Metal transition and superconductivity were observed in Li-doped $\alpha$-rhombohedral boron (α-B${}_{12}$). The authors have established a purification method and obtained a large amount of high-purity $\alpha$-B${}_{12}$ powder. Li doping into purified $\alpha$-B${}_{12}$ was attempted by vapor diffusion processing (VDP) in a Mo or Ta tube. Li-doped $\alpha$-B${}_{12}$ contained metallic glittering particles. Meissner effects were observed in such a compound with the nominal composition Li${}_x$B${}_{12}$ ($x$ $=$ 1.0, 1.4, 1.5, 1.7, or 2.5) ($T_{\mathrm{c}}$ $=$ 3.2–7 K). As for Li${}_{2.5}$B${}_{12}$, 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 $\alpha$-B${}_{12}$ 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 $\alpha$-B${}_{12}$ was successfully achieved. Our work also suggests that it is possible to dope a larger amount of Li into $\alpha$-B${}_{12}$ and to increase its $T_{\mathrm{c}}$.

• Received 8 December 2010

DOI:https://doi.org/10.1103/PhysRevB.83.184507