An interesting sol-gel route to prepare oxyfluoride Sr2CuO2F2+δ superconductors

The oxyfluoride superconductor, Sr 2 CuO 2 F 2+δ , was prepared by a sol-gel method. Two different routes were examined; one was an attempt to prepare the Sr 2 CuO 2 F 2+δ phase directly from fluorine-containing sols, and the other was the fluorination of sol-gel-derived Sr 2 CuO 3 fine powders by F 2 gas. The former route did not lead to the formation of oxyfluorides because of the precipitation of SrF 2 by the heat treatments of the gel. The latter route led to the formation of the Sr 2 CuO 2 F 2+δ phase. The amount of the Sr 2 CuO 2 F 2+δ phase and superconducting properties of the powders depended on the fluorinating condition such as F 2 gas concentration and fluorinating time. The T c values reached 45–47 K by annealing the fluorinated samples in vacuo. The magnetization hysteresis loop was measured for the first time in the oxyfluoride superconductors.