High-quality polycrystalline samples of LaO0.5F0.5BiS2 were obtained using high-pressure synthesis technique. The LaO0.5F0.5BiS2 sample prepared by heating at 700 C under 2 GPa showed superconductivity with superconducting transition temperatures (Tc) of Tconset = 11.1 and Tczero = 8.5 K in the electrical resistivity measurements and Tconset = 11.5 and Tcirr = 9.4 K in the magnetic susceptibility measurements, which are obviously higher than those of the LaO0.5F0.5BiS2 polycrystalline samples obtained using conventional solid-state reaction. It was found that the high-Tc phase can be stabilized under high pressure and relatively-low annealing temperature. X-ray diffraction analysis revealed that the high-Tc phase possessed a small ratio of lattice constants of a and c, c/a.
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