PREPARATION AND CHARACTERIZATION OF 80-K SUPERCONDUCTING PB2SR2Y1-XCAXCU3O8+DELTA SINGLE-CRYSTALS

Pb2Sr2YCu3O8+∂ becomes superconducting when some of the trivalent Y cations are replaced by Ca. Preparation of single phase powders of the calcium compound with a reasonable superconducting volume and sharp superconducting transitions has not yet been achieved. This is probably due to an inhomogeneous distribution of the Ca cations over the Y sublattice. Single crystals can be grown in platinum crucibles using a PbO-PbF2 flux according to the mole ratios: PbO : PbF2 : Pb2Sr2(Y,Ca)Cu3O8, 2:0.5:1. The powder mixture is melt at 1000 °C in a mild oxidizing atmosphere (0.5-2% O2 in N2), slowly cooled at 5-1 °C/minute to 800 °C and then furnace cooled. The crystals grown on the surface are square platelets of 2mm edge and 0.08mm thick. Those grown in the solution are thicker and some of them attain 1-2 mm3 in volume. Magnetic measurements at low temperature show sharp superconducting transitions and 100% superconducting volume at Tc near 80 K. Different crystals with values of Tc between 40 and 80 K have also been prepared by changing the preparation conditions and the Y Ca ratio. As determined by microprobe analysis, Tc increases with the Ca Y and Sr Pb ratios. Controled oxidation of crystals by thermogravimetric analysis has also been carried out in relation with superconductivity and x-ray measurements. In a superconducting crystal Tc does not change with oxygen uptake but the superconducting volume varies as the oxidized compound is not superconducting. Pb2Sr2YCu3O8+∂ is a good candidate to make detailed structural studies for the relations between structure and superconductivity since mm3 size single crystals of controled stoichiometry are now available for neutron diffraction and superconductivity measurements. © 1990.