High-temperature superconductivity (TC = 42 K) of Fe1.2[Fe(tepa)]0.8(β-Fe3Se4)4

Abstract Discovering new superconducting (SC) system, in particular with a high SC transition temperature (TC), helps for a better understanding of mechanism of high TC superconductors. Here we report a solution synthesis of a mixed valent hybrid superconductor, Fe1.2[Fe(tepa)]0.8(β-Fe3Se4)4 (tepa = tetraethylenepentamine), which exhibits superconductivity at TC ~42 K. This superconductor shows a √2×√2 superstructure the same as that of β-Fe3Se4. Besides ordered Fe vacancies, other iron sites in this structure are fully occupied by divalent and ferric irons as determined by X-ray absorption and Mössbauer spectra. The structure of Fe1.2[Fe(tepa)]0.8(β-Fe3Se4)4 is distinguished from the structures of previously reported FeSe-based superconductors with all Fe in the +2 oxidation state that was critical parameter in the promotion of superconductivity. We found that the SC order of this superconductor originates from a suppression of the long-range ferrimagnetic (FIM) order of the parent phase, [Fe(tepa)](β-Fe2Se3)4, through simultaneously tuning the host and the spacer layers.