Crystal Structure and Magnetism of (BEDT-TTF)2MCl4 (BEDT-TTF = Bis(ethylenedithio)tetrathiafulvalene; M = Ga, Fe)

The relation between crystal structure and bulk magnetic properties is investigated in the molecular charge transfer salts (BEDT-TF) 2 MCl 4 (M = Ga, Fe). (BEDT-TTF) 2 GaCl 4 crystallizes in the triclinic system. Its crystal structure consists of pairs of BEDT-TTF molecules arranged in layers with intermolecular S···S interactions. Band structure calculations predict semimetallic behavior contrary to the semiconductivity observed even under a pressure of 6 kbar (σ(300 K, 1 bar) = 10 - 1 S cm - 1 and E A = 0.2eV). The static (Faraday and SQUID magnetometry) and spin (EPR) susceptibilities indicate low-dimensional Heisenberg antiferromagnetic behavior with the susceptibility tending to zero as the temperature approaches zero. The data are analyzed using several low-dimensional magnetic models and are best fitted to a model consisting of two different spin dimers (Δ 1 = 108 K and Δ 2 = 212 K). The static magnetic susceptibility of (BEDT-TTF) 2 FeCl 4 is modeled by a sum of Curie−Weiss ( S = 5 / 2 for Fe(d 5 ) and ϑ = −4 K), χ tip, and single dimer (Δ = 45 K) parameters. The BEDT-TTF layers in these compounds thus behave as Mott−Hubbard-localized systems, and the interaction between the magnetic moment on the Fe with those on the organic layer is negligible.