Distorted FeSe4 unit in ammonium ion intercalated FeSe superconductor.

• The correlation between structures and the T c in FeSe-family is presented for the first time. • The distorted FeSe 4 tetrahedron plays an important role in understanding the superconductivity. • The electronic correlation and FS nesting between electron pockets are associated with distorted FeSe 4 tetrahedron. • The crystal structure of N 0.81(1) D 3.24(2) Fe 2 Se 2 is determined by neutron and x-ray powder diffraction. We demonstrate that unique cation, ND 4 (1−δ)+, exists between FeSe layers and induces a large distortion of the FeSe 4 tetrahedron in N 0.81(1) D 3.24(2) Fe 2 Se 2 by neutron and X-ray powder diffraction. The electronic correlations of the Fe 3d electrons and Fe d xy , d yz and d xz band widths could be tuned by changing the FeSe 4 tetrahedron distortion, revealing the importance of lattice distortion to the superconductivity of FeSe-based intercalates. [ABSTRACT FROM AUTHOR]