Hydrogen on the Fe(110) surface and near bulk bcc Fe vacancies - A comparative bonding study

The bonding of H to Fe is analyzed using qualitative band structure calcula tions in the framework of extended Huckel tight-binding theory and the ASED -MO cluster method. The changes in the electronic structure of bcc Fe upon the introduction of H at a vacancy are addressed, and a comparison drawn wi th H adsorption at the Fe(110) surface. H in bulk Fe with vacancies prefers a tetrahedral site shifted toward the vacancy. The vacancies act as a stro ng trap for H. The Fe atoms are initially more strongly bonded to each othe r as a consequence of vacancy formation; their Fe-Fe bond strength is then diminished as the new Fe-H bond is formed. The effect of H is limited to it s first Fe neighbor. An analysis of the orbital interactions reveals that t he Fe-H bonding involves mainly the Fe 4s and H 1s orbitals with less parti cipation of Fe 4p and 3d crystal orbitals. Relaxation of Fe atoms neighbori ng the vacancy is also addressed. A detailed atomistic mechanism for decohe sion and H embrittlement is provided. (C) 1999 Elsevier Science B.V. All ri ghts reserved.