ON A KINETIC APPROACH TO THE STUDY OF SUPERFICIAL EQUILIBRIUM SEGREGATION IN THE SB-CU(111) SYSTEM

Following a previous study performed in a system presenting a tendency to phase separation (Ag-Cu) we show here, to what extent one can also derive equilibrium segregation isotherms from dissolution and segrega tion kinetics measurements in the case of a system presenting a tenden cy to ordering at low temperature (Sb-Cu). The kinetics are recorded b y AES and the superficial superstructures determined from LEED pattern s. The dissolution kinetics of one monolayer of Sb/Cu(111) at 673 K pr esents a very surprising shape. The dissolution of Sb in the bulk is v ery fast at the beginning of the annealing but, contrary to the Ag/Cu case, stops for a given superficial concentration. No more dissolution is then observed even after a very long time of annealing. The LEED d iffraction pattern observed is characteristic of a p(square-root 3)R30 -degrees. The segregation kinetics of Sb, recorded at the same tempera ture but on the surface of a Cu(Sb)(111) solid solution (0.45 at%), re veals an increase of the superficial Sb concentration up to a constant value close to that obtained after dissolution kinetics. The superstr ucture observed is again p(square-root 3 x square-root 3)R30-degrees. These results are interpreted in terms of local equilibrium between th e surface and the selvedge and compared to those obtained for the Ag-C u system.