Exchange interactions in (Cr2Cl9)3-systems

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Abstract
The results of ESR studies of single crystals of the type M3Cr2Cl9 are reported for M=Cs and tetraethylammonium (Et4N). In both materials the anions (Cr2Cl9)3- exist as discrete units within the crystal structure. In the caesium compound the Cr3+ pairs are found to be coupled by an antiferromagnetic interaction which can be described by the isotropic form Delta S1S2, with Delta /k=17+or-1K. The corresponding value for the Et4N material is 20+or-2K. The caesium compound shows a spectral averaging effect at high temperatures. This is interpreted in some detail in terms of relatively weak exchange coupling between pairs. The Et4N compound is sufficiently magnetically dilute to show a resolved spectrum at all temperatures. This spectrum is however attributed entirely to the total spin J=2 manifold; no spectrum was detected from the J=1 or 3 manifolds of the pairs.