Diffusion of interstitial hydrogen into and through bcc Fe from first principles

We report periodic spin-polarized density functional theory (DFT) predictions of hydrogen adsorption, absorption, dissolution, and diffusion energetics on and in ferromagnetic (FM) body-centered cubic (bcc) iron. We find that H prefers to stay on the Fe surface instead of subsurfaces or in bulk. Hydrogen dissolution in bulk Fe is predicted to be endothermic, with hydrogen occupying tetrahedral (t) sites over a wide range of concentrations. This is consistent with the known low solubility of H in pure Fe. In the initial absorption step, we predict that H occupies the deep subsurface t-site for Fe(110) and the shallow subsurface t-site for Fe(100). Diffusion of H into Fe subsurfaces is predicted to have a much lower barrier for Fe(100) than Fe(110). For H diffusion in bulk Fe, we find that H diffuses through bcc Fe not via a straight line trajectory, but rather hops from one t-site to a neighboring t-site by a curved path. Moreover, we exclude a previously suggested path via the octahedral site, due to its higher barrier and the rank of the saddle point. Quantum effects on H diffusion through bulk Fe are discussed.

Figure 1

The interstitial sites in a bcc lattice.

Figure 2

(a) H in the t-site of the Fe(110) subsurface and (b) H in the t-site of the Fe(100) subsurface (H in dark and Fe light gray).

Figure 3

Energy landscape for H in bulk Fe, and the surfaces and subsurfaces of Fe(110) and Fe(100). Zero-point energies are excluded. The energy of a thick slab of Fe (labeled here simply as Fe) with either (110) or (100) surfaces plus one-half of an isolated ${\mathrm{H}}_2$ molecule is set as zero.

Figure 4

Minimum energy path for hydrogen diffusion into the Fe subsurface region and several intermediate structures along the path: (a) into Fe(110) and (b) into Fe(100) (H in dark and Fe in light gray). Note the difference in scales between axes in (a) and (b).

Figure 5

(a) o-sites and t-site on the (001) place of the bcc lattice, (b) energy profile and local structures for H diffusion from one t-site to a nearest-neighbor t-site, (c) structure of the transition state, and (d) initial guess and converged MEP.