COMPOSITION DEPENDENCE OF THE SUPERCONDUCTING PROPERTIES OF THE TL-BA-CA-CU-O SYSTEM

A series of superconducting samples with nominal composition Tl2Ba2Can-1CunO2n+4 (1 less-than-or-equal-to n less-than-or-equal-to 11) have been prepared by a two-step method using barium nitrate (instead of barium oxides or barium carbonate). The phase analysis and structural analysis were performed by X-ray diffraction and an electron microprobe. The four-probe resistance and AC susceptibility measurements show that all the samples are superconducting with T(c) above 105 K except the n = 1 sample, which has a T(c) of 88 K. The highest T(c) (T(c)(M) = 124 K) in this series is obtained in the n = 5 sample. It is found that there is a correlation between T(c) and lattice parameter a of the 2223 phase. T(c) increases when lattice parameter a decreases. Several new phases are found by the microprobe analysis. The reaction route and the stability of the samples are also discussed.