MAGNETOCHEMISTRY OF SOME IRON(III) COMPOUNDS

The magnetic properties of some of the compounds formed by the iron(III) ion are reviewed. The paramagnetism of the 6A1 state is straightforward and well-understood. The number of superexchange-coupled, ordered compounds of this ion is relatively small. Features common to most of these are the occurrence of small zero-field splittings and exchange constants, which result in low magnetic transition tempertures. As a specific example from a recent investigation, the compound of stoichiometry (4-chloropyridinium)3Fe2Cl9 is treated in detail. Structural analysis shows that the iron is present as the FeCl4 ion, and so the material should be formulated as [(4-chloropyridinium)FeCl4]2 . 4-chloropyridinium chloride. Magnetic measurements on single crystals show that the S = 5/2 material orders as a canted antiferromagnet at 2.725 K. Replacement of the 4-chloro cation by the larger 4-bromo analog shifts the transition temperature to 2.34 K. The phase diagrams in the H-T plane of both salts have also been determined and a variety of critical parameters are presented. These compounds behave approximately as S = 5/2, Heisenberg simple-cubic antiferromagnets.