SYNTHESIS AND NEUTRON POWDER DIFFRACTION STUDY OF THE SUPERCONDUCTOR HGBA2CA2CU3O8+DELTA BY TL SUBSTITUTION

Substitution of Tl for Hg was performed in the tig based 1223 phase HgBa2Ca2Cu3O8+delta (T-c = 135 K), resulting in an increase of the superconducting transition temperature to 138 K for samples with a nominal composition of Hg0.8Tl0.2Ba2Ca2Cu3O8+delta. The crystal structure of this solid solution has been investigated by neutron powder diffraction techniques at room temperature and at 10 K. The compound has the same crystal structure as Hg-1223 with the space group symmetry P4/mmm and lattice parameters a = 3.8489(1), c = 15.816(1) Angstrom. Rietveld analysis results indicate that Hg is partially replaced by Tl, and the oxygen content, delta, is 0.33. The lattice-parameter changes resulting from the Tl substitution are too small to account for the T-c change by mimicking the effect of pressure. No phase transition occurs down to 10 K.