Subsurface bonding of hydrogen in transition metals: dependence on surface orientation

The calculation of the surface-induced enhancement of the binding energy of hydrogen impurities in transition metals, previously reported for close-packed surfaces, are extended to open surfaces in the fee and bee structures. The effect is found to be stronger for close-packed surfaces than for open ones. Numerical values for the subsurface bonding energy in I% and Pd are given, from which changes in the kinetics of hydrogen absorption through specific surfaces are predicted. The interaction of hydrogen with the surface of ~ansition metals is a subject of ~ntinuing interest. Among the several aspects that are now at issue are the condensation of hydrogen in metal solution in the subsurface interstitial layers [l-6], and the effect of this in controlling the up take rate from the gas phase to the metal bulk [4,6,7]. Earlier we called attention to the fact that the contribution of the elastic distortion to the impurity solution energy is notably enhanced when approaching a surface [3,4,71. When filling an interstitial site, a hydrogen impurity exerts forces on the surrounding crystal ions, which move to new equilibrium positions in order to minim~e the total energy, stabilizing this way the impurity state. This energy reduction, of elastic origin, is typically of a few tenths of eV. If the defect is in an interstitial site just below the surface, where the neighboring crystal ions are * Corresponding author. Fax: + 1 619 534 0173; E-mail: [email protected]. freer to move than in the bulk, the stabilizing effect is expected to be enhanced. Estimates of the surface induced energy reduction of interstitial hydrogen below close-packed surfaces of fee and bee crystals yield values as large as 0.25-0.34 and 0.56-0.8 eV for palladium and niobium, respectively [4]. This effect is particularly interesting in metals like palladium, tantalum or niobi~, which spontaneously absorb hydrogen and have thus positive heats of solution. The energy of an interstitial hydrogen state in the subsurface layer, obtained by adding the surface contribution to the bulk solution energy, in these metals is lower, or comparable to that of the chemiso~tion states [3]. This causes observable phenomena, which have been reported to occur in niobium [1,6,8-lo] and palladium [5,11] in good agreement with theoretical estimates [3,4]. Direct experimental evidence of the existence of subsurface hydrogen states with energy close to that of the chemisorbed surface states has been reported for palladium and copper by the diffraction of atomic beams [l&12], LEED [12], the two techniques com0039-6028/94/$07.00