coverage. This adsorption system is important in a variety of technological processes such as the hydrogen embrittlement in ferritic steels, which motivated this work, and the Haber-Bosch process. We employed spin-polarised density functional theory to optimise geometries of a six-layer Fe slab, followed by frozen mode finite displacement phonon calculations to compute Fe-H vibrational frequencies. We have found that the quasi-threefold (3f) site is the most stable adsorption site, with adsorption energies of ∼3.0 eV/H for all coverages studied. The long-bridge (lb) site, which is close in energy to the 3f site, is actually a transition state leading to the stable 3f site. The calculated harmonic vibrational frequencies collectively span from 730 to 1220 cm −1 , for a range of coverages. The increased first-to-second layer spacing in the presence of adsorbed hydrogen, and the pronounced buckling observed in the Fe surface layer, may facilitate the diffusion of hydrogen atoms into the bulk, and therefore impact the early stages of hydrogen embrittlement in steels.

Magnetocrystalline effects on the subsurface hydrogen diffusion in γ-Fe(0 0 1)

“…This enhancement in the magnetic moment at the surface to a value of 2.85 µB has previously also been observed experimentally [58]. We have also reported an analogous effect in our previous DFT study of the (110) surface of ferromagnetic bcc α-Fe [59].…”

Adsorption site, orientation and alignment of NO adsorbed on Au(100) using 3D-velocity map imaging, X-ray photoelectron spectroscopy and density functional theory

“…We employed the Cambridge Serial Total Energy Package (CASTEP) which solves the Kohn-Sham equations with a plane wave basis set employing three-dimensional periodic boundary conditions. 27,28 The Perdew-Burke-Ernzerhof (PBE) version of the generalized gradient approximation (GGA) was used to represent the exchange and correlations, and on-thefly ultrasoft pseudopotentials for Au, N and O. The Monkhorst-Pack (MP) algorithm was selected for k point sampling, and a cut-off energy of 750 eV was employed.…”

Incipient FeO(1 1 1) monolayer formation during O-adsorption on Fe(1 1 0) surface

“…A slab model was used to represent a Fe(110) surface using six representative cell geometries; (2×2), (3×1), (2×1), (2×2)-2O, (3×1)-2O, and (1×1). Six layers and a vacuum gap of 20 Å were applied (see previous DFT study for further details [38]). The Monkhorst-Pack (MP) algorithm was employed for k point sampling [39].…”

“…Thus the adsorption of oxygen induces an expansion between the first two layers, relative to bulk Fe. The interlayer relaxation parameter shifts from ∆ 12 = -0.37 % for the O-free Fe(110) surface[38] to ∆ 12 = 2.02 % at 1 ML coverage. For coverages ranging 0 < θ O < 1 ML, there is noticeable buckling in the first layer, b 1 , in the [110] direction induced by the O atoms bound at the 3f sites, as illustrated inFigure 4b.…”