STRUCTURE STUDIES ON T1-2122 AND T1-2223 SUPERCONDUCTORS

Substantial differences in the superconducting transition, both in width and onset temperature, for the same structure-type have been observed in "pseudo-flux" grown crystals in the Tl-Ca-Ba-Cu-O system. These differences have been attributed to stoichiometry variations arising from cation site substitution, primarily the replacement of Ca2+ with Tl3+ between the Cu-O2 layers. Several crystals belonging to the Tl-2122 (Tl2CaBa2Cu2O8 with c congruent-to 29 angstrom) and the Tl-2223 (Tl2Ca2Ba2Cu3O10 with c congruent-to 36 angstrom) structure-type have been examined by single crystal X-ray diffraction. The Tl-2122 phase has 15 to 28% Tl for Ca substitution while the Tl-2223 phase has only 8-9% substitution in the particular crystals examined. In addition, both phases also show a large variation in the Tl site occupancy (88 to 100%). Such cation disorder is consistent with the observed small differences in lattice parameters reported by various investigators in crystals of this system.