NEW TERNARY IRON SULFIDES A(3)FE(2)S(4) (A=K, RB, CS) - SYNTHESES AND CRYSTAL-STRUCTURES

The compounds K3Fe2S4, Rb3Fe2S4 and Cs3Fe2S4 were synthesized by reaction of the respective alkali metal carbonate with iron and sulphur in a hydrogen stream charged with sulphur at 930 K. The crystal structures of these mixed-valent compounds were determined by single-crystal X-ray diffraction (space group: Pnma, Z=4). The characteristic structural unit of these sulphides is a (1)(infinity)[FeS4/2] zig-zag chain of edge-sharing, iron-centred sulphur tetrahedra. K3Fe2S4 and Rb3Fe2S4 are isotypic with Na3Fe2S4, whereas the structure of Cs3Fe2S4 shows significant differences. Cs3Fe2S4 is directly converted by water into CsFeS2 via oxidation of the iron ions and the evolution of hydrogen.