With single-crystal x-ray diffraction studies, we compare the structures of three samples showing optimal superconductivity: K0.775(4)Fe1.613(1)Se2, K0.737(6)Fe1.631(3)Se2, and Cs0.748(2)Fe1.626(1)Se2. All have an almost identical ordered vacancy structure with a (√5 × √5 × 1) supercell. The tetragonal unit cell, space group I4/m, possesses lattice parameters at 250 K of a = b = 8.729(2) Å and c = 14.120(3) Å, a = b = 8.7186(12) Å and c = 14.0853(19) Å, and at 295 K, a = b = 8.8617(16) Å and c = 15.304(3) Å for the three crystals, respectively. The structure contains two iron sites; one is almost completely empty while the other is fully occupied. There are similarly two alkali metal sites that are occupied in the range of 72.2(2)%–85.3(3)%. The inclusion of alkali metals and the presence of vacancies within the structure allows for considerable relaxation of the FeSe4 tetrahedron, compared with members of the Fe(Te, Se, S) series, and the resulting shift of the Se–Fe–Se bond angles to less distorted geometry could be important in understanding the associated increase in the superconducting transition temperature. The structure of these superconductors are distinguished from the structure of the nonsuperconducting phases by an almost complete absence of Fe on the (0 0.5 0.25) site, as well as lower alkali metal occupancy that ensures an exact Fe2+ oxidation state, which are clearly critical parameters in the promotion of superconductivity.
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