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, 4(2) center dot FeX4 and 4(2) center dot GaX4, are isostructural to each other and showed a beta-type packing of the donor molecules where they form a uniform-stacked structure with an interplanar distance of 3.50 angstrom. These salts exhibited metallic behavior down to 140 - 170 K, but at these temperatures (similar to TM-I) an abrupt increase in the resistivities (rho) 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 4(2) center dot 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 pi 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 4(2) center dot FeCl4 and -37 K for 4(2) center dot FeBr4), giving rise to antiferromagnetic orderings at 1.6 K for 4(2) center dot FeCl4 and 9.3 K for 4(2) center dot FeBr4. The magnitudes of the d - d and pi - d interactions in 4(2) center dot FeBr4 are calculated to be J(dd) = 2.06 K and J(pi d) = 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 4(2) center dot FeBr4 is stronger than the pi - 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 pi electrons to the antiferromagnetic ordering of the Fe( III) d spins in order to mediate the magnetic interaction between two-dimensional magnetic anion layers.