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

The compounds K3Fe2S4, Rb3Fe2S4 and Cs3Fe2S4 were synthesized by react ion of the respective alkali metal carbonate with iron and sulphur in a hydrogen stream charged with sulphur at 930 K. The crystal structure s of these mixed-valent compounds were determined by single-crystal X- ray diffraction (space group: Pnma, Z=4). The characteristic structura l unit of these sulphides is a (1)(infinity)[FeS4/2] zig-zag chain of edge-sharing, iron-centred sulphur tetrahedra. K3Fe2S4 and Rb3Fe2S4 ar e isotypic with Na3Fe2S4, whereas the structure of Cs3Fe2S4 shows sign ificant differences. Cs3Fe2S4 is directly converted by water into CsFe S2 via oxidation of the iron ions and the evolution of hydrogen.