Structural motifs, mixing, and segregation effects in 38-atom binary clusters

Thirty eight-atom binary clusters composed of elements from groups 10 and 11 of the Periodic Table mixing a second-row with a third-row transition metal (TM) (i.e., clusters composed of the four pairs: Pd-Pt, Ag-Au, Pd-Au, and Ag-Pt) are studied through a combined empirical-potential (EP)/density functional (DF) method. A "system comparison" approach is adopted in order to analyze a wide diversity of structural motifs, and the energy competition among different structural motifs is studied at the DF level for these systems, mainly focusing on the composition 24-14 (the first number refers to the second-row TM atom) but also considering selected motifs with compositions 19-19 (of interest for investigating surface segregation effects) and 32-6 (also 14-24 and 6-32 for the Pd-Au pair). The results confirm the EP predictions about the stability of crystalline structures at this size for the Au-Pd pair but with decahedral or mixed fivefold-symmetric/closed-packed structures in close competition with fcc motifs for the Ag-Au or Ag-Pt and Pd-Pt pairs, respectively. Overall, the EP description is found to be reasonably accurate for the Pd-Pt and Au-Pd pairs, whereas it is less reliable for the Ag-Au and Ag-Pt pairs due to electronic structure (charge transfer or directionality) effects. The driving force to core-shell chemical ordering is put on a quantitative basis, and surface segregation of the most cohesive element into the core is confirmed, with the exception of the Ag-Au pair for which charge transfer effects favor the segregation of Au to the surface of the clusters.