MAGNETIC-INTERACTIONS IN TERNARY COBALT C HALCOGENIDES CONTAINING ISOLATED TETRAHEDRAL COBALT ANIONIC GROUPS - THE SPIN STRUCTURES OF NA6COS(4) AND NA6COSE4

The sodium cobalt chalcogenides Na6CoS4 and Na6CoSe4 are characterized by isolated [CoX(4)]-units. Despite the large distances of more than 6 Angstrom between the cobalt ions magnetic inter-actions at low tempe ratures lead to threedimensionally ordered spin structures, that were determined from neutron diffraction experiments. The magnetic structur e can be described in the Shubnicov group P(2a)bc2(1) with a unit cell that four times as large as the crystallographic cell. The magnetic m oments of both compounds correspond to the value expected for three un paired electrons per Co2+ ion.