Pb-doping effect on irreversibility fields of HgBa₂Ca ₂Cu₃O_8+δ superconductors

We prepared nearly single-phase polycrystalline samples of HgBa ₂Ca₂Cu₃O_8+δ(Hg-1223) without and with Pb doping, i.e., Pb-free Hg-1223 and (Hg_0.8Pb _0.2)-1223, by means of a sample encapsulation technique. According to quantitative energy dispersive x-ray spectroscopy analyses and iodometric oxygen titration, the Pb-free Hg-1223 and the (Hg_0.8Pb _0.2)-1223 sample were identified to be Hg_0.97Ba _2.00Ca_2.04Cu_3.00O_8.33 and (Hg _0.79Pb_0.29)Ba_2.00Ca_1.98Cu _2.94O_8.42, respectively. The highest Meissner-signal onset temperatures for the Pb-free Hg-1223 (after post-annealing in O₂ gas) and the (Hg,Pb)-1223 (as-sintered) sample were observed at 135.4 and 132.0 K, respectively. The difference in the irreversibility fields (H_irr) for both samples was very small at temperatures above 80 K, while, at temperatures below 70 K, the H_irr versus T curve for the (Hg,Pb)-1223 sample shifted toward the higher field side. Thus, Pb doping was effective for the improvement of magnetic property of Hg-1223 superconductors, at least, at temperatures below 70 K.