Metallic/semiconducting behaviors and an antiferromagnetic ordering of FeBr4−d spins in (Benzo-TTFVS)2·MX4 (M = Fe, Ga; X = Cl, Br)

Abstract

The 2 : 1 salts of a new donor molecule, benzotetrathiafulvalenothioquinone-1,3-dithiolemethide (Benzo-TTFVS) with magnetic FeCl₄⁻ and FeBr₄⁻ ions, and with non-magnetic GaCl₄⁻ and GaBr₄⁻ ions were obtained as single crystals by an electrochemical oxidation method. All of the crystals have very similar structures to each other, in which two different layers of the donor molecules and the counteranions are stacked alternately, although the separation distance between the neighboring donor layers in the FeCl₄⁻ and GaCl₄⁻ salts is shorter by 0.43–0.47 Å than that in the FeBr₄⁻ and GaBr₄⁻ salts. The donor molecules formed a β-like stacking structure with almost the same interplanar distances and effective overlaps along the stacking direction, such that these salts showed high electrical conductivities (9–33 S cm⁻¹) at room temperature, as well as metallic behavior. However, owing to a quasi one-dimensional character of the calculated band structures, a transition of metal-to-semiconductor with a very small activation energy of <30 meV occurred at comparatively high temperatures of 100–150 K. The Fe(III) d spins of the FeCl₄⁻ ions were subject to weakly antiferromagnetic interaction (Weiss temperature, θ = −4.3 K). While, the interaction between the Fe(III) d spins of the FeBr₄⁻ ions was fairly strong (θ = −16.1 K), eventually giving rise to an antiferromagnetic ordering at the temperature of 5.8 K. In the magnetization measurement of the FeBr₄⁻ salt up to 320 kOe at 0.5 K, a spin-flop occurred near 20 kOe and a saturation near 156 kOe.