First-Principles Study on Hydrogen Diffusivity in BCC, FCC, and HCP Iron

The hydrogen diffusion behavior in BCC, FCC, and HCP iron has been investigated by means of first-principles calculations. Diffusion coefficients were estimated quantitatively from the migration energy derived by the Nudged elastic band method, and phonon calculations including the vibrations of all atoms at every stable and metastable site. Our calculations on the BCC structure show good agreement with those in the previous report. In the FCC structure as well, the calculated diffusion coefficients are in good agreement with experimental data. Our results suggest that the consideration of the antiferromagnetic state in FCC is important for the reproduction of experimental results. For the HCP structure, although there was a lack of systematic experimental results, our calculations predict that the diffusion coefficient is smaller than that in the case of the FCC sample. In the HCP lattice, there are two diffusion paths: one parallel to the c-axis and the other in the c-plane. The direction and the diffusion coefficient can be controlled by the tuning of c/a, which is the ratio of the lattice constants.