Optimization of superconducting properties of F-doped SmFeAsO by cubic anvil high-pressure technique

Abstract We report the optimization of the synthesis conditions for SmFeAsO 0.80 F 0.20 (Sm1111) bulk superconductor using a cubic-anvil high-pressure (CA-HP) technique through both ex situ and in situ processes, under pressures up to 4 GPa and heating temperatures up to 1600 °C. A comprehensive characterization has been performed, including structural, microstructural, transport, and magnetic measurements. Our findings indicate that a modest growth pressure of approximately 0.5 GPa is sufficient for the formation of the Sm1111 phase via the ex situ process. In contrast, the in situ process requires higher synthesis pressure (4 GPa) and temperature (1400 °C for 1 h) to achieve the Sm1111 phase with enhanced superconducting properties. Notably, the optimized in situ process significantly reduces the reaction time needed for the formation of the Sm1111 phase compared to conventional synthesis process at ambient pressure (CSP), leading to an increase in the transition temperature by 3 K and improvements in the critical current density ( J c ). Conversely, the optimized ex situ process maintains an onset transition temperature ( T c ) of approximately 53 K, similar to that of CSP, while enhancing the J c from ∼10 3 A cm −2 to ∼10 4 A cm −2 at 5 K. Despite these improvements, small amounts of impurity phases, as observed in CSP samples, persists in all Sm1111 samples prepared through either the in situ or ex situ CA-HP processes. These results suggest that the in situ process under optimized conditions (1400 °C, 4 GPa for 1 h) can effectively improve the superconducting properties of Sm1111, resulting an increased T c to 56 K. Furthermore, a comprehensive comparison with high gas pressure techniques, spark plasma sintering, and CSP methods suggests that the formation of small amounts of impurity phases in Sm1111 cannot be completely eliminated by various pressure techniques, even under the applied pressures up to 4 GPa. These results provide valuable insights for both fundamental studies and applied research, contributing to the further advancement of iron-based superconductors (FBS).