Surface relaxation and induced stress accompanying the adsorption of H upon Be(0001)

Abstract

A series of self-consistent field, molecular orbital cluster calculations has been conducted to ascertain some of the substrate relaxation that would accompany the adsorption of a hydrogen atom by the (0001) surface of beryllium. The total HF energies of clusters of either seven, ten, or fifteen Be atoms, both with and without the adsorbed hydrogen atom, have been determined as functions of the basal plane lattice parameter of the clusters. Comparison of the locations of the minima of these energy functions shows that the adsorbed H atom has the effect of drawing surface Be atoms in with great force towards the perpendicular from the H atom to the surface. There is some indication that this pinching is stronger in the case of a hydrogen atom adsorbed at a bridging site, although the effect is still pronounced for a hydrogen atom adsorbed directly above a beryllium atom. From the predicted relaxation one might infer the possibility of large stresses arising in the surface layer of beryllium (or other metal) when it is exposed to hydrogen. A simple model calculation shows that, in the absence of plastic flow or other deformation, these stresses roughly equal the yield stress of Be metal when the adsorbed H constitutes a few percent of a monolayer. The implications of such stresses in hydrogen damage phenomena are discussed, and various experiments recalled which corroborate the arguments presented.