NEW TERNARY IRON CHALCOGENIDES A9FE2X7 (A-EQUIVALENT-TO-K, RB, CS X-EQUIVALENT-TO-S, SE) - SYNTHESIS, CRYSTAL-STRUCTURE AND MAGNETIC-PROPERTIES

The ternary iron chalcogenides A9Fe2X7 with A=K, Rb, Cs and X=S, Se were synthesized by reaction of alkali carbonates with iron and the respective chalcogen in a stream of hydrogen. The crystal structures of the potassium and rubidium compounds were determined by single-crystal diffractometer data (spacegroup P2(1)3, Z=4). The atomic arrangement of the isotypic compounds is characterized by isolated trigonal nearly planar [FeX3] and tetrahedral [FeX4] units. The structure corresponds to that of K9Ni2O7. The magnetic susceptibilities of K9Fe2S7 reveal Curie-Weiss behaviour. The moment is due to a high spin state of both Fe3+ and Fe2+. Magnetic ordering occurs below 6 K. Mossbauer experiments prove that the Fe3+ ions must be assigned to the tetrahedral coordination and the Fe2+ ions to the trigonal planar coordination.