On the various terminations occurring in CuPt-ordered alloys: the TBIM approach

Abstract Although surface segregation is well documented in the disordered state, its relation with the stable termination of the ordered alloys is not so clear. In this paper we determine the stable (and metastable) terminations of the Cu c Pt 1− c alloys in the ordered state using an energetic model based on the electronic structure: the tight-binding Ising model. In particular for the observed experimentally Ll 1 -ordered structure in the cquiatomic CuPt alloy we study the relative stability of the three possible terminations for the (111) face, the more stable being the more Cu-enriched. Moreover, the bulk concentration range of existence of the two other terminations is smaller than the domain where bulk ordering exists. This has strong consequences on the existence of single or double steps at the surface. The (100) surface is simpler: only one termination exists, the surface plane differing appreciably from the corresponding bulk plane.