Increase in Tc and change of crystal structure by high-pressure annealing in BiS2-based superconductor CeO0.3F0.7BiS2

Recently, several types of BiS2-based superconductor such as Bi4O4S3, REO1-xFxBiS2 (RE: rare earth) and Sr1-xLaxFBiS2 have been discovered. In this study, we have investigated the crystal structure and the superconducting properties for two kinds of polycrystalline samples (As-grown and high-pressure-annealed samples) of the BiS2-based superconductor CeO0.3F0.7BiS2. We found that both the As-grown and the high-pressure-annealed CeO0.3F0.7BiS2 samples show bulk superconductivity. A higher Tc was observed in the high-pressure-annealed sample. The Tc of CeO0.3F0.7BiS2 increased from Tczero = 2.7 K to Tczero = 3.7 K by high-pressure annealing. The lattice constant of a and c axis did not show remarkable differences between the As-grown and the high-pressure-annealed samples. Nevertheless, the peak symmetry of the (200) peak seemed to become more symmetric while the (004) peak did not show such a difference, indicating that the crystal structure within the ab plane changed to a higher-symmetric phase, a perfect tetragonal, by high-pressure annealing in CeO0.3F0.7BiS2.