Effect of hydrogen on the electronic structure of a grain boundary in iron

LMTO-ASA calculations were performed on a 26-atom supercell model of a Σ3 (111) grain boundary (GB) in bcc Fe. The supercell emulated two GB's with 11 (111) planes of Fe atoms between the GB planes. One of the GB's was clean, with a structural vacancy at the GB core in the center of a trigonal prism of Fe atoms, while on the other GB this site was occupied by a H atom. The interplanar spacings of the supercell were relaxed using a modified embedded atom method. As in the case of P and S in a similar GB environment in Fe there is only a weak interaction between H and nearest Fe atoms. Almost all the Fe d-states are nonbonding. A very weak covalent bond exists between H and Fe due to s-pd hybridization, the hybrid bonding band located far below the Fermi energy. This bond is mostly of α-type, connecting H with Fe atoms in the GB plane; the δ- component of this bond across the GB is weaker. Contrary to a general belief, H on the GB does not contribute any significant electron charge to Fe valence d-bands. A weak electrostatic interaction attracts Fe-atoms across the clean GB, but results in repulsion if a H atom is present. The magnetic contribution to intergranular cohesion is decreased when H is present due to the suppression of the magnetic moments of the nearest Fe atoms both in the GB plane and directly across the GB.