Li- and Mg-doping into icosahedral boron crystals, α- and β-rhombohedral boron, targeting high-temperature superconductivity:: structure and electronic states

The possibility of high T-C superconductivity is suggested for lithium- and magnesium-doped icosahedral boron crystals, alpha- and beta-rhombohedral boron. The doping of these elements was attempted by a vapor diffusion processing. Both lithium and magnesium are hardly doped into the alpha-rhombohedral boron, although small amounts of metallic parts are found in the sample. In only one Li-doped sample, the metallic part contained 0.02 vol% of the superconductive phase (T-C similar to 36 K). Magnesium was successfully doped into beta-rhombohedral boron homogeneously up to 4 at% (Mg4.1B105), although considerable amount of impurity silicon was introduced together with magnesium. The structures of the doped samples were analyzed assuming co-doping of magnesium and silicon. The relation between the site occupancies of the dopants and the lattice expansion is discussed. The estimation of the density of states near the Fermi energy by EELS and magnetic susceptibility measurements suggested a metal transition of the beta-rhombohedral boron by the doping of magnesium and silicon. The relation between the metal transition and the intrinsic acceptor level is also discussed. (C) 2003 Elsevier Inc. All rights reserved.