Ground and excited states of an icosahedral B12H12 cluster simulating the B-12 cluster in beta-rhombohedral boron

From many experimental studies, a localized intrinsic acceptor level is assumed to exist at about 0.2 eV above the valence band edge in beta-rhombohedral boron. The acceptor level has been tentatively originated from the unoccupied intracluster bonding level split away from the valence band due to the distortion of the electron-deficient B-12 icosahedral cluster. We investigated numerically the ground and the excited states of the B12H12 cluster simulating the B-12 cluster in beta-rhombohedral boron. One stable and two metastable geometries for the B-12 cluster at a vertex of the rhombohedral unit cell are found. The energy of the first excited state for only one of the metastable structures is lowered toward a valence band close to that of the acceptor level, while the energies of the other two structures have values corresponding to the band gap. The origin of localization is assumed to be the random distribution of the intrinsic acceptor levels originating from the metastable geometry of the B-12 cluster. (C) 1997 Academic Press.