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

摘要

The effect of Sr/Ba and Tl/Pb substitutions has been investigated for Tl1-xPbx(Sr1-yBay)(2)CaCu2O7-delta (0 less than or equal to x and/or y less than or equal to 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 less than or equal to 960 degrees C. For x = 0.5, the highest weight fraction of the (Tl0.5Pb0.5)(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 B a stoichiometry. The unit cell parameters of (Tl0.5Pb0.5)(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. (C) 1998 Elsevier Science B.V. [References: 21]