Superconductivity at 108 K in the simplest non-toxic double-layer cuprate of Ba2CaCu2O4(O,F)2

Abstract

We report the superconductivity in apical fluorine system of Ba 2CaCu2O6-yFy: the second member of Ba2Can-1CunO2n(O,F)2 homologous series. The polycrystalline samples of Ba2CaCu 2O6-yFy (F-0212) were synthesized under high pressure as a parameter of nominal fluorine content (y). Samples with y 2.0 ∼ 1.2 elucidating the very sharp superconducting transitions in temperature dependence of susceptibility from under doping state to slightly over doping state via optimal doping state. A remarkable highest Tc of 108 K has been achieved for the sample synthesized from a nominal composition of Ba 2CaCu2O4.4F1.6. This Tc is highest among the double CuO2 layered system except for that including toxic elements such as Hg and T1. The Tc has been systematically controlled from 57 K to 108 K by controlling the doping state, by designing the starting composition of fluorine and oxygen. The strong dependence of the 'a' and 'c' lattice constants were found on the nominal F content. We propose the Ba2CaCu2O4(O,F) 2 as a promising material for practical use due to its high T c and non-toxicity with a simple crystal structure. © 2008 IOP Publishing Ltd.