Structures and relative stability of medium-sized silicon clusters.: IV.: Motif-based low-lying clusters Si21-Si30 -: art. no. 054304

Structures and relative stability of four families of low-lying silicon clusters in the size range of Si-n(n=21-30) are studied, wherein two families of the clusters show prolate structures while the third one shows near-spherical structures. The prolate clusters in the first family can be assembled by connecting two small-sized magic clusters Si-n (n=6, 7, 9, or 10) via a fused-puckered-hexagonal-ring Si-9 unit (a fragment of bulk diamond silicon), while those in the second family can be constructed on the basis of a structural motif consisting of a puckered-hexagonal-ring Si-6 unit (also a fragment of bulk diamond silicon) and a small-sized magic cluster Si-n (n=6, 7, 9, or 10). For Si-21-Si-29, the predicted lowest-energy clusters (except Si-27) exhibit prolate structures. For clusters larger than Si-25, the third family of near-spherical clusters becomes energetically competitive. These near-spherical clusters all exhibit endohedral cagedlike structures, and the cages are mostly homologue to the carbon-fullerene cages which consist of pentagons and hexagons exclusively. In addition, for Si-26-Si-30, we construct a new (fourth) family of low-lying clusters which have "Y-shaped" three-arm structures, where each arm is a small-sized magic cluster (Si-6, Si-7, or Si-10). Density-functional calculation with the B3LYP functional shows that this new family of clusters is also energetically competitive, compared to the two prolate and one near-spherical low-lying families. (c) 2006 American Institute of Physics.