Structure and chemical ordering in CoPt nanoalloys.

The structure and chemical ordering of CoPt nanoclusters in the size range of 1 to 3 nm are investigated by global optimization methods and Monte Carlo simulations using a many body potential derived from the tight binding model. For the smaller systems (number of atoms N < 100), the optimized clusters display a polyicosahedral-like atomic structure with a little core-shell chemical ordering characterized by a particular surface chemical configuration: some pentagonal or hexagonal Pt rings centered, respectively on a Co atom or a Co dimer. A transition to the decahedral symmetry occurs at about N = 100 atoms, with a pseudo L1(0) ordered phase in each tetrahedral unit. For larger cluster sizes, 201 < N < 1289, the L1(0)-ordered/disordered transition on the face centered cubic truncated octahedron is studied by canonical Monte Carlo simulations showing that the critical disordering temperature decreases with the cluster size. We also notice a Co surface segregation especially at edges and, possibly, (100) facets, depending on the cluster size, on both cubic and fivefold symmetry structures.