A new method for structural characterization of atomic clusters in solidification processes of liquid metals

A molecular dynamics simulation has been performed on the solidification process of liquid Na, and the atomic clusters in the solidification process have been identified by means of a new characterization method -- cluster-type index method (CTIM) proposed by us. In order to evaluate the reliability and efficiency of CTIM, it is compared with the widely used Voronoi polyhedron method (VPM). Our results show that when the atomic clusters in the solidification structures of liquid Na are characterized by the VPM and CTIM, respectively, their structural features are identical. The distributions of atomic clusters in the amorphous structure characterized by the two different methods both display several broad peaks, and each peak corresponds to one favorable cluster type. The favorable cluster types both are icosahedron and its defective cases. The dominated cluster type in the crystal structure characterized by the two different methods both is bcc crystal unit. It is also found that the VPM is difficult to distinguish the small differences between cluster configurations according to the shapes of Voronoi polyhedra, and different cluster configurations may be classified into the same type. The CTIM directly describes cluster configurations according to the position relations of neighbor atoms. CTIM simplifies the representation format of atomic clusters by means of outstanding the principal ones. This will be efficient in the structural analysis of larger-scale simulation systems. (authors)