Synthesis and superconducting properties of the strontium copper oxy-fluoride Sr₂CuO₂F_2+δ

Abstract

HIGH-PRESSURE synthesis has proved a useful technique for obtaining new, metastable copper oxide superconductors; for example, oxygen insertion into Sr₂CuO₃ at 6 GPa (ref. 1) yields superconducting Sr₂CuO₃.i, with transition temperature T_c = 70 K, in which the superconducting CuO₂ 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+1Cu_nO_{2n + 1+ δ}) 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 superconducting Sr₂CuO₂F_{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 La₂CuO₄F_x (ref. 2) and Nd₂CuO_4−xF_y (ref. 3).