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

The intermediate-valent sulfido ferrate Cs-7[FeS2](2)[Fe2S3](2), which forms needles with metallic greenish lustre, was synthesized in pure phase from Cs2S2, elemental iron and sulfur at a maximum temperature of 1000 degrees 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, beta = 95.829(4)degrees, 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 [(FeS4/2)-S-II/III](1.5-) of [FeS4] tetrahedra connected by opposite edges, which are similarly found as sole anions in mixed-valent chain ferrates(II/III) like e.g. Cs-3[Fe2S4]. The second sulfido ferrate ion, double chains [(Fe2S3)-S-II](2-) of two fused single chains, is comparable to the building blocks in the structure of the intermediate-valent mineral pautovite, Cs[(Fe2S3)-S-II,III], and the ferrate(II) Ba[(Fe2S3)-S-II]. 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.