Thermal behavior of core-shell and three-shell layered clusters: Melting of Cu_1Au_(54) and Cu_(12)Au_(43)

摘要

The meltinglike transition of the Cu_1 Au_(54) and Cu_(12)Au_(43) clusters is investigated by canonical Monte Carlo simulations, based on the second-moment approximation of the tight-binding potentials. The structures of both the Cu_1Au_(54) and Cu_(12)Au_(43) clusters, shown to be icosahedral, are obtained from the so-called semi-grand-canonical ensemble Monte Carlo simulation at 100 K. A core-shell structure is found in Cu_1Au_(54), with a single Cu atom in the center and 54 Au atoms on the surface and in interior shells of the cluster. On the other hand, Cu_(12)Au_(43) possesses a three-shell onionlike structure, with a single Au atom located in the center, 12 Cu atoms in the middle shell, and 42 Au atoms occupying the surface shell of the cluster. Melting characteristics are observed by the changes in the caloric curve, heat capacity, root-mean-square bond-length fluctuation, and deformation parameter. It is found that doping of Au_(55) with a single Cu atom can sharply raise the melting point of the cluster. In addition, the three-shell onionlike structure can be transformed into the core-shell structure at the higher temperatures after melting. It is also found that surface segregation of Au atoms in Cu_1Au_(54) and Cu_(12)Au_(43) occurs in the liquid phase. In addition, the simulation results show that the melting point increases with the concentration of Cu in the 55-atom Cu-Au bimetallic cluster.