Synthesis and thermal stabilization of nearly single-phase superconductor HgBa₂Ca₂Cu₃O_8+δ by TI substitution

Nearly single-phase Hg(TI)Ba₂Ca₂Cu₃O_8+δ superconductors have been fabricated by the solid state reaction method. Resistance and magnetization measurements indicated that the samples annealed at 300°C for 6 h in O₂ had a superconducting transition temperature with T_c(onset) = 140 K and T_c(zero) = 134 K. The thermal stability of the Hg(TI)-1223 phase was studied with the help of thermogravity, differential thermal analysis and the Raman spectrum. The thermal processes of Hg(TI)-1223 phase decomposition indicated that the percentage of reversible oxygen evaporation was smaller and the onset temperature (825°C) of Hg(TI)-1223 phase complete decomposition was much higher than those of the undoped Hg-system superconductor. Raman measurements showed that there existed a strong peak at 596 cm^-1 with a small shoulder peak at 571 cm^-1 and several other weak bands at 126, 142, 232 and 665 cm^-1. The strongest peak at 596 cm^-1 may originate from the A_1g vibration mode of both interstitial site oxygens and apical oxygens. The addition of thallium introduced more oxygen atoms and stronger bonding into HgO planes, raising the vibration frequency of oxygen atoms and stabilizing the crystal structure of the Hg-1223 phase.