Magnetic suppression for a possible Fe-poor organic–inorganic hybrid superconductor Fe14Se16(tepa)0.8 (tepa = tetraethylenepentamine) with a superconducting transition at ∼42 K

Composition/structure-dependent superconductivity for FeSe-based superconductors attracted great attention not only due to their high superconducting transition temperatures ( T C ), but also for understanding the origin of iron-based superconductivity. Here, we report a new Fe-poor organic-inorganic hybrid material Fe 14 Se 16 (tepa) 0.8 with a paramagnetic-diamagnetic transition at ∼42 K grown by a high-temperature organic-solution-phase method with soluble iron/selenium sources in a tepa solution, alternative to previous intercalation strategies. The Fe 14 Se 16 (tepa) 0.8 phase is in a tetragonal layered hybrid structure with a nanoplate shape. Composition analyses reveal a Fe-poor characteristic of the hybrid in contrast to previous FeSe-intercalated superconductor, and selected area electron diffraction pattern is featured by Fe 3 Se 4 superstructures with a √2 × √2 of Fe vacancy order. Ab initio density functional calculations show that minus Fe 3 Se 4 ions are stable in the hybrid and ∼0.25e − /Fe 0.75 Se is obviously larger than the reported values of approximately 0.2e − /FeSe in other FeSe-intercalated superconductors. Typical hysteresis loops and temperature dependence of dc/ac susceptibilities of the Fe 14 Se 16 (tepa) 0.8 measured below ∼42 K suggest a presence of the Meissner effect in this material. Effects of synthesis conditions on structures and magnetic properties of the hybrids show a magnetic evolution from a long-range ferrimagnetic (FIM) order of Fe 14 Se 16 (tepa) to a coexistence of FIM and superconducting (SC) orders of Fe 14 Se 16 (tepa) 0.9 and an SC order of Fe 14 Se 16 (tepa) 0.8 . X-ray absorption spectrum (XAS) confirms the presence of ferric/ferrous irons. Mössbauer studies reveal that the high- T C superconductivity originates from a suppression of the FIM order through tuning the spin states of irons from high-spin Fe 3+ ( S = 5/2) and Fe 2+ ( S = 2) in the Fe 14 Se 16 (tepa) to low-spin Fe 3+ ( S = 1/2) and Fe 2+ ( S = 0) in the Fe 14 Se 16 (tepa) 0.8 . Although no zero resistance is detected even at a temperature of 2 K, the resistivity at 2 K decreases by more than 1600 times compared to that in a normal state calculated by a variable range hopping (VRH) model, suggesting that the high- T C superconductivity of Fe 14 Se 16 (tepa) 0.8 is possible.