STRUCTURAL REFINEMENTS ON HIGH-T-C SUPERCONDUCTOR TL0.5PB0.5SR2-XBAXCA2CU3O9-DELTA

Samples of nominal composition Tl0.5Pb0.5Sr2-xBaxCa2Cu3O9-delta with 0 less than or equal to x less than or equal to 0.6 were characterized by structural refinements on X-ray powder diffraction data. The weight fraction of the 1223 phase ranged from 85% (x=0) to 91% (x=0.4); 1212 phase, (Ca,Sr)(2)PbO4, (Ca,Sr)(2)CuO3 and BaPbO3 being detected in sm all quantities. The refined Ba content in the 1223 phase corresponded to the nominal compositions and the cell parameters increased with inc reasing x. The distance between the Cu atom and the apical O atom of t he square pyramid increased from 2.29 to 2.50 Angstrom as the Ba conte nt was raised. A displacement (up to 0.26 Angstrom) of the Tl atoms fr om the ideal position on the I-fold axis was observed for all samples. Structural refinements on neutron diffraction data indicated related displacement of the O atoms in the TlO layer. T-c was not significantl y affected by the changes in composition and structure and ranged from 118 K (x=0) to 116 K (x=0.6).