Institute of High Pressure Physics

Experimental data collected for the preparation of the manuscript: "Optimization of superconducting properties of F-doped SmFeAsO by cubic anvil high-pressure technique"

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

Authors: Mohammad Azam, Tatiana Zajarniuk, Hiraku Ogino and Shiv J Singh

Mater. Res. Express 12 (2025) 116001

Abstract:

We report the optimization of the synthesis conditions for SmFeAsO_0.80F_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 ∼10³ A-cm⁻² to ∼10⁴ 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).

Note: In the published article,

• Figure 1 is the block diagram. (No dataset for this figure)
• Figure 2 is the flow chart. (No dataset for this figure)
• Figure 4 is the elemental mapping for the constituent elements. (No dataset for this figure)
• Figure 5 is the scanning electron microscope (SEM) image of SmFeAsO_0.80F_0.20, samples. (No dataset for this figure)

Figure 3. (a) Powder x-ray diffraction (XRD) patterns and (b) An enlarged view of the main (102) diffraction peak for SmFeAsO_0.80F_0.20 samples prepared by the CSP and CA-HP ex-situ process under different pressures. (c) Powder x-ray diffraction (XRD) patterns and (d) An enlarged view of the main (102) diffraction peak for SmFeAsO_0.80F_0.20 samples prepared by the CSP and CA-HP in situ process under various heating temperatures at an applied pressure of 4 GPa.

Figure 6. (a) The variation of normal state resistivity (ρ) with temperature up to room temperature (b) Low-temperature variation of the resistivity up to 60 K for various Sm1111 bulks by the ex situ CA-HP process. The inset image of figure (a) depicts the temperature dependence of the resistivity of the parent P sample up to the room temperature. (c) The variation of normal state resistivity (ρ) with temperature up to room temperature (d) Low-temperature variation of the resistivity up to 60 K for various Sm1111 bulks prepared by the in-situ CA-HP process.

Figure 7. The temperature dependence of the normalized magnetic moment measured in ZFC and FC modes under an applied magnetic field of 20 Oe for Sm1111 bulks prepared by (a) the ex-situ CA-HP process and (b) the in-situ CA-HP process, compared with the parent P sample. The variation of the critical current density (J_c) at a temperature of 5 K with the applied magnetic field up to 9 T for the parent and Sm1111 bulks prepared by (c) the ex-situ CA-HP process and (d) the in-situ CA-HP process.

Figure 8. The variations of (a) the onset transition temperature (T_c^onset), (b) the transition width (ΔT), (c) the room temperature resistivity (ρ_300K), (d) the residual resistivity ratio (RRR = ρ_{300 K} / ρ_{60 K}), and (e) the critical current density (J_c) of SmFeAsO_0.80F_0.20 bulks prepared by the ex-situ CA-HP process under the different growth pressures (0.5 – 4 GPa).

Figure 9. The variations of (a) the onset transition temperature (T_c^onset), (b) the transition width (ΔT), (c) the room temperature resistivity (ρ_{300 K}), (d) the residual resistivity ratio (RRR = ρ_{300 K} / ρ_{60 K}), and (e) the critical current density (J_c) of SmFeAsO_0.80F_0.20 bulks prepared by the in situ CA-HP process under the different growth temperatures (1100 °C–1600 °C) and reaction times.

Figure 10. Magnetic field dependent critical current density (J_c) for the best SmFeAsO_0.80F_0.20 samples prepared by different synthesis techniques: CSP, SPS, HP-HTS (HIP), and both ex-situ and in-situ CA-HP processes. For comparison, SmFeAsO_0.80F_0.20 sample prepared by SPS (45 MPa, 900 °C for 5–10 min) and HP-HTS (0.5 GPa, 900 °C for 1 h) methods are included, along with the data from other groups using CSP method reported by Wang et al [Supercond. Sci. Technol. 23, 055002 (2010)] and Singh et al. [IEEE Transactions On Applied Superconductivity 23, 7300605 (2013)].