Tl/Pb and Sr/Ba cuprates of type 1212: compositional effect on the purity and on the superconducting properties

• Published in: Physica. C, Superconductivity Superconductivity, 1998-03, Vol.297 (3), p.201-210
• Main Authors: Lebbou, K, Cohen-Adad, M.Th, Abraham, R, Trosset, S, Gladyshevskii, R.E, Flükiger, R, Galez, P, Schulz, G.W, Weber, H.W, Couach, M
• Format: Article
• Language: English
• Subjects: Crystal structure; High-temperature superconductors; Phase equilibria; Tl,Pb-1212
• ISSN: 0921-4534; 1873-2143
• DOI: 10.1016/S0921-4534(97)01868-6

The effect of Sr/Ba and Tl/Pb substitutions has been investigated for Tl 1− x Pb x (Sr 1− y Ba y ) 2CaCu 2O 7− δ (0≤ x and/or y≤1) compounds produced by conventional solid state reaction. The observed phases were characterized by X-ray powder diffraction, scanning electron microscopy and their nominal composition has been controlled by plasma emission spectroscopy. A tentative single phase field is proposed at T≤960°C. For x=0.5, the highest weight fraction of the (Tl 0.5Pb 0.5)(Sr/Ba)-1212 phase was found to be 96.4% and 94% when Ba increases from 0 to 0.2. It drastically decreases when Ba content is over 0.3. For Ba over 0.3, (Tl/Pb)(Sr/Ba)-1223 and perovskite (Tl/Pb)(Sr/Ba)O 3 are observed as main impurities and their content increases with Ba stoichiometry. The unit cell parameters of (Tl 0.5Pb 0.5)(Sr/Ba)-1212 phase increase when Sr atoms are progressively replaced by Ba atoms. Structural refinement leads to conclude that 10 at% of Tl would be substituted on Ca sites and less than 2 at% of Ca could be substituted on Sr sites. The distance between the Cu atom and the apical O atom of the square pyramid increases only slightly when the Ba content raises. The critical temperature remains between 80 and 90 K and seems to be affected by the substitution Sr/Ba.