Melting and crystallization in Ni nanoclusters: The mesoscale regime

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 ~500 atoms to a mesoscale nanocrystal regime (well-defined bulk and surface properties) above ~750 atoms (2.7 nm). We find that the mesoscale nanocrystals melt via surface processes, leading to Tm,N = Tm,bulk–alphaN^–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.