Superconducting La1.5-xBa1.5+x-yCayCu3Oz solid solutions I. Phase diagram, cation stoichiometry and Tc data

The phase diagram for the LaBCCO solid solutions La_1.5-xBa_1.5+x-yCa_yCu₃O_z has been determined in air at 950°C. Solid solution form over the complete composition range 0≤x≤0.5, 0≤y≤0.5 apart from composition at high x and low y, centred on the La-123 composition LaBa₂Cu₃O_z. The LaBCCO solid solutions are tetragonal at all oxygen contents and are structurally distinct from tetragonal/orthorhombic La-123, which can be prepared as a stoichiometric phase by reaction in Ar. T_c values of the LaBCCO solid solutions increase linearly with x at constant y and with y at constant x for samples prepared under the same conditions. Maximum T_c is ∼78 K for x=y=0.5 for samples that were oxygen annealed at 400°C. The melting behaviour of the LaBCCO solid solutions has been determined. Melting temperatures increase with both decreasing x and y; the La-336 composition has the highest melting temperature (∼1180°C) and melts incongruently to La₂CuO₄ + liquid.