Superconductivity of 30.4 K and its Reemergence under Pressure in Fe1.11Se Synthesized via Ion-exchange and De-intercalation Reaction

Binary stoichiometry FeSe (s-FeSe) is a well-known parent of high-temperature unconventional superconductors owing to its charge-neutral layer, highly tunable structure and electronic properties, and rich interplays among multiple electronic phases. Yet the s-FeSe synthesized via high-temperature equilibrium reactions bears the notorious interstitial Fe,where merely 3% of them is sufficient to kill the superconductivity. Here, we successfully synthesized a new non-stoichiometric Fe1.11Se single crystal with a superconducting onset temperature (Tconset) of 30.4 K through a hydrothermal ion-exchange and de-intercalation route. 11% interstitial Fe ions exceed the equilibrium phase diagram limit. Intriguingly, under physical pressure, the Tconset of exhibits a “V”-shaped evolution with a minimum at 2-2.6 GPa, and then upturning into a second superconducting region, reminiscent of the behaviors in FeSe intercalates. Furthermore, a pressure-induced possible magnetic order, previously only observed in pressurized s-FeSe, shows up. These results offer fresh insights into the role of interstitial Fe in governing superconducting and transport properties under non-equilibrium synthesis and tuning strategies.

💡 Research Summary

The authors report the synthesis of a non‑stoichiometric Fe₁.₁₁Se single crystal by a two‑step hydrothermal ion‑exchange followed by selective de‑intercalation. This route introduces 11 % interstitial Fe (Fe₂) into the van der Waals gap, a concentration far beyond the solubility limit of the Fe–Se binary phase diagram. Single‑crystal X‑ray diffraction shows that Fe₂ occupies the 2c Wyckoff site with random distribution, while Mössbauer spectroscopy confirms its non‑magnetic S = 2 (d⁶) character. Chemical analyses (ICP‑AES, EDX) verify the Fe:Se ratio of ≈1.11:1, and gas‑analysis confirms the complete removal of K⁺/Li⁺ species.

Electrical transport measurements at ambient pressure reveal a sharp superconducting transition with an onset temperature T_c^onset = 30.4 K, nearly four times higher than the 8.5 K of stoichiometric FeSe. The normal‑state resistivity follows a ρ = ρ₀ + AT^α law with α ≈ 1, indicating non‑Fermi‑liquid behavior. Hall effect data show a single‑band, electron‑dominated conduction with a carrier concentration n_e ≈ 1.3 × 10²¹ cm⁻³, comparable to that of FeSe intercalates and far larger than that of s‑FeSe. Magnetization measurements confirm bulk type‑II superconductivity with strong flux pinning.

Under hydrostatic pressure, T_c exhibits a pronounced V‑shaped dependence. From 0 to ~2.5 GPa, T_c decreases to a minimum of ~15 K (SC‑I region). Above 2.5 GPa, T_c rises again, reaching 31 K near 10 GPa (SC‑II region). In the pressure range 5.5–12 GPa, an additional kink appears in the resistivity, identified as a possible stripe‑type antiferromagnetic transition (T_m), reminiscent of the magnetic phase observed in pressurized stoichiometric FeSe. Both T_c and T_m increase together in this high‑pressure regime, suggesting coexistence of superconductivity and magnetism.

Synchrotron X‑ray diffraction under pressure shows no structural transition within the SC‑I and SC‑II zones; only at higher pressures does the lattice transform to a NiAs‑type hexagonal phase, coinciding with the disappearance of both superconductivity and the magnetic anomaly. The unit‑cell volume and c/a ratio decrease smoothly up to ~6 GPa, then deviate from the Birch‑Murnaghan equation, implying the formation of interlayer bonding under pressure.

Density‑functional theory calculations using the experimentally determined lattice parameters reveal that the Fermi surface evolves from a quasi‑2D cylinder at ambient pressure to a more three‑dimensional shape at 10 GPa, as bands along the A–M direction cross the Fermi level. The total density of states at the Fermi energy decreases with pressure, but this reduction does not correlate directly with the observed T_c enhancement, indicating that changes in Fermi‑surface topology and electron‑phonon coupling likely drive the pressure‑induced superconducting behavior.

Comparative analysis with stoichiometric FeSe, KₓFe₂Se₂, (Li₁₋yFe_y)OHFeSe, and Liₓ(NH₃)ᵧFe₂Se₂ shows that Fe₁.₁₁Se achieves a carrier density comparable to high‑T_c intercalates while retaining a simple binary structure. Its V‑shaped pressure phase diagram mirrors that of intercalated compounds, yet the emergence of a magnetic phase at higher pressure aligns it with the behavior of pressurized FeSe. This duality bridges the gap between the two families.

In conclusion, the work demonstrates that a substantial amount of interstitial Fe, introduced via non‑equilibrium synthesis, can act as an effective electron dopant that suppresses nematicity, enhances carrier density, and raises T_c to 30 K. The pressure‑induced re‑emergence of superconductivity and the concomitant magnetic order highlight the delicate interplay between lattice, charge, and spin degrees of freedom in FeSe‑based systems. The findings open avenues for further tuning of interstitial Fe content, combined chemical and physical pressure, and exploration of quantum criticality to achieve even higher superconducting transition temperatures.