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 small quantities. The refined Ba content in the 1223 phase corresponded to the nominal compositions and the cell parameters increased with increasing x. The distance between the Cu atom and the apical O atom of the square pyramid increased from 2.29 to 2.50 Angstrom as the Ba content was raised. A displacement (up to 0.26 Angstrom) of the Tl atoms from 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 significantly affected by the changes in composition and structure and ranged from 118 K (x=0) to 116 K (x=0.6).