Superconducting La-123 materials and related rare earth cuprates
Solid solutions of general formula La1.5-xBa1.5+x-yAyCu3Oz (A=Ca, Sr) have been studied. These form over the complete composition range 0x0.5, 0y0.5, excluding compositions centred on the La-123 composition, LaBa2Cu3Oz. The solid solutions have properties which range from semiconducting (x=y=0) to superconducting, with Tc (max.) = 78K for the composition LaBa1.5Ca0.5Cu3O7.01, and were found to be tetragonal across their entire range. Rietveld refinement of X-ray powder data has shown that whereas Sr occupies the Ba site, Ca occupies the La site with concomitant displacement of La onto the Ba site. The magnetic behaviour and oxygen contents of these solid solutions are discussed. The variation of Tc with oxygen content has been studied for the composition LaBa1.5Ca0.5Cu3Oz; for quenched samples, Tc varies linearly with oxygen content, but slow cooled samples show a "Double Plateau" behaviour, and thus Tc depends critically upon the cooling regime. Analogues RE1.5-xBa1.5+x-yCayCu3Oz (RE = Nd, Pr, Sm, Eu, Gd and Dy) have also been studied; the extent of the solid solution decreases with decreasing size of rare earth. For Dy, no appreciable solid solubility was observed. For Nd and Pr, the solid solutions centre on the 336 composition La1.5Ba1.5Cu3Oz; for the smaller rare earths this composition cannot be prepared in air, and the solid solutions are centred on the orthorhombic, REBa2Cu3Oz phase. All were found to be superconducting except the Pr-based solid solutions; results suggest that these contain Pr4+ as well as Pr3+. In the solid solutions RE1+xBa2-xCu3Oz (x0, RE = Sm, Eu and Gd), critical temperatures exhibit a marked dependence on the size of the A-site cations.