TL PB AND SR/BA CUPRATES OF TYPE-1212 - COMPOSITIONAL EFFECT ON THE PURITY AND ON THE SUPERCONDUCTING PROPERTIES/

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 sta te reaction. The observed phases were characterized by X-ray powder di ffraction, scanning electron microscopy and their nominal composition has been controlled by plasma emission spectroscopy. A tentative singl e phase field is proposed at T less than or equal to 960 degrees C. Fo r 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. Th e unit cell parameters of (Tl0.5Pb0.5)(Sr/Ba)-1212 phase increase when Sr atoms are progressively replaced by Ba atoms. Structural refinemen t 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 dista nce between the Cu atom and the apical O atom of the square pyramid in creases only slightly when the Ba content raises. The critical tempera ture remains between 80 and 90 K and seems to be affected by the subst itution Sr/Ba. (C) 1998 Elsevier Science B.V.