THE EFFECT OF THE OXYGEN-CONTENT IN PB2SRLACU2O6+DELTA AND PB2BA2YCU3O8+DELTA ON THE STRUCTURAL AND SUPERCONDUCTING PROPERTIES

The effect of the change in the oxygen content of Pb2SrLaCu2O6+δ and Pb2Ba2YCu3O8+δ on their structure and the superconducting properties has been studied. A large miscibility gap exists, whereby δ is either close to 0 or close to 1.4. Pb2SrLaCu2O6+δ with δ=0 has a Tc of 32 K, whereas with δ=1.4 no superconductivity is obtained. Pb2Ba2YCu3O8+δ shows no superconductivity. Both compounds are orthorhombic for δ being close to 0. When heated in oxygen at 450°C Pb2SrLaCu2O6+δ undergoes a phase transition to quasi-tetragonal associated with the incorporation of oxygen up to about δ=1.4. Electron diffraction on Pb2SrLaCu2O6+δ shows the presence of a superstructure with an incommensurate modulation along the b axis with a length of about 3.8b. Oxygen rich Pb2Ba2YCu3O8+δ, obtained by heating in oxygen at 400°C, shows a superstructure with a 4ap, 4bp, c unit cell, where ap is the a axis of a primitive perovshite unit cell. The superstructures of both compounds disappear when the materials are heated in reducing atmospheres. Heating in air at 450°C leads to a partial collapse of the structures. © 1990.