Surface Reconstruction and Decahedral Structure of Bimetallic Nanoparticles

We report on energetic surface reconstruction phenomena observed on bimetallic nanoparticle systems of AuPd and AuCu, similar to a resolidification effect observed during the cooling process in lead clusters. These binary alloy nanoparticles show the fivefold edges truncated, resulting in { 100 } facets on decahedral structures, an effect largely envisioned and reported theoretically, with no experimental evidence so far. We demonstrate experimentally as well as by computational simulations that this new eutectic structure is favored in such nanoalloy systems.Fil: Rodríguez López J. L.. Instituto Potosino de Investigación Científica y Tecnológica ; MéxicoFil: Montejano Carrizales, J. M.. Universidad Autónoma de San Luis Potosí; MéxicoFil: Pal, U.. Universidad Auto´noma de Puebla; MéxicoFil: Sánchez Ramírez, J. F.. Universidad Auto´noma de Puebla; MéxicoFil: Troiani, Horacio Esteban. Comision Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Física de Metales; ArgentinaFil: García, D.. The University of Texas; Estados UnidosFil: Miki Yoshida, M.. The University of Texas; Estados UnidosFil: José Yacamán, Miguel. The University of Texas; Estados Unido

An orbital-free molecular dynamics study of melting in K_20, K_55, K_92, K_142, Rb_55 and Cs_55 clusters

The melting-like transition in potasium clusters K_N, with N=20, 55, 92 and 142, is studied by using an orbital-free density-functional constant-energy molecular dynamics simulation method, and compared to previous theoretical results on the melting-like transition in sodium clusters of the same sizes. Melting in potasium and sodium clusters proceeds in a similar way: a surface melting stage develops upon heating before the homogeneous melting temperature is reached. Premelting effects are nevertheless more important and more easily established in potasium clusters, and the transition regions spread over temperature intervals which are wider than in the case of sodium. For all the sizes considered, the percentage melting temperature reduction when passing from Na to K clusters is substantially larger than in the bulk. Once those two materials have been compared for a number of different cluster sizes, we study the melting-like transition in Rb_55 and Cs_55 clusters and make a comparison with the melting behavior of Na_55 and K_55. As the atomic number increases, the height of the specific heat peaks decreases, their width increases, and the melting temperature decreases as in bulk melting, but in a more pronounced way.Comment: LaTeX file. 6 pages with 17 pictures. Final version with minor change

Structural, electronic and magnetic properties of ConPtM-n for M=13, 19, and 55, from first principles

The geometrical structure, chemical ordering, electronic and magnetic properties of ConPtm-n for M=13, 19, and 55, are investigated, within the framework of the density functional theory as implemented in the SIESTA code. For the exchange and correlation potential, we used the Perdew-Burke-Ernzerhof (PBE) form of the generalized gradient approximation (GGA). We considered the hexagonal, the cubo-octahedral (CO), the centered pentagonal prism (CPP), and the icosahedral (ICO) geometries. We found that the CO, ICO, and CPP structures have similar energies and the lowest energy configuration changes as a function of the Co concentration and cluster size. In most of the cases, around the equiatomic concentration, the Co and Pt atoms form chemically ordered structures, but most often not the L1(0) structure stable in bulk. We observed that the Pt atoms are segregated to the surface in all the concentration range. A detailed analysis of the dependence of the magnetic moment of the components, and their average, as a function of structure and of chemical composition is reported. A strong polarization of the Pt atoms by the nearest neighbor Co atoms is observed. (C) 2013 Elsevier B.V. All rights reserved

Magnetism in segregated bimetallic CoRh nanoclusters

Abstract The magnetic properties of free-standing Co n Rh m clusters (N ¼ n þ m % 110 and n % m) of three different symmetries: cubo-octahedral, icosahedral and HCP, were investigated in the segregated case. The initial geometrical structures constructed at Rh bulk distances were relaxed with a many-body Gupta potential to obtain the cluster geometries and energies. We find that the lowest energy is associated with the HCP structure. The relaxed interatomic distance for all the structures is slightly lower than the Rh bulk distance. The spin-polarized electronic structure and related magnetic properties of these optimized geometries were calculated by solving self-consistently a spd tightbinding Hamiltonian. The magnetic moment of the Rh atoms shows strong dependence on the position and environment, whereas the Co atoms show a smoother dependence. Results are compared with the experimental data and with other theoretical calculations available in the literature.