Superconductivity of 30.4 K and Its Reemergence under Pressure in Fe_1.11Se Synthesized via Ion-Exchange and...

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_1.11Se 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+δSe_1-xS_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.