Superconductivity in WO_2.6F_0.4 synthesized by reaction of WO₃ with teflon

WO_3-xF_x (x ≤ 0.45) perovskite-like oxyfluorides were prepared by a chemically reducing fluorination route using the polymer polytetrafluoroethylene (teflon). The symmetry of the crystal structures of WO_3-xF_x changes from monoclinic to tetragonal to cubic as the fluorine content increases. Fluorine doping changes insulating WO ₃ to a metallic conductor, and superconductivity (T_c = 0.4 K) was discovered in the samples with fluorine contents of 0.41 ≤ x ≤ 0.45. This easy fluorination method may be applicable to other systems and presents an opportunity for finding new oxyfluoride superconductors.