Synthesis and properties of fluorine-doped Tl(1223): bulk materials and Ag-sheathed tapes

Fluorine-doped TI(1223) superconductors were prepared starting from mixtures of oxides (carbonates) and fluorides (in particular TIF) under moderately high gas pressure (50 bar). The thermodynamics of the phase formation were found to be modified so that the melting point was lowered and the region where a liquid phase coexists enlarged. 90% phase purity was obtained for bulk materials with the composition TI0.6 Pb(0.5)Sr(1.8)Ba(0.2)Ca(1.9)Cu(3)O(8.5-)delta F-0.5, prepared by a two-step synthesis including TIF among the starting materials. The critical temperature remained approximately the same as for F-free samples (similar to 115 K), but a remarkable increase of the irreversibility line was observed below 75 K. The samples showed superconducting properties even for the highest amount of fluorine introduced (about one atom per formula unit). Ag-sheathed tapes were prepared by the conventional PIT method, using ex situ reacted TI(1223) powders or producing the reaction in situ. Transport critical current densities of about 10 000 A cm(-2) were measured. This value is slightly lower than generally observed for F-free samples, but the magnetic-field dependence was slightly improved. The tape anisotropy could be increased to a factor of 2.5 by performing cycles of mechanical pressing and heating above the formation temperature, The results (in particular neutron scattering diffraction) indicate that, with high-pressure synthesis, F remains in the sample and substitutes for O in the TIO layers in the TI(1223) phase.