Superconductivity in lanthanum barium cuprate perovskites

Samples in the system Ba_2-xLa_1+xCu₃O_7-δ (0.0 ≤ x ≤ 0.5; 0 < δ < 0.25) have been prepared and characterized by X-ray powder diffraction, magnetic susceptibility, electrical resistivity, and for x = 0.5 powder neutron diffraction. This neutron diffraction study on Ba_1.5La_1.5Cu₃O_7.25 indicates that the structure is similar to the 92 K superconductor Ba₂YCu₃O₇ with two exceptions. First, the "Ba site" contains 75% Ba and 25% La and second, the symmetry is tetragonal. At x = 0.5 the material is a semiconductor, but as x decreases metallic behavior and superconductivity are observed. Compositions with x < 0.2 contain traces of BaCuO₂, but exhibit T^onset_c ≈ 90 K and R = 0 near 80 K. For x = 0.2-0.3, T^onset_c ≈ 60 K and R = 0 near 50 K are observed . For x ≤ 0.1 orthorhombic symmetry is observed. These results provide further evidence that oxygen stoichiometry and details of the oxygen ordering are important to superconductivity.