Study on enhancement ofTc(⩾130 K) in TlBa₂Ca₂Cu₃Oysuperconductors

A TlBa 2 Ca 2 Cu 3 O y (Tl-1223) superconductor with an ideal composition has a much higher T c than the established value (~120 K). The intrinsic T c arises by suppression of Tl substitution for Ba and Ca sites, which inherently occurs in samples prepared by conventional methods. The substitution of Tl was suppressed by preserving the samples in a heavily over-doped state ( T c ~100 K) which was realized by a high-pressure (solid and oxygen) synthesis method. In addition, we found several synthesis conditions for which we could obtain high- T c samples with good reproducibility. Residual carbon in the precursor was found to be harmful for superconductivity, so that it had to be reduced as much as possible before the high-pressure synthesis. The synthesis temperature and starting concentration of Tl, which also affect T c , had to be controlled properly to enhance T c . Samples show that T c ⩾130 K by post-annealing in a reducing atmosphere at 650-700 °C for 0.5 h to 12 h. The sample prepared with the starting composition of Tl 0.75 Ba 2 Ca 2 Cu 3.25 O y had an almost ideal grain composition and showed a T c of 133.5 K. This T c is comparable with that of HgBa 2 Ca 2 Cu 3 O y (Hg-1223) which is known to have the highest T c in superconductors.