Cs[FeSe₂], Cs₃[FeSe₂]₂, and Cs₇[Fe₄Se₈]: Missing Links of Known Chalcogenido Ferrate Series

The three cesium selenido ferrate title compounds with an Se:Fe ratio of 2:1 were synthesized from stoichiometric samples reacting elemental Cs either (A) with Fe and Se in a double‐crucible setup (Cs[FeSe2], Cs3[FeSe2]2) or (B) with previously prepared FeSe2 (Cs3[FeSe2]2, Cs7[Fe4S8]) (Tmax = 800–1000 °C). The pure FeIII ferrate Cs[FeSe2] crystallizes in the Tl[FeSe2] type [monoclinic, space group C2/m, a = 1392.95(10), b = 564.43(3), c = 737.44(6) pm, β = 119.163(5)°, Z = 4, R1 = 0.0550]. It is thus not isotypic to all other alkali ferrates(III) A[FeS2] and A[FeSe2] containing chains of edge‐sharing tetrahedra, but crystallizes in a t2 subgroup of the Immm structure of Cs[FeS2]. The mixed‐valent chain compound Cs3[FeSe2]2 is isotypic to its sulfido analogue [orthorhombic, space group Pnma, a = 777.88(6), b = 1151.02(6), c = 1341.61(7) pm, Z = 4, R1 = 0.0470]. In contrast to the isopunctal Na3[FeSe2]2 type K/Rb compounds the chains are only slightly corrugated. The monoclinic, likewise mixed‐valent FeII/III selenido ferrate Cs7[Fe4Se8] [monoclinic, space group C2/c, a = 1953.79(10), b = 879.71(5), c = 1717.03(10) pm, β = 117.890(2)°, Z = 4, R1 = 0.0816] is isostructural both to the cesium sulfido and tellurido compound. The structure contains oligomeric moieties of four edge sharing [FeSe4] tetrahedra forming slightly distorted tetrahedral clusters [Fe4Se8]7–, which are surrounded by a cube of 26 Cs cations. Based on a structure map, the crystal chemistry of the three title compounds is discussed together with all chain/cluster ferrates of the general series A1+x[FeIII1–xFeIIxQ2] (x = 0–1; A = Na, K, Rb, Cs; Q = S, Se, Te).