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.