Superconductivity in Li-doped α-rhombohedral boron

Metal transition and superconductivity were observed in Li-doped α-rhombohedral boron (α-B₁₂). The authors have established a purification method and obtained a large amount of high-purity α-B₁₂ powder. Li doping into purified α-B₁₂ was attempted by vapor diffusion processing (VDP) in a Mo or Ta tube. Li-doped α-B₁₂ contained metallic glittering particles. Meissner effects were observed in such a compound with the nominal composition Li _xB₁₂ (x = 1.0, 1.4, 1.5, 1.7, or 2.5) (T_c = 3.2-7 K). As for Li_2.5B₁₂, the temperature dependence of its electrical conductivity indicates a metallic character and its electrical resistivity drop is detected near the Meissner temperature. The existence of Li and Fermi edges in Li-doped α-B₁₂ crystals was verified by transmission electron microscopy-electron energy loss spectroscopy (TEM-EELS). Lattice expansion, which is a well-known indicator of metal doping into a crystal, was also observed. Thus, Li doping into α-B₁₂ was successfully achieved. Our work also suggests that it is possible to dope a larger amount of Li into α-B₁₂ and to increase its T _c.