Cation Composition and Oxygen Content Dependence of Crystal Structure and Tc for Tl2-xBa2Ca2Cu 3+zOy

Abstract

Tl2-xBa2Ca2Cu3+zO y(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. T c was about 120 K 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-xBa2Ca2Cu 3+zOy. The Tc (zero) dependence of Cu/Tl ratio, showed a maximum value of 122 K 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-xBa2Ca2Cu3O y and Tl2-xBa2Ca2Cu 3+zOy. However, the structural change dependence on average Cu valence of Tl2-xBa2Ca2Cu 3Oy was large as compared to that of Tl 2-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-xBa2Ca2Cu 3Oy and Tl2-xBa2Ca 2Cu3+zOy. Tc(zero) reached the highest value of 122 K at ∼2.07 of average Cu valence. Tc was related to the average Cu valence and flatness of the CuO2 and BaO planes.