Na7[Fe2S6], Na2[FeS2] and Na2[FeSe2]: New 'reduced' sodium chalcogenido ferrates

Three new 'reduced' Fen containing sodium chalcogenido ferrates were obtained applying a reductive synthetic route. The mixed-valent sulfido ferrate Na-7[Fe2S6], which forms bar-shaped crystals with metallic greenish luster, was synthesized in pure phase from natural pyrite and elemental sodium at a maximum temperature of 800 degrees C. Its centrosymmetric triclinic structure (SG PI, a = 764.15(2), b = 1153.70(2), c = 1272.58(3) pm, alpha = 62.3325(7), beta= 72.8345(8), gamma = 84.6394(8)degrees, Z = 3, R1 = 0.0185) exhibits two crystallographically different [Fe2S6](7-) dimers of edge-sharing [FeS4] tetrahedra, with somewhat larger Fe-S distances than in the fully oxidized Fe-III dimers of e.g. Na-6[(Fe2S6)-S-III]. In contrast to the localized AFM ordered pure di-ferrates(III), the Curie-Weiss behavior of the magnetic susceptibility proves the rarely observed valence-delocalized S = 9/2 state of the mixed-valent Fe-III/Fe-II dimer. The nearly spin-only value of the magnetic moment combined with the chemical bonding not generally differing from that in pure ferrates(II) and (III), provides a striking argument, that the reduction of the local Fe spin moments observed in all condensed sulfido ferrate moieties is connected with the AFM spin ordering. The two isotypic ferrates(II) Na-2[FeS2] and Na-2[FeSe2] with chain-like structural units (SG Ibam, a = 643.54(8)/660.81(1), b = 1140.2(2)/1190.30(2) c = 562.90(6)/585.59(1) pm, Z = 4, R1 = 0.0372/0.0466) crystallize in the K-2[ZnO2]-type structure. Although representing merely further members of the common series of chalcogenido metallates(II) Na-2[M(II)Q(2)], these two new phases, together with Na-6[FeS4] and Li-2[FeS2], are the only examples of pure Fe-II alkali chalcogenido ferrates. The new compounds allow for a general comparison of di- and chain ferrates(II) and (III) and mixed-valent analogs concerning the electronic and magnetic properties (including Heisenberg super-exchange and double-exchange interactions) based on the comprehensive literature and own DFT band structure calculations.