First-principles density-functional theory has been used to investigate equilibrium geometries, total energies, and diffusion barriers for H as an interstitial impurity absorbed in α-Fe. Internal strains/stresses upon hydrogen absorption are a crucial factor to understand preferred absorption sites and diffusion. For high concentrations, H absorbs near the octahedral site favoring a large tetragonal distortion of the BCC lattice. For low concentration, H absorbs near the tetrahedral site minimizing the elastic energy stored on nearby cells. Diffusion paths depend on the concentration regime too; hydrogen diffuses about ten times faster in the distorted BCT lattice. External stresses of several GPa modify barriers by ≈ 10%, and diffusion rates by ≈ 30%.
@article{arxiv.1106.0383,
title = {Hydrogen in $\alpha$-iron: stress and diffusion},
author = {J. Sanchez and J. Fullea and C. Andrade and P. L. de Andres},
journal= {arXiv preprint arXiv:1106.0383},
year = {2011}
}