Structural evolution of anionic silicon clusters Si _N (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.