Melting and crystallization in Ni nanoclusters: The mesoscale regime

Yue Qi,1 Tahir Çagin,1 William L. Johnson,2 and William A. Goddard III1
1Materials and Process Simulation Center, California Institute of Technology, Pasadena, California 91125
2Keck Laboratory of Engineering Materials, California Institute of Technology, Pasadena, California 91125

We studied melting and freezing of Ni nanoclusters with up to 8007 atoms (5.7 nm) using molecular dynamics with the quantum-Sutten-Chen many-body force field. We find a transition from cluster or molecular behavior below similar to 500 atoms to a mesoscale nanocrystal regime (well-defined bulk and surface properties) above similar to 750 atoms (2.7 nm). We find that the mesoscale nanocrystals melt via surface processes, leading to Tm,N=Tm,bulk - aN(-1/3), dropping from Tm,bulk, =1760 K to Tm,336=980 K. Cooling from the melt leads first to supercooled clusters with icosahedral local structure. For N > 400 the supercooled clusters transform to FCC grains, but smaller values of N lead to a glassy structure with substantial icosahedral character.

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