Diffusion processes of Au and Cu adatoms on the Cu3Au(001) surface

We present molecular dynamics results on the diffusive properties of Au and Cu adatoms on the ordered Cu3Au(0 0 1) surface. We found that for both adatoms there are two available equilibrium positions: one over the fourfold cite and another one on top of a Cu surface atom. It turns out that although this second position is rather a stopover than a stable adatom cite, it plays an important role in adatom diffusive behavior. The Cu adatom diffuses via a variety of Arrhenius type mechanisms, among which a new type of exchange mechanism, occurring at temperatures higher than T-s = 500 K, involving second layer Cu atoms. In addition, some interlayer Cu diffusion events were observed, induced by the Cu adatom when localized at the on top Cu position. Moreover, we found that the Cu adatom stimulates the creation of diffusive vacancies that contribute in the rearrangement of the surface atoms. Concerning the Au adatom, we found that its diffusive behavior is substantially different from the Cu adatom case. Indeed, although the Cu and Au adatoms have similar reaction paths for diffusion, the Au adatom's behavior is inscribed by the new exchange mechanism that takes place exclusively with the participation of Cu surface atoms. This mechanism, occurring more often above T-s, results in a local disorder of the surface layer, rendering the Au adatom unstable. Therefore, it seems that above T-s, the presence of Cu or Au adatoms on the Cu3Au(0 0 1) face stimulates segregation and diffusive phenomena that could be related with the order-disorder transition occurring at higher temperatures. (C) 2001 Elsevier Science B.V. All rights reserved.