Superconductivity of 30.4 K and Its Reemergence under Pressure in Fe 1.11 Se Synthesized via Ion-Exchange and Deintercalation 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 interplay 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 nonstoichiometric Fe 1.11 Se single crystal with a superconducting onset temperature ( T c onset ) of 30.4 K through a hydrothermal ion-exchange and deintercalation route. 11% interstitial Fe ions exceed the equilibrium phase diagram limit. Intriguingly, under physical pressure, the T c onset of Fe 1+δ Se 1– x S x 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 observed only in pressurized s -FeSe, shows up. These results offer fresh insights into the role of interstitial Fe in governing superconducting and transport properties under nonequilibrium synthesis and tuning strategies.