Correlation between structure, energy and surface tension for free surfaces in b.c.c. metals

Abstract

A many-body potential of the Finnis-Sinclair type for Mo and a Johnson-type pair potential for α-Fe are used to determine the zero-temperature structures, energies and surface tensions of a variety of free surfaces in b.c.c. metals. The variation in the surface energy and tension as a function of the two variables characterizing the surface normal A is found to be similar for the two potentials, and our results for the principal surfaces of Mo agree with those presented recently by Ackland and Finnis. However, while the pair potential yields an outward relaxation of the surface plane in some cases and an inward relaxation in others, the many-body potential yields an inward relaxation for all surfaces considered. This difference is shown to arise from the very different magnitudes of the surface tensions obtained for the two types of potential which, in turn, are shown to be closely connected with the volume dependence of the related perfect-crystal cohesive energies. For both potentials a strong correlation is found between the number of broken nearest- and second-nearest neighbour bonds on the one hand, and the surface tensions and energies on the other.