Metal-semiconductor structural phase transitions and antiferromagnetic orderings in (Benzo-TTFVO)₂·MX₄ (M = Fe, Ga; X = Cl, Br) salts

Crystals of the 2: 1 salts of a new donor molecule, benzotetrathiafulvalenoquinone-1,3-dithiolemethide (4, Benzo-TTFVO) with magnetic FeX₄^- and non-magnetic GaX₄ ^- (X = Cl, Br) ions, 4₂·FeX₄ and 4 ₂·GaX₄, are isostructural to each other and showed a β-type packing of the donor molecules where they form a uniform-stacked structure with an interplanar distance of 3.50 Å. These salts exhibited metallic behavior down to 140-170 K, but at these temperatures (∼T _M-I) an abrupt increase in the resistivities (ρ) occurred and thereafter semiconducting behavior with an activation energy of 40-100 meV was observed. A structural change in the donor column from uniform to tetramer-unit stacks was observed in the 4₂·FeBr₄ crystal before and after T_M-I. By application of pressures up to 1.0 GPa, the metallic behavior in the higher temperature region was gradually strengthened and T_M-I gradually became lower with increasing pressure, but the transitions could not be suppressed at all. In response to the metal-semiconductor transition at T_M-I, there was a sharp decrease in the paramagnetic susceptibility of the π electron system, where the transition from Pauli paramagnetism due to the metal-conducting behavior to the spin singlet state caused by tetramer formation of the donor molecules was observed. In addition, the FeX₄^- (X = Cl, Br) salts showed comparatively strong antiferromagnetic interactions between the Fe(iii) d spins of the FeCl₄^- and FeBr₄^- ions (Weiss temperature: -11 K for 4₂·FeCl₄ and -37 K for 4₂·FeBr₄), giving rise to antiferromagnetic orderings at 1.6 K for 4₂·FeCl₄ and 9.3 K for 4₂·FeBr₄. The magnitudes of the d-d and π-d interactions in 4₂·FeBr₄ are calculated to be J_dd = 2.06 K and J_πd = 2.32 K, respectively. The comparison of these J values with the other magnetic conductors based on our system suggests that the d-d interaction of 4₂·FeBr ₄ is stronger than the π-d interaction. Since the three-dimensional antiferromagnetic ordering appears at the comparatively high temperature of 9.3 K, there is an important contribution of the π electrons to the antiferromagnetic ordering of the Fe(iii) d spins in order to mediate the magnetic interaction between two-dimensional magnetic anion layers.