Synthesis, Crystal and Electronic Structure of the New Sulfido Ferrate Cs7[FeS2]2[Fe2S3]2

Abstract The intermediate‐valent sulfido ferrate Cs 7 [FeS 2 ] 2 [Fe 2 S 3 ] 2 , which forms needles with metallic greenish lustre, was synthesized in pure phase from Cs 2 S 2 , elemental iron and sulfur at a maximum temperature of 1000 °C in an argon atmosphere. The monoclinic structure [space group C2 / m , a = 3083.8(2), b = 559.03(3), c = 761.57(4) pm, β = 95.829(4)°, Z = 2, R 1 = 0.0715] represents a new structure type. It contains two different type of chain anions running along the monoclinic b direction, one of them single chains [Fe II/III S 4/2 ] 1.5– of [FeS 4 ] tetrahedra connected by opposite edges, which are similarly found as sole anions in mixed‐valent chain ferrates(II/III) like e.g. Cs 3 [Fe 2 S 4 ]. The second sulfido ferrate ion, double chains [Fe II 2 S 3 ] 2– of two fused single chains, is comparable to the building blocks in the structure of the intermediate‐valent mineral pautovite, Cs[Fe II, III 2 S 3 ], and the ferrate(II) Ba[Fe II 2 S 3 ]. Corrected for the local disorder of two cesium sites, the Cs cations separating the two chain ferrate ions are coordinated by 7 to 8 sulfur atoms. The assignment of the iron charges in the two different chain anions is consistent, regardless whether geometric features like Fe–S and Fe–Fe distances (as compared with related compounds) or lattice energy calculations are taken into account. It is further supported by the Fe pDOS and the Bader charges obtained from an FP‐LAPW spin‐polarized GGA+ U bandstructure calculation of the title compound.