Structural evolution of anionic silicon clusters SiN (20 ≤ n ≤ 45)

Results of a combined photoelectron spectroscopy and first-principles density-functional study Of Si-N(-) clusters in the size range 20 <= N <= 45 are reported and discussed. Evidence for a prolate-to-near-spherical shape transition at N = 27 is presented. It is shown that the tricapped-trigonal-prism (TTP) structural motif Si-9 found in most low-lying clusters Si-N(-), 9 <= N <= 19, is replaced or augmented by a series of structural motifs consisting of a bulklike "adamantane" fragment plus a magic-number cluster (Si-6. Si-7, Si-10) or TTP Si-9 in low-lying prolate clusters Si-N(-), N <= 20. For 28 <= N <= 45, almost all low-lying near-spherical clusters Si-N(-) adopt "stuffed-cage"-like structures where the cages are homologous to carbon fullerenes in the sense that they are composed of only five- and six-membered rings. However the arrangement of the "stuffing" atoms is not yet diamondlike.