  With single crystal X-ray diffraction studies, we compare the structures of
three sample showing optimal superconductivity, K0.774(4)Fe1.613(2)Se2,
K0.738(6)Fe1.631(3)Se2 and Cs0.748(2)Fe1.626(1)Se2. All have an almost
identical ordered vacancy structure with a ({\sqrt}5 x {\sqrt}5 x 1) super
cell. The tetragonal unit cell, space group I4/m, possesses lattice parameters
at 250K of a = b = 8.729(2) {\AA} and c = 14.120(3) {\AA}, a = b = 8.7186(12)
{\AA} and c = 14.0853(19) {\AA} and at 295 K, a = b = 8.8617(16) {\AA} and c =
15.304(3) {\AA} for the three crystals, respectively. The structure contains
two iron sites; one is almost completely empty, whilst the other is fully
occupied. There are similarly two alkali metal sites that are occupied in the
range of 72.2(2) % to 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 - F - 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
distinguishes themselves from the structure of the non-superconducting 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.
