POLYCRYSTALLINE SURFACE-PROPERTIES FROM SPHERICAL CRYSTALLITES - AG, AU, CU AND PT

We have performed a series of atomistic simulations of nearly spherica l, crystalline (fcc) clusters of Ag, Au, Cu and Pt as a function of te mperature and cluster size. Since both a spherical cluster and a rando m polycrystal expose all possible surfaces equally, this provides a pl ausible approach for determining the surface properties of random (non -textured) polycrystalline metals and to find a simple expression to r elate these average surface properties to the oft calculated propertie s of high symmetry/low index surfaces. Atomic clusters with radii grea ter than approximately 4a0 yield cluster average surface energies and surface stresses are within a few percent of those obtained from very large clusters. The variation of the cluster average surface propertie s with cluster size is dominated by a geometrical effect associated wi th the discrete spacing between atomic planes and that the differences associated with differences in the atomic bonding between different e lements is small, at least for the four elements considered herein. Co mparison of the cluster average surface free energy with those of the more commonly studied high symmetry flat {100}, {110}, and the {111} s urfaces suggest two useful approximations for the average surface free energy: (1) equating it to the surface free energy of a {110} surface and (2) using a linear fit to the {100}, {110}, and the {111} surface free energies. Conversely, the first approximation provides an accura te estimate of the {110} surface energy from experimentally measured p olycrystalline surface energies.