Synthesis and superconducting properties of the strontium copper oxy-fluoride Sr2CuO2F2+δ

HIGH-PRESSURE synthesis has proved a useful technique for obtaining new, metastable copper oxide superconductors; for example, oxygen insertion into Sr 2 CuO 3 at 6 GPa (ref. 1) yields superconducting Sr 2 CuO 3 .i, with transition temperature T c = 70 K, in which the superconducting CuO 2 layers are generated by pressure-induced oxygen migration from apical to equatorial sites. Although the simple structure and high transition temperatures make this family (general formula Sr n+1 Cu n O 2n + 1+ δ ) of interest, the stringent synthesis conditions limit its value for applications. Here we report that fluorine insertion into Sr 2 CuO 3 at ambient pressure causes related structural rearrangements to give superconducting Sr 2 CuO 2 F 2 + δ with a maximum T c of 46 K. In this synthesis, the structural changes previously initiated by the thermodynamic effects of high pressure are induced chemically under ambient conditions. The result is a superconducting oxy-fluoride in which fluorine plays a dominant structural role, rather than merely being an electronic dopant as in La 2 CuO 4 F x (ref. 2) and Nd 2 CuO 4−x F y (ref. 3).