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.