Superconducting La_1.5-xBa_{1.5 + x-y}Ca_yCu₃O_z solid solutions II. Oxygen content and crystal structure

The La_1.5-xBa_{1.5 + x-y}Ca_yCu₃O_z solid solutions have properties that vary from semiconducting (x=y=0) to super conducting with T_c=78 K (x=y=0.5). The latter are of special interest since they are tetragonal. Rietveld refinement of powder X-ray diffraction data confirms that Ca substitutes onto the La sites, with concomitant displacement of La onto the Ba sites and that the solid solutions are based on the 336 formula, La_1.55Ba_1.5Cu₃O_z rather than on 123, LaBa₂Cu₃O_z. The oxygen con tents have been determined by H₂ reduction. With increasing x, the oxygen content is unchanged as Ba substitutes for La and therefore charge compensation is achieved by hole creation. With substitution of Ba by Ca, i.e. increasing y, additional oxygen is incorporated into the structure and, again, holes are created to preserve charge balance. A good correlation between T_C and average Cu valence is found.