SYNTHESIS AND CHARACTERIZATION OF A NEW MERCURY-BASED SUPERCONDUCTOR,HG0.5TL0.5BA2CA1-XSRXCU2O6+DELTA

A new mercury-based superconductor Hg0.5Tl0.5Ba2Ca1-xSrxCu2O6+delta (x = 0.14, denoted as Hg,Tl-1212) was synthesized from the reaction of H gO, Tl2O3, Ba2CuO3+x, and Ca1-xSrxCuO2. X-ray powder diffraction analy sis showed that the thallium completely substitutes for the mercury si tes in the rock-salt [HgO delta] layer. The as-synthesized Hg,Tl-1212 has a T-c of 122 K. Surprisingly the T-c decreased to about 100 K upon annealing in an oxygen atmosphere at 350 degrees C for 8 h while the argon annealing did not show any substantial change in T-c under the s ame condition. The crystal symmetry was found to be tetragonal (P4/mmm ) with lattice parameters a = 3.8496(1) Angstrom and c = 12.6536(4) An gstrom. The Hg,Tl-1212 is quite stable up to 450 degrees C. The intra- critical current density of Hg,Tl-1212 is about twice larger than that of HgBa2Ca1-xSrxCu2O(6+delta) (x = 0.14). The substitution of Tl for Hg clearly stabilized the Hg,Tl-1212 phase as well as enhanced the pin ning property.