  We report systematic studies of structural, microstructural and transport
properties of (Hg_0.80 Sb_0.2-x Pb_x)Ba_2 Ca_2 Cu_3O_8+\delta (where x = 0.0,
0.05, 0.1, 0.15, 0.2) compounds. Bulk polycrystalline samples have been
prepared by two-step solid-state reaction route at ambient pressure. It has
been observed that simultaneous substitution of Sb and Pb at Hg site in oxygen
deficient HgO_\delta layer of HgBa2Ca2Cu3O8+\delta cuprate high-Tc
superconductor leads to the formation of Hg-1223 as the dominant phase.
Microstructural investigations of the as grown samples employing scanning
electron microscopy reveal single crystal like large grains embodying spiral
like features. Superconducting properties particularly transport current
density (Jct) have been found to be sensitive to these microstructural
features. As for example (Hg0.80Sb0.05Pb0.15)Ba2Ca2Cu3O8+\delta compound which
exhibits single crystal like large grains (~ 50 micrometer) and appears to
result through spiral growth mechanism, shows highest Jct (~ 1.85 x 103 A/cm2)
at 77K. A possible mechanism for the generation of spiral like features and
correlation between microstructural features and superconducting properties
have been put forward.
