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 Fe1.11Se single crystal with a superconducting onset temperature (Tconset) 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 Tconset of Fe1+δSe1-xSx 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.
Yang et al. (Wed,) studied this question.