HETEROGENEOUS ADATOM DIFFUSION ON FCC(100) SURFACES - NI, CU, RH, PD, AND AG

We present activation barriers and prefactors for the migration of heterogeneous adatoms on fcc(100) surfaces. Two mechanisms are considered in this paper: (1) hopping of the adatom from one four-fold hollow site to an adjacent four-fold hollow site via a two-fold bridge site; and (2) exchange of the adatom with an atom in the first surface layer. Twenty heterogeneous combinations of Ni, Cu, Rh, Pd, and Ag were treated using transition state theory, and select comparisons were made to the results of finite temperature molecular dynamics simulations. The interaction potentials were generated using the molecular dynamics/Monte Carlo corrected effective medium (MD/MC-CEM) theory throughout. We find that the final state energies differ due to the variation of metallic bonding with coordination for the different types of metal atoms. This variation with coordination is reflected in the surface energies of the two metals, and thus this macroscopic quantity can be used to correlate the amount of energy gained or released when the adatom displaces a surface atom. Due to the non-directional character of metallic bonding in the fee metals, this difference in energetic stability of final configurations is also found to generally determine whether bridge hopping diffusion or atomic displacement is the dominant kinetic process in these heterogeneous systems.