Superconducting La 1.5− xBa 1.5 + x−yCa yCu 3O z solid solutions II. Oxygen content and crystal structure

Abstract

The La 1.5− x Ba 1.5 + x−y Ca yCu 3O 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 1.5Cu 3O z rather than on 123, LaBa 2Cu 3O z . The oxygen con tents have been determined by H 2 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.