SYNTHESIS AND CHEMISTRY OF THE NEW Y-BASED AND Bi-BASED HIGH TEMPERATURE SUPERCONDUCTING PEROVSKITES

Abstract
Chemical synthesis of the new high T_c oxides using solid state reactions or solution techniques is shown. A solution process which allows the preparation of both homogeneous and dense ceramics and of superconducting thick films has been developed for the YBa₂Cu₃O₇ system. Physical measurements performed on homogeneous YBa₂Cu_3-xCo_xO_y samples have shown that the Co substitution, associated with an uptake of oxygen, takes place on the Cu-O chains and that T_c is depressed, leading ultimately to antiferromagnetic insulators whose magnetic structures are shown. Three phase of general formula Bi₂Sr₂Ca_n-1Cu_nO_y with n = 1,2 and 3 have been isolated in the Bi system and characterized for their structural and physical properties. These phases crystallize in the same pseudotetragonal unit cell differing one from the next by the stacking sequence along the c-axis, with the main feature being the presence of a sheared Bi-O double layer. The T'_cs of the n = 1,2 and 3 phases are 10 K, 85 K and 110 K respectively, but because of phase intergrowth, these values depend upon sample processing, making chemical substitutions within the Bi system quite complex as is discussed. Finally, we succeeded in making superconducting thick films (having zero resistance around 100 K) of the Bi-based and T1-based materials via the decomposition of aqueous-glycerol solutions containing the salts of the elements.