Structural and physical properties of the tetragonal superconductors LaBaACu₃O_7-δ (A = Ca and Sr)

The compounds LaBaACu₃O_7-δ (A = Ca and Sr) have been prepared and characterized by X-ray diffraction and electron microscopy. Their structure is similar to that of YBa₂Cu₃O₇, except that they are tetragonal. Electron diffraction of LaCaBaCu₃O_7-δ reveals the presence of a weak superstructure having doubled, a, b and c axes. The oxygen content and superconducting transition temperature were found to vary significantly, depending on the post-annealing conditions. Oxygen- and air-annealed samples have T_c values of 76 K and 60 K for A = Ca and 50 K and 33 K for A = Sr respectively. Air-quenched samples exhibit semiconductor-like behaviour in both cases. The d.c. resistivity and a.c. susceptibility vs. temperature data for LaBaSrCu₃O_7-δ are presented. The absence of an orthorhombic unit cell for these compounds is discussed and a model for the oxygen-vacancy ordering is proposed to explain the observed superstructure in LaBaCaCu₃O_7-δ.