An interesting sol-gel route to prepare oxyfluoride Sr₂CuO₂F_2+δ superconductors

The oxyfluoride superconductor, Sr₂CuO₂F_2+δ, was prepared by a sol-gel method. Two different routes were examined; one was an attempt to prepare the Sr₂CuO₂F_2+δ phase directly from fluorine-containing sols, and the other was the fluorination of sol-gel-derived Sr₂CuO₃ fine powders by F₂ gas. The former route did not lead to the formation of oxyfluorides because of the precipitation of SrF₂ by the heat treatments of the gel. The latter route led to the formation of the Sr₂CuO₂F_2+δ phase. The amount of the Sr₂CuO₂F_2+δ phase and superconducting properties of the powders depended on the fluorinating condition such as F₂ 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.