SYNTHESIS AND SUPERCONDUCTING PROPERTIES OF THE STRONTIUM COPPER OXY-FLUORIDE SR2CUO2F2+DELTA

HIGH-PRESSURE synthesis has proved a useful technique for obtaining ne w, metastable copper oxide superconductors; for example, oxygen insert ion into Sr2CuO3 at 6 GPa (ref. 1) yields superconducting Sr2CUO3.1, w ith transition temperature T-c = 70 K, in which the superconducting Cu O2 layers are generated by pressure-induced oxygen migration from apic al to equatorial sites. Although the simple structure and high transit ion temperatures make this family (general formula Sr-n+1CUnO2n+1+delt a) of interest, the stringent synthesis conditions limit its value for applications. Here we report that fluorine insertion into Sr2CuO3 at ambient pressure causes related structural rearrangements to give supe rconducting Sr2CuO2F2+delta with a maximum T-c of 46 K. In this synthe sis, the structural changes previously initiated by the thermodynamic effects of high pressure are induced chemically under ambient conditio ns. The result is a superconducting oxy-fluoride in which fluorine pla ys a dominant structural role, rather than merely being an electronic dopant as in La2CuO4Fx (ref. 2) and Nd2CuO4-xFy (ref. 3).