Surface segregation in alloys, i.e., concentration modulation in the surface selvedge at thermodynamical equilibrium, can be viewed as resulting from two kinds of competition or synergy. The first one is between surface and bulk interactions, i.e., surface energy versus bulk alloying interactions, whereas the second one is between these chemical forces and the atomic size-mismatch. Modelling the phenomenon then requires to account for all these forces on the same level. This leads to use both realistic energetic models derived from electronic structure and efficient statistical tools, from mean–field approximation to Monte Carlo simulations. A particular attention must be payed to the possible multisolution character of the problem. Actually, the possible coexistence of stable and metastable solutions may be at the origin of superficial phase transitions, in which the surface acts as a precursor of bulk order–disorder transitions: layering transitions, concentration profile transitions, surface induced order or disorder. All these phenomena are illustrated here on some specific systems and analyzed with tools derived from tight-binding formalism. Finally, the strong coupling between surface segregation and atomic reconstructions is illustrated in the case of strong size-mismatch by means of molecular dynamics calculations.

Using an energetic model derived from the electronic structure, we study the stability of the Various terminations of Pt1-cSnc (111) for 0<c<0.3, both in the disordered and in the ordered L1(2) structure. Moreover, using a kinetic Version of the model we reproduce the formation of different surface alloys obtained experimentally by depositing Sn/Pt(lll). The relation between the equilibrium isotherm and the surface alloys is then discussed via the local equilibrium concept.

We study the kinetics of formation and dissolution of surface alloy obtained by depositing one monolayer of Pd on Cu(111) at room temperature. This is performed within the kinetic tight-binding Ising model. We also present the equilibrium segregation isotherm for PdcCu1-c(111) and show the relation between kinetics (surface alloy) and equilibrium (alloy surface) via the local equilibrium concept.

The theoretical segregation isotherm in CucPd1-c(111) has been studied both in the disordered and in the ordered state within an energetic model derived from the electronic structure. A segregation reversal is obtained as a function of the bulk concentration in the disordered state. In the ordered state, we obtain a great variety of surface critical behaviours: whereas the bulk order-disorder or order-order transitions are of first order, the surface ones can be of first or second order. In this last case, a wetting behaviour is shown.

Nous avons etudie les cinetiques de formation et de dissolution d'alliage de surface obtenus lors de depot de Pd / Cu (111) dans le cadre d'un modele cinetique (KTBIM). Selon l'epaisseur du depot initial, nous mettons en evidence une succession spatio-temporelle d'alliages ordonnes de surface au cours de la cinetique. Le concept d'equilibre local nous permet de relier les profils de concentration obtenus lors de la cinetique a des profils d'equilibre de couches minces supportees. L'extension spatio-temporelle du concept d'equilibre local sera egalement discutee.