Synthesis and neutron powder diffraction study of the superconductor HgBa 2Ca 2Cu 3O 8 + δ by Tl substitution
Substitution of Tl for Hg was performed in the Hg based 1223 phase HgBa 2Ca 2Cu 3O 8 + δ ( Tc = 135 K), resulting in an increase of the superconducting transition temperature to 138 K for samples with a nominal composition of Hg 0.8Tl 0.2Ba 2Ca 2Cu 3O 8 + δ. 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 as Hg-1223 with the space group symmetry P4/mmm and lattice parameters a = 3.8489(1), c = 15.816(1) Å. Rietveld analysis results indicate that Hg is partially replaced by Tl, and the oxygen content, δ, is 0.33. The lattice-parameter changes resulting from the Tl substitution are too small to account for the Tc change by mimicking the effect of pressure. No phase transition occurs down to 10 K.