Structure and electronic properties of Mg-doped β-rhombohedral boron constructed from icosahedral clusters

High-T-C superconductivity is possible upon metallic-element doping into beta-rhombohedral boron (beta-B-105), which is one of the boron icosahedral cluster solids. We attempted magnesium (Mg) doping into beta-B-105 and discussed the possibility of metal transition and superconductivity. We achieved Mg doping into beta-B-105 at a high Mg concentration of up to MgB11.5 (8.6 Mg/cell), i.e., electron doping sufficient for the Fermi energy (E-F) to reach the conduction band over the intrinsic acceptor level (IAL) and trapping levels. However, neither metal transition nor superconductivity was observed. The changes in the structure and electronic properties are discussed on the basis of the results of x-ray powder diffraction using the Rietveld method and electrical conductivity and magnetic susceptibility measurements, respectively. We estimated the density of states near E-F and discussed the electronic states of beta-B-105. From the result, it is suggested that a localized state exists above the IAL probably originating from the B-28 cluster with structural defects.