A CRYSTAL STUDY OF THE 128 K SUPERCONDUCTOR TL2BA2CA2CU3O10-DELTA - SYNTHESIS AND ANISOTROPIC MAGNETIC-PROPERTIES

Tl-2223 (Tl2Ba2Ca2CU3O10) millimeter-size (in the ab-plane) crystals, exhibiting sharp transitions with T-c's ranging from 125 K to 128 K, have been grown for the first time. The combination of ED, EDX and XRD techniques has allowed us to select large crystals with the composition ''Tl1.95Ba2Ca2.5Cu3'', i.e. close to the nominal composition. An annealing in a reducing atmosphere at low temperature decreases T-c from 128 K to 116 K with a broadening of the transition; T(c)is partially restored (124 K) by annealing under oxygen flow. Magnetic measurements have been carried out in both geometries, H parallel to ab-plane and H parallel to c with 0 less than or equal to H less than or equal to 5.5 T at different temperatures. The values of J(c), in the ab-plane and the irreversibility line position for H parallel to c have been deduced from the magnetization loops. lambda(ab)(O)=2400 Angstrom and lambda(c)(0)=11 000 Angstrom have been extracted from the reversible part of these isothermal M(H) curves, leading to an anisotropy y=(m(c)/m(ab))(1/2)=4.6. This latter value demonstrates that Tl2Ba2Ca2Cu(3)O(10) is less anisotropic than the n=1 and n=2 members of the thallium series Tl2Ba2Can-1CunO(2n+4).