It was found that Tl-Sr-Ca-Cu-O system gives rise to at least two superconducting phases, a high-T
c phase at 100 K and a low-T
c phase at about 75 K. The two phases are isostructural to those of Bi-Sr-Ca-Cu-O system. Comparing the T
c's and crystal structures of the Tl and Bi systems, it was suggested that T
c is mainly determined by the combination of two alkaline earth elements, i.e., Sr/Ca or Ba/Ca, when the repetition unit in the layered perovskites is equal.
We report on the superconductivity and crystal structures in Tl-(Ba, Sr)-Ca-Cu-O (TBSCC) system. In the case of TBSCC system, the 2223 phase was obtained as a single phase more easily compared with the Tl-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems. As a result, we achieved a high superconducting volume fraction, 95% above 100 K. The crystallographic transformation was caused by changing the ratio of Ba to Sr. By the X-ray diffraction and TEM analysis, the crystal structure was determined as a 2223 structure consisting of Tl-O bilayers in the Ba-rich side, and a 1223 structure consisting of Tl-O monolayer in the Sr-rich side.
Electrical properties of the Sr-V-O systems doped with Tl, La, and Ce were extensively studied. The Sr-V-O system was prepared in the reduced form, for example, SrVO3-δ, and was doped with Tl. In the Tl-Sr-V-O system a sharp drop in the resistivity was observed at around 30 and 130 K in several samples. A diamagnetic signal was clearly observed below 30 K. The superconducting states of the Tl-Sr-V-O system are found to be metastable in the ambient atmosphere. It is suggested that there are at least two superconducting phases or crystal structures giving rise to the T
c's. The highest onset temperature observed for the sharp resistivity drop was 160 K.
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