Simplest nontoxic double-layered cuprate Ba2CaCu2O4(O,F)2 superconductor with a transition temperature of 108K

We report synthesis and properties of the simplest double-layered cuprate superconductor Ba2CaCu2O4(O,F)2. A superconducting transition temperature (Tc) has been achieved as high as 108K for a sample with a nominal fluorine content y=1.6 in Ba2CaCu2O6−yFy. The Tc is the highest among double-layer cuprates that are free of toxic elements such as Hg, Tl, and Pb so that the nontoxic Ba2CaCu2O4(O,F)2 will be promising for application. As y decreases from approximately 2, the a-axis length decreases and the Tc increases from 40to108K. The material is underdoped at large y and as y decreases, the doping moves toward an optimally doped state.