Preparation and Characterization of Y-Ba-Cu-O-F Superconductors

Fluorine (F) substitution effects on the superconducting properties are investigated with YBa2Cu3OxFy prepared by two different methods. At first, the sintered samples were prepared with the appropriate mixture of Y2O3, Ba2O3, CuO and several fluorides. Since the samples had strong peaks of BaF2 phase together with the perovskite one in the X-ray diffraction patterns, it was difficult to obtain information on the F-substitution effects on the physical properties. Next, the samples were prepared by annealing YBa2Cu3Ox sintered pellets and CoF3 powder in a sealed quartz tube. In this case, the crystalline state was a single phase of the orthorhombic perovskite structure. It was found that the diamagnetic susceptibility χ decreased concurrently to the increase of the F content and the tempreture dependence of χ was similar regardless of the presence of F-substitution. This indicates that the volume fraction of the superconductor phase decreased. In the resistivity measurement, the superconductivity was not observed down to 4.2K. These results suggests that the F atoms enter the crystalline grains from the surfaces and change the superconductivity state into a normal one in the vicinity of the crystalline grain surfaces.