The effect of Pb doping in HgBa₂Ca₂Cu₃O_8+δ superconductor

Polycrystalline samples with the starting compositions of (Hg_0.8Pb_0.2)_xBa₂Ca₂Cu₃O_8+δ (1.0≤x≤2.5) were prepared under various synthetic conditions. Both powder X-ray-diffraction analysis and energy dispersive X-ray spectroscopy revealed that the Pb ions were successfully doped into the Hg-1223 phase. High-quality Pb-doped Hg-1223 samples for the starting composition with x=1.5 were obtained when heated at 675°C only for 1 h. The resistivity (ρ{variant}) of such a sample was low, being ∼8×10³ Ω cm at 300 K, and the ρ{variant} versus T curve was linearly extrapolated to zero resistance at 0 K, being similar to the cases of high-quality high-T_c cuprates. The zero resistance due to superconductivity started at 122.2 K. When the sample was reduced in flowing argon gas, it exhibited a Meissner signal below 132 K and its Meissner volume fraction exceeded 20% at 5 K. According to thermoelectric power measurements, the carrier density in the sample was reversibly dependent on the oxygen partial pressure, and the sample post-annealed in flowing oxygen gas was likely overdoped.