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 (-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 -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 FeSe single crystal with a superconducting onset temperature () 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 of FeSeS 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 -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.