Superconducting Phase Diagram of Li Metal in Nearly Hydrostatic Pressures up to 67 GPa

The dependence of the superconducting transition temperature $T_c$ on nearly hydrostatic pressure has been determined to 67 GPa in an ac susceptibility measurement for a Li sample embedded in helium pressure medium. With increasing pressure, superconductivity appears at 5.47 K for 20.3 GPa, $T_c$ rising rapidly to $\sim 14\mathrm{K}$ at 30 GPa. The $T_c(P)$ dependence to 67 GPa differs significantly from that observed in previous studies where no pressure medium was used. Evidence is given that superconductivity in Li competes with symmetry breaking structural phase transitions which occur near 20, 30, and 62 GPa. In the pressure range 20–30 GPa, $T_c$ is found to decrease rapidly in a dc magnetic field, the first evidence that Li is a type I superconductor.