Structural Evolution of Anionic Silicon Clusters SiN (20 ≤ N ≤ 45)

Results of a combined photoelectron spectroscopy and first-principles density-functional study of SiN- clusters in the size range 20 <or= N <or= 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 Si9 found in most low-lying clusters SiN-, 9 <or= N <or= 19, is replaced or augmented by a series of structural motifs consisting of a bulklike "adamantane" fragment plus a magic-number cluster (Si6, Si7, Si10) or TTP Si9 in low-lying prolate clusters SiN-, N >or= 20. For 28 <or= N <or= 45, almost all low-lying near-spherical clusters SiN- 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.