Phase transition induced by superficial segregation: the respective role of the size mismatch and of the chemistry

Segregation at the (0 0 1) surface in Cu(Ag) alloys has been studied using Monte Carlo (MC) simulations in conjunction with a semi-empirical N-body potential. The simulations show that the surface undergoes a first order phase transition at 600 K. This phase transition corresponds to a discontinuous increase of the surface concentration of Ag from 0.3 to 0.9 for a critical value of the bulk concentration well within the domain of existence of the bulk Cu(Ag) solid solution. This chemical transition is coupled with a change of the surface structure. The Ag-rich phase presents a pseudo-hexagonal c(10×2) structure, in complete agreement with experiments. This structural transition requires that some of the surface atoms are squeezed out into adatom positions. We present also a matching of the MC results on a lattice gas model consistent with the N-body interatomic potential used in the simulations. This matching allows us to determine the segregation driving forces and to detail their dependence on the surface structure. Surprisingly, the presence of the superstructure does not change the main driving forces for the surface plane but has a large influence for the first underlayer. In addition, the matching allows a discussion on the respective role of the structure and of the chemistry for the occurrence of the surface phase transition.