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

Abstract

A series of samples with nominal compositions ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ba}}_{\mathrm{x}}$${\mathrm{Cu}}_{1}$${\mathrm{O}}_{3\mathrm{\ensuremath{-}}\mathrm{y}}$ was prepared by solid-state reaction. The subsolidus phase relation in the CuO-rich region of the La-Ba-Cu-O system has been determined by an x-ray diffraction method. Two ternary compounds were confirmed to exist in the system. The tetragonal or pseudotetragonal phase (${\mathrm{Ba}}_{2\mathrm{\ensuremath{-}}\mathit{z}}$${\mathrm{La}}_{\mathit{z}}$)${\mathrm{La}}_{1}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7+\mathrm{\ensuremath{\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{\ensuremath{-}}\mathrm{\ensuremath{\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.