Crystal structure and physical properties of (BEDT-TTF)2BF4Sol0.5 (Sol = 1,1,2-trichloroethane, 1,2-dibromoethane, 1,2-dichroloethane)

The crystal structures and phase transition nature of (BEDT-TTF)(2)BF(4)Sol(0.5) (Sol = TCE, DBE, and DCE) are investigated by means of X-ray diffraction, electrical conductivity, ESR, and magnetic susceptibility measurements. As the size of the neutral molecules become smaller (TCE > DBE > DCE), the metal-insulator transition temperature rises (60 K < 100 K < 110 K), showing that their transitions are governed mainly by the structural degrees of freedom. For the DBE and DCE salts, these transitions are simple metal-insulator transitions, whereas the transition of the TCE salt occurs only when the sample is cooled slowly, and the Fermi surfaces survive in the resulting low-temperature phase. For this salt, the anomaly of the interplanar resistivity appears at ca. 160 K for the slow cooling process, and the difference of the ESR line width between the slow and rapid cooling processes is also observed below this temperature. These phenomena are due to the disorder enhanced by the generation of the in-plane superlattice.