Structural characterization and superconducting properties of (Tl0.5Pb0.5)(Sr2-xBax)Ca2Cu3O9-delta

Samples of nominal composition (Tl0.5Pb0.5)(Sr2-xBax)Ca2Cu3O9-delta 0 less than or equal to x less than or equal to 0.6, and (Tl0.6Pb0.2Bi0.2)(Sr1.8Ba0.2)Ca2Cu3O9-delta were characterized by X-ray powder diffraction, scanning electron microscopy and AC susceptibility measurements. In the Bi-free samples, the highest weight fraction of the 1223 phase, 91.3%, was observed for x = 0.4. Above this Ba content, BaPbO3 perovskite appeared as main impurity phase. In the Bi-containing sample the weight fraction was even higher, 92.6%. The Ba content of the 1223 phase, as determined from X-ray diffraction data, was in agreement with the nominal composition of the samples. The unit cell parameters increased when the Sr atoms were progressively replaced by the larger Ba atoms. The refinement of the crystal structure of the 1223 phase revealed a significant off-centering of the Tl site. The distance from the splitted site to the ideal position on the four-fold axis was found to be similar to 0.26 Angstrom for the Ba-containing samples, but only 0.07 Angstrom for the Ba-free sample. The distance from the Cu atom to the apical atom of the square pyramid formed by the oxygen atoms increased from 2.29 to 2.50 Angstrom as the Ba content was raised. The critical temperature remained between 116 and 120.5 K and was not significantly affected by changes in composition or structure.