EFFECT OF LEAD SUBSTITUTION IN BISMUTH-BASED CUPRATE SYSTEMS CONTAINING BOTH LOW-T(C) AND HIGH-T(C) PHASES

We report the results of resistivity, ac-susceptibility, and specific-heat measurements carried out on samples of Bi2-xPbxSr2Ca2Cu3O10+x (0 less-than-or-equal-to x less-than-or-equal-to 0.7) that were characterized by x-ray diffraction and high-resolution scanning tunneling microscopy. The zero resistance temperature, T(c) (R = 0), shows a peak at x = 0.3 and the susceptibility data indicate that although a high-T(c) 2:2:2:3 phase is present at 110 K, there is a second transition at 90 K that seems to indicate the existence of an additional low-T(c) 2:1:2:2 phase. The specific-heat data unambiguously indicate a clear and distinct anomaly around T(c) for samples with x = 0.3-0.4 and smearing of the anomaly for x > 0.5. Based on the results obtained from these combinations of experiments, the role of lead substitution in Bi samples, particularly in the stabilization of the 2:2:2:3 phase has been discussed.