Abstract
Tl2-xBa2Ca2Cu3+zOy (Tl-2223 system) superconducting oxides with various Tl contents and Cu contents were prepared. We investigated the relation between crystal structure and superconducting property dependence of Tl and Cu content. First, we obtained an almost single-phase material at a Tl content 2-x=1.7 and the Tl-2223 phase remained the main phase in the region of 2-x≥0.631 of Tl2-xBa2Ca2Cu3Oy. Tc was about 120K for the Tl-2223 in a wide range of 0.631≤2-x≤1.761. Second, we obtained a single-phase at 2-x=1.777 and Cu content 3+z=3.284 of Tl2-xBa2Ca2Cu3+zOy. The Tc(zero) dependence of Cu/Tl ratio, showed a maximum value of 122K at Cu/Tl ratio=2.026. Based on results of crystal structure refinements by powder neutron diffraction, it was found that the change in bond length and angle dependence of average Cu valence were similar for Tl2-xBa2Ca2Cu3Oy and Tl2-xBa2Ca2Cu3+zOy. However, the structural change dependence on average Cu valence of Tl2-xBa2Ca2Cu3Oy was large as compared to that of Tl2-xBa2Ca2Cu3+zOy, because the substitution of Cu for Tl causes only a limited distortion of the crystal structure. Moreover, the Tc dependence on average Cu valence shows the same tendency for Tl2-xBa2Ca2Cu3Oy and Tl2-xBa2Ca2Cu3+zOy. Tc(zero) reached the highest value of 122K at-2.07 of average Cu valence. Tc was related to the average Cu valence and flatness of the CuO2 and BaO planes.
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