Cs[FeSe 2 ], Cs 3 [FeSe 2 ] 2 , and Cs 7 [Fe 4 Se 8 ]: 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[FeSe 2 ], Cs 3 [FeSe 2 ] 2 ) or ( B ) with previously prepared FeSe 2 (Cs 3 [FeSe 2 ] 2 , Cs 7 [Fe 4 S 8 ]) ( T max = 800–1000 °C). The pure Fe III ferrate Cs[FeSe 2 ] crystallizes in the Tl[FeSe 2 ] type [monoclinic, space group C 2/ m, a = 1392.95(10), b = 564.43(3), c = 737.44(6) pm, β = 119.163(5)°, Z = 4, R 1 = 0.0550]. It is thus not isotypic to all other alkali ferrates(III) A [FeS 2 ] and A [FeSe 2 ] containing chains of edge‐sharing tetrahedra, but crystallizes in a t 2 subgroup of the Immm structure of Cs[FeS 2 ]. The mixed‐valent chain compound Cs 3 [FeSe 2 ] 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, R 1 = 0.0470]. In contrast to the isopunctal Na 3 [FeSe 2 ] 2 type K/Rb compounds the chains are only slightly corrugated. The monoclinic, likewise mixed‐valent Fe II/III selenido ferrate Cs 7 [Fe 4 Se 8 ] [monoclinic, space group C 2/ c, a = 1953.79(10), b = 879.71(5), c = 1717.03(10) pm, β = 117.890(2)°, Z = 4, R 1 = 0.0816] is isostructural both to the cesium sulfido and tellurido compound. The structure contains oligomeric moieties of four edge sharing [FeSe 4 ] tetrahedra forming slightly distorted tetrahedral clusters [Fe 4 Se 8 ] 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 A 1+ x [Fe III 1– x Fe II x Q 2 ] ( x = 0–1; A = Na, K, Rb, Cs; Q = S, Se, Te).