Theoretical study of segregation of Zn and Pd in Pd–Zn alloys

Surface segregation of Zn and Pd in PdZn alloys was investigated based on density functional calculations using periodic slab models. The segregation propensity of Pd–Zn films was found to be support-dependent: Zn showed a stronger propensity to segregate on Zn support than on Pd support; likewise, for Pd the propensity was stronger on Pd support than on Zn support. In self-supported 1:1 alloy films, segregation of neither Zn nor Pd was favorable. Segregation was suppressed due to the fact that Pd–Zn heteronuclear bonds are stronger than the corresponding homonuclear bonds Pd–Pd and Zn–Zn. However, in films rich in Zn or Pd, segregation of the dominant component was possible. For instance, when a monolayer of Pd is deposited on a Zn(0 0 0 1) film, segregation of Zn is calculated energetically favorable, in agreement with experimental observations. With increasing Zn concentration in the top surface layer, the corrugation of alloy films was reduced.