Electronic magnetic resonance in a series of antiferromagnetic molecular perovskites

(Received 11 December 1992, accepted 27 January 1993)

Abstract
The magnetic properties of a series of ternary tetrathiafulvalenium (TTF ⁺) salts incorporating, within a unique antiperovskite structure, a discrete halide (Y = Cl ^-, Br ^- or I ^-) in addition to an octahedral molybdenum halide cluster dianion Mo ₆X ₁₄^2- (with X = Cl or Br), are described. The preparation and structural chemistry of the materials of general formula (TTF ⁺) ₃ (Mo ₆X ₁₄^2-)(Y ^-) are briefly recalled. The single-crystal electronic magnetic resonance is explored in details using an X-band ESR spectrometer, both in the paramagnetic state where a narrow EPR signal is observed, as well as in the low temperature antiferromagnetic phase where antiferromagnetic resonance (AFMR) is detected. A quantitative discussion of the resonance field and linewidth is given in both regimes. The EPR line characteristics are shown to be strongly sensitive to the TTF ⁺ environment, namely, the nature of the halide on both anion sites and small variations in intermolecular interactions, which reveal the importance of the spin-orbit coupling. On the other hand, AFMR data are discussed in terms of a dominant dipole-dipole interaction as found previously for other TTF-based cation radical salts. Finally, similar albeit contrasted results are obtained with the isomorphous, chalcohalide rhenium cluster dianion-based ternary salt (TTF ⁺) ₃(Re ₆S ₆C ^2-₈)(Cl ^-), which allow for a discussion of the AFMR lineshape.