New ternary iron chalcogenides A 9Fe 2X 7 (A  K, Rb, Cs; X  S, Se): synthesis, crystal structure and magnetic properties

Abstract

The ternary iron chalcogenides A 9Fe 2X 7 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 13, Z = 4). The atomic arrangement of the isotypic compounds is characterized by isolated trigonal nearly planar [FeX 3] and tetrahedral [FeX 4] units. The structure corresponds to that of K 9Ni 2O 7. The magnetic susceptibilities of K 9Fe 2S 7 reveal Curie-Weiss behaviour. The moment is due to a high spin state of both Fe 3+ and Fe 2+. Magnetic ordering occurs below 6 K. Mössbauer experiments prove that the Fe 3+ ions must be assigned to the tetrahedral coordination and the Fe 2+ ions to the trigonal planar coordination.