Description of the hydrogen-metal interaction by a morse potential function

Abstract

The interaction between hydrogen atoms adsorbed on the surface or dissolved in the interstices of a metal and the atoms of the host crystal is represented by a pairwise additive Morse potential function. This interaction is summed over all metal atoms surrounding the hydrogen atom. Interactions between hydrogen atoms are not considered. The three force constants of the potential function are determined from three experimental values of the hydrogen-metal system: the heat of solution, the heat of adsorption and the activation energy for bulk migration. Relaxation of the metal lattice surrounding the dissolved hydrogen and reconstruction of the surface metal atoms next to the adsorbed hydrogen are considered in the calculation. The potentials so determined are long rangemetal atoms many lattice constants distant from the hydrogen atom contribute to the binding and the metal atoms closest to a dissolved hydrogen atom are repulsive. Four metals are treated: b.c.c. iron and tantalum, and f.c.c. nickel and copper. For each metal, two surface planes are investigated. Although both tetrahedral and octahedral interstitial sites were tested as solution sites, only occupancy of octahedral sites satisfied the interaction energies upon which the calculation is based. Knowledge of the potential function permits other characteristics of the hydrogen-metal interaction, such as the activation energy for surface migration and the vibration frequency of dissolved hydrogen, to be computed. Agreement of these calculated properties with experimental data is satisfactory, but not quantitative. The extreme sensitivity to crystal structure (b.c.c. or f.c.c.) and the assumption that the nature of the binding of surface hydrogen is the same as that of bulk hydrogen limit the utility of the pairwise model to that of an interpolation scheme—as a means of utilizing known characteristics of the hydrogen-metal system to estimate parameters not experimentally available.