Crystal Structure and Physical Properties of Conducting Molecular Antiferromagnets with a Halogen-Substituted Donor:  (EDO-TTFBr₂)₂FeX₄ (X = Cl, Br)

The crystal structure and physical properties of radical ion salts (EDO-TTFBr₂)₂FeX₄ (X = Cl, Br) based on halogen-substituted organic donor and magnetic anions are investigated, including the comparison with the isomorphous compounds (EDO-TTFBr₂)₂GaX₄ with nonmagnetic anions. The crystal structure of these four salts consists of uniformly stacked donor molecules and tetrahedral counter anions, and the Br substituents of the donor molecules are connected to halide ligands of anions with remarkably short intermolecular atomic distances. These salts show metallic behavior around room temperature and undergo a spin-density-wave transition in the low-temperature range, as confirmed with the divergence of the electron spin resonance (ESR) line width. Although close anion−anion contacts are absent in these salts, the FeCl₄ salt undergoes an antiferromagnetic transition at T_N = 4.2 K, and the FeBr₄ salt shows successive magnetic transitions at T_N = 13.5 K and T_C2 = 8.5 K with a helical spin structure as a candidate for the ground state of the d-electron spins. The magnetoresistance of the FeCl₄ salt shows stepwise anomalies, which are explained qualitatively using a π−d interaction-based frustrated spin system model composed of the donor π-electron and the anion d-electron spins. Although on the ESR spectra of the FeX₄ salts signals from the π- and d-electron spins are separately observed, the line width of the π-electron spins broadens under the temperature where the susceptibility deviates from the Curie−Weiss behavior, showing the presence of the π−d interaction.