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 ada toms there are two available equilibrium positions: one over the fourfold c ite 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 pl ays an important role in adatom diffusive behavior. The Cu adatom diffuses via a variety of Arrhenius type mechanisms, among which a new type of excha nge mechanism, occurring at temperatures higher than T-s = 500 K, involving second layer Cu atoms. In addition, some interlayer Cu diffusion events we re observed, induced by the Cu adatom when localized at the on top Cu posit ion. Moreover, we found that the Cu adatom stimulates the creation of diffu sive vacancies that contribute in the rearrangement of the surface atoms. C oncerning the Au adatom, we found that its diffusive behavior is substantia lly different from the Cu adatom case. Indeed, although the Cu and Au adato ms have similar reaction paths for diffusion, the Au adatom's behavior is i nscribed by the new exchange mechanism that takes place exclusively with th e participation of Cu surface atoms. This mechanism, occurring more often a bove T-s, results in a local disorder of the surface layer, rendering the A u adatom unstable. Therefore, it seems that above T-s, the presence of Cu o r Au adatoms on the Cu3Au(0 0 1) face stimulates segregation and diffusive phenomena that could be related with the order-disorder transition occurrin g at higher temperatures. (C) 2001 Elsevier Science B.V. All rights reserve d.