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 42·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 A. These salts exhibited metallic behavior down to 140–170 K, but at these temperatures (∼TM−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 TM−I. By application of pressures up to 1.0 GPa, the metallic behavior in the higher temperature region was gradually strengthened and TM−I gradually became lower with increasing pressure, but the transitions could not be suppressed at all. In response to the metal–semiconductor transition at TM−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 Jdd = 2.06 K and Jπd = 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·FeBr4 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.