Optimization of Synthesis Parameters and Superconducting Properties of GdFeAsO1-xFx

REFeAsO (RE1111; RE: rare earth) belongs to the 1111 family of iron-based superconductors (FBS), which illustrates the enhancement of the superconducting transition (Tc) with smaller radii of RE. However, the synthesis of the 1111 phase with a heavy rare-earth is always challenging. In this paper, we report the optimization of the growth and superconducting properties of F-doped GdFeAsO1-xFx bulks by preparing the samples in a wide temperature range (700-1100{\deg}C) at ambient pressure. The optimized synthesis parameters are concluded based on structural, microstructural, transport, and magnetic measurements. These findings suggest that the optimal conditions for preparing F-doped Gd1111 bulks involve a two-step process at 900{\deg}C for 61 hours at ambient pressure, which is lower than previously reported. The optimized samples have revealed the superconducting transition temperature (Tconset) of 43 K for GdFeAsO0.83F0.17. The first-time reported critical current Jc value for this Gd1111 is observed of the order of 10^3 (A/cm^2) at 0 T and 5 K. Our investigation also concluded that highly pure precursors, particularly gadolinium metal, are required to achieve the superconducting properties of F-doped Gd1111. A high growth pressure of 1 GPa reduces the superconducting properties of F-doped Gd1111.