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  • Physics Letters A 374 (2010) 17691772

    Contents lists available at ScienceDirect

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    03doRecently, the phenomena of metallic clusters melting has at-acted considerable experimental [1,2] and theoretical [3] interestse to their extensive applications in catalysts, sensors, micro-ectronic and optoelectronic devices [4]. It is more important formetallic clusters as their melting maybe tuned by varying theirzes, compositions and structures [5]. A particular example is thate melting temperature of bimetallic clusters has been changed bylectively doping with a single impurity [6]. The melting was de-ndent on the concentrations and sizes of clusters [7]. It was alsound that the solid phase changes without diffusion induced bye variation of atomic distribution will result in a sharp decline ofe temperature-dependent energy curve [810]. In addition, at themperatures near the melting point of cluster, premelting and dif-sion become obvious, except for structural evolutions. This maycrease the possibility to induce a special melting phenomenon.erefore, a controllable melting of the clusters may be designeddoping hetero atoms with different physical parameters and

    ntrolling their distributions. It plays a key role in synthesizingw materials with fascinating potential applications. However, lit-attention has been paid to this up to now.In this Letter, the cuboctahedral clusters, including all 309

    oms, were set-up as the objectives. They were truncated from30a0 30a0 30a0 large bulk. This can make the cluster have amilar lattice structure with its bulk. On the other hand, the simu-tion can be continued without considering the periodic boundaryndition. However, the icosahedral cluster with the good low-

    Corresponding author. Tel.: +86 24 83681726; fax: +86 24 83681758.E-mail address:wangq@mail.neu.edu.cn (Q. Wang).

    energy structure cannot be truncated directly from the bulk. But,in our previous study [10], we found that the cuboctahedral clus-ter can transform to icosahedra at low temperature. Therefore, inorder to obtain the low-energy icosahedral cluster, cuboctahedralclusters were selected as the initial objectives due to the exis-tence of cuboctahedraicosahedra transformation at low temper-ature. The effect of composition and atomic distribution on themelting of bimetallic clusters was studied. Firstly, in order to denethe inuence of compositions on melting, Co, Cu, and Ni elementswere selected to form onion-like CoCuCo and CoNiCo clus-ters. These clusters were constructed with 55 Co atoms in thecore (the 3rd-layer), 92 Cu or Ni atoms in the middle (the 2nd-layer) and 162 Co atoms in the surface (the 1st-layer). Co, Cu,and Ni were chosen on account of the following reasons: (1) Thesurface energy of Cu (1592 mJm2) is much lower than that ofCo (2197 mJm2) and the energy of Ni (2104 mJm2) is sim-ilar to that of Co [11]. From the previous study, we know thatthe surface energy is strongly related to surface orientation. How-ever, any change of atomic situation will induce the distortion oflattice for small clusters. This will lead to diculties in den-ing the segregated position during heating process. Therefore, theorientation-dependent surface energy is not distinguished and av-eraged surface energy is used to dene the inuence of segregationon melting process. Because most of the Cu atoms in the 2nd-layerwill segregate to the CoCu surface during the heating process,while only a few Ni atoms will segregate to the CoNi surface.This will lead to the difference of energy variations during themelting of clusters and can be used to explore the composition-dependent melting. (2) The small differences in atomic radii be-tween Co (0.1385 nm), Cu (0.1412 nm), and Ni (0.1378 nm) can

    75-9601/$ see front matter 2010 Elsevier B.V. All rights reserved.i:10.1016/j.physleta.2010.02.027Physics L

    www.elsevier.c

    ontrollable irregular melting induced byith fabricating different initial congurat

    uojian Li, Tie Liu, Qiang Wang , Xiao L, Kai Wang,y Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeaster

    r t i c l e i n f o a b s t r a c t

    ticle history:ceived 17 November 2009ceived in revised form 6 February 2010cepted 11 February 2010ailable online 13 February 2010mmunicated by R. Wu

    ywords:eltingmetallic clusterolecular dynamics

    The melting process of Co,in molecular dynamics by adecreases as the temperaturclusters, but not in the mixdistributions and energy varsegregation. Furthermore, thienergies and controlling theirters A

    /locate/pla

    omic segregation in bimetallic clustersns

    heng He

    iversity, Shenyang 110004, China

    Cu and CoNi clusters with different initial congurations is studiederal embedded atom method. An irregular melting, at which energycrease near the melting point, is found in the onion-like CoCuCoCoCu and onion-like CoNiCo clusters. From the analysis of atomicn, the results indicate the irregular melting is induced by Cu atomicelting can be controlled by doping hetero atoms with different surfacetributions.

    2010 Elsevier B.V. All rights reserved.

  • 1770 G.J. Li et al. / Physics Letters A 374 (2010) 17691772

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    Fi miing. 1. Curves of the heat capacity Cp , the temperature-dependent energy for mixedCu, the onion-like CoCuCo and CoNiCo clusters, and the tted energy ofCuCo during the heating process.

    g. 2. Atomic numbers of Co and Cu in different layers of the onion-like CoCuCo andthe gure indicates to which layer the atom belongs to.n also be found that the energy of solid CoCu is lower thanat of solid CoCuCo at the same temperature, but that aftere cluster melting, the energy of CoCu is larger than that ofCuCo at the same temperature. Since both the concentrationsCu in the clusters and the heating processes are same, it is rea-nable to conclude that the atomic distribution plays a key role inis melting phenomenon. However, for the CoNiCo cluster, thearp energy increase is obvious at 1300 K and no similar irreg-ar melting appeared. Since only the composition changes duringe heating processes of CoCuCo and CoNiCo, the variation of

    xed CoCu clusters at different temperatures near the melting points. The layer noid the formation of polyicosahedral structures in the clusterse to the large lattice mismatch [12,13]. Secondly, the meltingmixed CoCu cluster (CoCu), which was obtained by randomlybstituting 92 Co atoms with 92 Cu atoms, was simulated. Then,s melting was compared with that of CoCuCo to reveal thelationship between the atomic distribution and melting. Finally,e molecular dynamics with a general embedded atom methodAM) was used to study the melting of all the clusters mentionedove. The construction and accuracy of this model were describedour previous papers [7,14]. The melting processes were simu-

    ted as follows: the clusters were heated up to 1800 K from 100 Kith 0.2 ns equilibrated simulation time for each temperature step0 K) and a time step of 1 fs.In this study, the melting points were dened by using the heatpacity Cp(T ), which was obtained as Cp(T ) = d(Energy)/dT [15].e premelting occurs before melting, which could affect the clus-r surface and lead to the change of heat capacity. The energy

    and heat capacity curves during the heating processes were shownin Fig. 1. There is a sharp increase in the heat capacity curve ofCoCuCo (at 120 K) existing due to the structural transformationfrom cuboctahedron to icosahedron [9]. However, similar cases didnot appear in the clusters of CoCu because its structural trans-formations occurred during the relaxation process at 100 K. Thisalso leads to its heat capacity curves being kept unchanged un-til the premelting occurred. The melting points of the CoCuCoand CoCu clusters can be obtained from the maximum apparentheat capacity, which values are 1240 K and 1120 K, respectively.Although the same Cu concentrations were used in both clusters,their melting points are different due to the different distributionsof the Cu atoms. Furthermore, their temperature-dependent en-ergy curves were also different and two phenomena can be found:(1) For CoCu, the energy increases with the increase in tempera-ture and there is a sharp energy increase at 1120 K which indicatesthat the cluster melts completely. (2) For CoCuCo, after the pre-melting at 1140 K, the energy decreases with the increase in tem-perature until 1220 K and the cluster melts completely at 1240 K.However, by comparing the tted energy curve of CoCuCo with-out considering premelting and segregation (obtained by ttingthe temperature-dependent energy curve of CoCuCo from 1001020 K with a third order exponential decay), we found that, theenergy at 1240 K increases as expected, while it was still lessthan the tted energy at the same temperature. This melting isdifferent from those of general clusters [8,16] and indicates thatan irregular melting occurred in CoCuCo. In the meantime, it

  • G.J. Li et al. / Physics Letters A 374 (2010) 17691772 1771

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    pethlaofwnugawnuwCoocengamfasethbumcapewsithatmismatch induced by segregation and premelting increases the in-n be calculated by using the values mentioned above. Further-

    ore, the relationship between melting and segregation can alsoobtained. The tted temperature-dependent energy of cluster

    ithout segregation and premelting was shown in Fig. 3 (the blacklid line). Only the near-melting cases were shown because nogregation occurred at l

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