Effect of Pb addition on microstructure, transport properties, and the critical current density in a polycrystalline FeSe${}_{0.5}$Te${}_{0.5}$

We have investigated the effects of lead (Pb) additions (x) up to 40 wt% (x = 0-0.4) on the structure, electrical properties, and magnetic properties of FeSe${}_{0.5}$Te${}_{0.5}$ superconductor. The samples were prepared by the solid-state reaction method and were characterized by various techniques. The parent compound (x = 0) showed the onset temperature T${}_c^{onset}$ of 15 K, and zero-resistance temperature, T${}_c^{offset}$ of 12 K. The addition of Pb enhances the metallic characteristics of FeSe${}_{0.5}$Te${}_{0.5}$, but both T${}_c^{onset}$ and T${}_c^{offset}$ are decreased to the lower temperature with the broadened transition width. The T${}_c^{onset}$ is nearly the same (10.3 K) at higher additions, such as x = 0.3 and 0.4, but zero resistivity is not observed up to 7 K. Microstructural analysis and transport studies suggest that for x > 0.05, Pb additions weakened the coupling between grains and suppressed the superconducting percolation, leading to a broad transition. More importantly, the inclusion of a relatively small amount of Pb (x = 0.05) increased the critical current density, J${}_c$, in the entire magnetic field, which might be attributed to better phase uniformity as well as good grain connectivity.