Superconductivity and crystal structure in the La-Ba-Cu-O system

A series of samples with nominal compositions ${\mathrm{La}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Ba}}_{\mathrm{x}}$ ${\mathrm{Cu}}_1$ ${\mathrm{O}}_{3\mathrm{-}\mathrm{y}}$ was prepared by solid-state reaction. The subsolidus phase relation in the CuO-rich region of the La-Ba-Cu-O system has been determined by an x-ray diffraction method. Two ternary compounds were confirmed to exist in the system. The tetragonal or pseudotetragonal phase (${\mathrm{Ba}}_{2\mathrm{-}\mathit{z}}$ ${\mathrm{La}}_{\mathit{z}}$)${\mathrm{La}}_1$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7+\mathrm{\delta }}$ has a wide homogenous phase range from z=0.095 to z=0.578, and a crystal structure closely related to the structure of ${\mathrm{Ba}}_2$ ${\mathrm{YCu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. Superconductivity of compounds in this system was studied by resistivity and ac susceptibility measurements. The relation between crystal structure and the superconductivity is discussed.