Crystal Structure and Superconductivity in the La-Ba-Ca-Cu-O System

Resistivity and X-ray measurements have been done on ceramic samples which have nominal compositions of La_<2-x>A_xCaCu_2O_y or La_<2-x>A_xSrCu_2O_y (A=Ba and Ca). The samples with nominal compositions of La_<2-x>Ca_<1+x>Cu_2O_y (x=0.4) and La_<2-x>Ba_xCaCu_2O_y (x=0.2 and 0.4) are metallic but not superconducting down to 22 K. They are found to have the same crystal structure as that for La_2CaCu_20_6. However, when the amount of Ba doping is further increased, superconductivity appears near 80 K in samples with nominal compositions of La_<2-x>Ba_xCaCu_2O_y (x=1.0, 1.2 and 1.4). The X-ray results show that the superconductivity is not caused by the Ba doping but by a structural change to CaBaLaCu_3O_<7-δ> which is tetragonal at all temperatures.