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 diffra ction, scanning electron microscopy and AC susceptibility measurements . In the Bi-free samples, the highest weight fraction of the 1223 phas e, 91.3%, was observed for x = 0.4. Above this Ba content, BaPbO3 pero vskite appeared as main impurity phase. In the Bi-containing sample th e weight fraction was even higher, 92.6%. The Ba content of the 1223 p hase, as determined from X-ray diffraction data, was in agreement with the nominal composition of the samples. The unit cell parameters incr eased when the Sr atoms were progressively replaced by the larger Ba a toms. The refinement of the crystal structure of the 1223 phase reveal ed a significant off-centering of the Tl site. The distance from the s plitted 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 t he apical atom of the square pyramid formed by the oxygen atoms increa sed from 2.29 to 2.50 Angstrom as the Ba content was raised. The criti cal temperature remained between 116 and 120.5 K and was not significa ntly affected by changes in composition or structure.