A series of FeSe_0.5Te_0.5 bulk samples have been prepared using the high gas pressure and high-temperature synthesis (HP-HTS) method to optimize the growth conditions for the first time and investigated for their superconducting properties using structural, microstructure, transport, and magnetic measurements to reach the final conclusions. Ex situ and in situ processes are used to prepare bulk samples under a range of growth pressures using Ta-tube and without Ta-tube. The parent compound synthesized by convenient synthesis method at ambient pressure (CSP) exhibits a superconducting transition temperature of 14.8 K. Our data demonstrate that the prepared FeSe_0.5Te_0.5 sealed in a Ta-tube is of better quality than the samples without a Ta-tube, and the optimum growth conditions (500 MPa, 600 °C for 1 h) are favorable for the development of the tetragonal FeSe_0.5Te_0.5 phase. The optimum bulk FeSe_0.5Te_0.5 depicts a higher transition temperature of 17.3 K and a high critical current density of the order of >10⁴ A/cm² at 0 T, which is improved over the entire magnetic field range and almost twice higher than the parent compound prepared using CSP. Our studies confirm that the high-pressure synthesis method is a highly efficient way to improve the superconducting transition, grain connectivity, sample density, and pinning properties of a superconductor.

The high-pressure growth technique generally plays an important role in the improvement of sample quality and the enhancement of various physical and magnetic properties of materials. The high gas pressure technique provides a large sample space (10–15 cm) to grow various kinds of materials. In this paper, we introduce the high gas pressure and high-temperature synthesis (HP-HTS) technique that is present at our institute and is applied to the growth process of different kinds of superconducting materials, particularly iron-based superconductors. More details and the working principle of this HP-HTS technique are discussed. We have also demonstrated the current results based on iron-based superconductors by using this unique HP-HTS technique. These results demonstrate the enhancement of the superconducting properties with improved sample quality compared to the conventional synthesis process at ambient pressure.

High-pressure growth is a unique method to improve the sample quality and size. Here, we have used the high gas pressure and high-temperature synthesis (HP-HTS) method to grow CaKFe ₄ As ₄ (1144) bulks and investigated their superconducting properties using structural, microstructural, transport, and magnetic studies. The microstructural analysis demonstrates that 1144 samples prepared by HP-HTS have improved the sample density and grain connectivity. The transition temperature $({{T}_c^{onset}})$ of 1144 bulks prepared by HP-HTS is increased up to 35.2 K with a transition width ( ΔT ) of 1 K, which is remarkably comparable to the reported 1144 single crystal. Additionally, the critical current density ( J_c ) is enhanced by almost one order of magnitude compared with the parent compound prepared by the conventional synthesis process at ambient pressure (CSP), which could be attributed to the improved sample density and effective pinning centers. Our study demonstrates that the sample quality and superconducting properties of various iron-based superconductors can be enhanced by applying the HP-HTS approach, and further research is demanded in this direction.

The enhanced performance of superconducting FeSe_0.5Te_0.5 materials with added micro-sized Pb and Sn particles is presented. A series of Pb- and Sn-added FeSe_0.5Te_0.5 (FeSe_0.5Te_0.5 + xPb + ySn; x = y = 0–0.1) bulks are fabricated by the solid-state reaction method and characterized through various measurements. A very small amount of Sn and Pb additions (x = y ≤ 0.02) enhance the transition temperature (T_c^onset) of pure FeSe_0.5Te_0.5 by ~1 K, sharpening the superconducting transition and improving the metallic nature in the normal state, whereas larger metal additions (x = y ≥ 0.03) reduce T_c^onset by broadening the superconducting transition. Microstructural analysis and transport studies suggest that at x = y > 0.02, Pb and Sn additions enhance the impurity phases, reduce the coupling between grains, and suppress the superconducting percolation, leading to a broad transition. FeSe_0.5Te_0.5 samples with 2 wt% of cometal additions show the best performance with their critical current density, J_c, and the pinning force, F_p, which might be attributable to providing effective flux pinning centres. Our study shows that the inclusion of a relatively small amount of Pb and Sn (x = y ≤ 0.02) works effectively for the enhancement of superconducting properties with an improvement of intergrain connections as well as better phase uniformity.

We report the synthesis and characterization of a series of antimony-doped SmFeAs _{1- x} Sb _x O _0.8 F _0.2 ( x = 0, 0.01, 0.03, 0.05, 0.1, 0.2, and 0.3) bulks to investigate the twin doping effects on the superconducting properties of SmFeAs(O,F) caused by fluorine (F) incorporation at O-site in SmO layer and antimony (Sb) substitution at As-site in the conducting layer (FeAs). Since the antimony (Sb) has a larger size than arsenic (As), the enhancement of lattice parameters has been confirmed by the XRD analysis. Microstructural analysis confirms that Sb-doping leads to a small improvement in the sample density and an increase in the inhomogeneity of the constituent elements, especially at higher Sb-doping levels. The parent compound SmFeAsO _0.8 F _0.2 has shown the superconducting transition ( T_c ) at ∼54 K, which is systematically reduced with the antimony doping contents ( x ). Our investigation indicates that the Sb-doped SmFeAs(O,F) phase at low levels is less prone to the multiphase formation than at high levels, which affects the inter- and intragranular behaviour originating from the microstructure nature of 1111 bulks. The critical current density ( J_c ) of the parent compound has almost the same value as previously reported, which is suppressed slowly with increased Sb-doping. It could be due to the reduced grain connections and the effective pinning centers. This study confirms that the superconducting FeAs layer doping with larger ions at arsenic sites does not support the superconducting properties of Sm1111, which is a distinct behavior from that of Sb-doped CeFeAs(O,F) and LaFeAs(O,F).

RE FeAsO ( RE 1111; RE : rare earth) belongs to the 1111 family of iron-based superconductors (FBS), which illustrates the enhancement of the superconducting transition ( T_c ) 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 GdFeAsO _{1- x} F _x bulks by preparing the samples in a wide temperature range (700–1100 °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 °C for 61 hours at ambient pressure, which is lower than previously reported. The optimized samples have revealed the superconducting transition temperature ( T_c^onset ) of 43 K for GdFeAsO _0.83 F _0.17 . The first-time reported critical current J_c 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.