STRUCTURAL MODELS FOR INTERMEDIATE-SIZED SI CLUSTERS

We have investigated two generic structural models for Si clusters in the size range 20-33 atoms. The first model consists of stacks of puck ered six-fold rings, with capping atoms on the two ends. The second mo del comprises compact structures with as many bulk-like four-fold coor dinated atoms as possible. We have found the cohesive energy of the ri ng structures to be nearly constant as a function of cluster size, whi le that of the compact structures decreases dramatically with increasi ng size as more four-fold coordinated atoms are incorporated into the structures. We compare our results with recent experiments which sugge st a crossover in stability between two cluster types in the size rang e studied here.