Rossi G, Ferrando R, Mottet C
" INFM and IMEM/CNR, Dipartimento di Fisica dell'Universitá di Genova, via Dodecaneso 33, 16146 Genova, Italy.
Faraday Discuss. 2008;138:193-210; discussion 211-23, 433-4. doi: 10.1039/b705415g.
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.
通过全局优化方法和蒙特卡罗模拟，利用从紧束缚模型导出的多体势，研究了尺寸在1至3纳米范围内的CoPt纳米团簇的结构和化学有序性。对于较小的体系（原子数N < 100），优化后的团簇呈现出类似多二十面体的原子结构，具有少量的核壳化学有序性，其特征是特定的表面化学构型：一些分别以Co原子或Co二聚体为中心的五角形或六角形Pt环。在约N = 100个原子时发生向十面体对称性的转变，每个四面体单元中存在伪L1(0)有序相。对于更大的团簇尺寸，201 < N < 1289，通过正则蒙特卡罗模拟研究了面心立方截顶八面体上的L1(0)有序/无序转变，结果表明临界无序温度随团簇尺寸减小。我们还注意到，根据团簇尺寸的不同，在立方和五重对称结构上，特别是在边缘以及可能的(100)晶面上，存在Co表面偏析现象。
