PB-0212, NEUTRON POWDER DIFFRACTION STUDY OF A SUPERCONDUCTING CUPRATE WITH REDUCED ANISOTROPY

The crystal structure of Pb-0212 was refined from neutron powder diffr action data at room temperature and 8K as tetragonal (space group I4/m mm). The transition to superconductivity is not accompanied by a struc tural rearrangement. The disadvantage of the lacking charge reservoir in the 0212 type is compensated by the introduction of Pb2+/Pb4+ as se cond redox couple in addition to the system Cu2+/Cu3+. An alternative charge reservoir is created between the CuO5 double pyramids by incorp oration of Pb in 2a and O in 2b. Preparation of powdered materials at ambient pressure results in the formation of superconductivity. Howeve r, the superconducting volume fraction remains low due to the formatio n of grain boundaries acting as weak links. A significant improvement is obtained by use of Ca2PbO4 or CaBi2O4 as flux. The highest T-c is a bout 75 K.