Abstract
Composition/structure-dependent superconductivity for FeSe-based superconductors attracted great at-tention not only due to their high superconducting transition temperatures(Tc),but also for under-standing the origin of iron-based superconductivity.Here,we report a new Fe-poor organic-inorganic hybrid material Fe14Se16(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,alter-native to previous intercalation strategies.The Fe14Se16(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 Fe3Se4 superstructures with a √2 × √2 of Fe vacancy order.Ab initio density functional cal-culations show that minus Fe3Se4 ions are stable in the hybrid and～0.25e-/Fe0.7sSe is obviously larger than the reported values of approximately 0.2e-/FeSe in other FeSe-intercalated superconductors.Typ-ical hysteresis loops and temperature dependence of dc/ac susceptibilities of the Fe14Se16(tepa)0.8 mea-sured below～42 K suggest a presence of the Meissner effect in this material.Effects of synthesis condi-tions on structures and magnetic properties of the hybrids show a magnetic evolution from a long-range ferrimagnetic(FIM)order of Fe14Se16(tepa)to a coexistence of FIM and superconducting(SC)orders of Fe14Se16(tepa)0.9 and an SC order of Fe14Se16(tepa)0.8.X-ray absorption spectrum(XAS)confirms the pres-ence of ferric/ferrous irons.Mössbauer studies reveal that the high-Tc superconductivity originates from a suppression of the FIM order through tuning the spin states of irons from high-spin Fe3+(S=5/2)and Fe2+(S=2)in the Fe14Se16(tepa)to low-spin Fe3+(S=1/2)and Fe2+(S=0)in the Fe14Se16(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-Tc superconductivity of Fe14Se16(tepa)0.8 is possible.
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