The iron–hydrogen system: Lattice location of hydrogen, heat of formation, and hydrogen–hydrogen binding energy

Abstract

The electronic structure of the iron–hydrogen system has been performed in terms of band structure calculation based on the extended Hückel formalism. Heat of formation for a hydrogen atom at octahedral and tetrahedral positions in bcc and hcp iron lattice has been estimated. Allowing the iron lattice to relax, we minimize the energy of the system in terms of a variable lattice parameter. The heat of formation is less endothermic for hydrogen located at tetrahedral interstitial position than for hydrogen at octahedral position. Moreover, the effect of hydrogen is to stabilize the hcp phase. We have performed calculations with different geometrical arrangements, in terms of the distance between hydrogen atoms. Binding energies between hydrogen atoms at nearest and next-nearest interstitial positions are always found unstable. Numerical results obtained in the case of hcp structure are discussed in terms of recent Mössbauer experiments. The physical origin of the sign of the heat of formation in terms of charge transfer is presented. For this we use a model of a Fe4H cluster.