Absence of superconductivity in a new FeS-based compound (FeOOH)FeS

We successfully synthesized the tetragonal FeS-based compound (FeOOH)FeS through a hydrothermal method and optimized the ratio of the starting reaction materials Fe and Na₂S·9H₂O. Magnetic susceptibility and specific heat measurements revealed that (FeOOH)FeS undergoes a ferromagnetic transition at 15 K, whereas the peak observed at approximately 10 K in the magnetic susceptibility curve is closely related to the system entering a superparamagnetic state. Importantly, on the basis of the specific heat results, the Debye temperature and average phonon velocity in (FeOOH)FeS are calculated to be 108.2 K and 1026.3 m·s⁻¹, respectively. We suggest that the absence of superconductivity in (FeOOH)FeS may not be linked to the Fe vacancy in the FeS layer, offering a new explanation for the absence of superconductivity in most FeS-based compounds.