The La1.5-xBa1.5 + x-yCayCu3Oz solid solutions have properties that vary from semiconducting (x=y=0) to super conducting with Tc=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, La1.55Ba1.5Cu3Oz rather than on 123, LaBa2Cu3Oz. 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 TC and average Cu valence is found.
Bibliographical noteAcknowledgements We thank the Science and Engineering Research Council for research grants (to ARW), the International Centre for Diffraction Data for a scholarship (JMSS) and S.M. Fray for her assistance with TGA work