SUPERCONDUCTIVITY UP TO 64-K IN THE COPPER OXYFLUORIDES SR2-XAXCUO2F2+DELTA (A=CA, BA) PREPARED USING NH4F AS A FLUORINATING REAGENT

We report here a simple and efficient synthetic route to high-temperature superconducting oxyfluorides. Solid phase fluorination of Sr2CuO3 with NH4F at relatively low temperatures produces superconducting Sr2CuO2F2+delta; this simple preparation route eliminates the need for F2 gas. Importantly, as-synthesised samples from the NH4F route appear to have the optimally doped value of the superconducting transition temperature, T(c), of 46 K. Moreover, Ba doping for Sr results in an increase in T(c), for both F2 gas and NH4F routes, to a maximum value of 64 K for Sr1.4Ba0.6CuO2F2+delta; this represents the highest ever recorded value for a material with the confirmed La2CuO4 structure.