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 $\alpha \text{-Fe}$. 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 body-centered-tetragonal (bct) lattice. External stresses of several GPa modify barriers by $\approx 10\%$, and diffusion rates by $\approx 30\%$.

• Received 17 April 2008

DOI:https://doi.org/10.1103/PhysRevB.78.014113