Bidimensional phases in Co-Pt surface alloys: A theoretical study of ordering and surface segregation

Surface segregation and bulk ordering are characterized by Monte Carlo simulations using a Tight-Binding interatomic potential within the Second Moment Approximation of the density of states (TB-SMA potential) which reproduces the bulk ordered phases and the surface segregation reversal between the dense (111) and (100) surfaces (Pt segregation) and the more opened (110) one (Co segregation). Among the different surface superstructures, most are simply the surface termination of bulk ordered phases and have been observed experimentally, but we predict also purely bidimensional phases such as the (root 3 x root 3)R30(111) or the c(2 x 2)(110) ones with no equivalence in the bulk alloy. Such (root 3 x root 3)R30(111) phase should be stable on nanoalloys where the (111) facets are prevailing. Finally, we show that the c(2 x 2) superstructure of the CoPt(100) surface, which represents the mixed variant of the L1(0) phase, remains stable above the bulk order/disorder critical temperature.