A unified picture for icosahedral cluster solids in boron-based and aluminum-based compounds

Discussions are presented to get a unified picture for the boron-based and aluminum-based compounds constructed from icosahedral clusters, Twelve-atom centerless icosahedra, Al-12 and B-12, have a covalent bonding nature, though 13-atom centered ones, Al-13 and B-13, are metallic, Stabilization mechanisms for the both compounds, the Hume-Rothery mechanism and the Jahn-Teller effect, are closely related, There are recant sites in the second shell of the multi-shell structure for boron compounds, When these sites are occupied by dopant metal atoms, the cluster structure, dc conductivity, and optical conductivity of the boron compounds approach those of the aluminum compounds, Photo-conduction is observed in aluminum compounds as roell as in boron compounds, Photocurrent arising from carrier excitation through the localized states in the pseudogap and the intrinsic acceptor levels are important for aluminum and boron compounds, respectively. (C) 1997 Academic Press.