Molecular dynamics simulation of crystallization of liquid copper clusters

Crystallization of liquid copper clusters of 13 to 4033 atoms has been studied using the effective-medium theory and molecular dynamics simulation methods. Small clusters with 13, 55, and 147 atoms are observed to crystallize directly to icosahedral structure, which is the most stable structure for copper clusters with less than 2000 atoms. Larger clusters crystallize to twinned fcc structures having typically three to five grains of fcc structure but no regular geometry. The solidification transition of a cluster is a fast internal process and is independent of external conditions. Regardless of the cluster size, the time taken to achieve crystallization to a twinned structure was less than 100 ps, and only the core of the cluster crystallizes during the transition. One third of the atoms, mainly on the surface area, remain liquid, but no reordering due to diffusion of the crystallized core was observed after the transition on a timescale of several nanoseconds.