Superconductivity in Lanthanum Barium Cuprate Perovskites.

Samples in the system Ba sub (2-x) La sub (1-x) Cu sub 3 O sub (7-delta) (0.0 = x = 0.5; 0 delta 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 sub (1.5) La sub (1.5) Cu sub 3 O sub (7.25) indicates that the structure is similar to the 92K superconductor Ba sub 2 YCu sub 3 O sub 7 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 x0.2 contain traces of BaCuO sub 2, but exhibit T onset above c ~ 90K and R=0 near 80K.