Novel BEDT-TTF salts containing rare earth ions, (ET)4Ln(NCS)6·CH2Cl2

We have prepared single crystals of a novel BEDT-TTF salts. (ET)4Ln(NCS)6·CH2Cl2, where ET is an abbreviation of BEDT-TTF, bis(ethylenedithio)tetrathiafulvalene and En is a heavy rare earth element, Ho, Er, Yb and Y. They are semiconductors below room temperature. From the crystal structure and the magnetism of the Y compound, it is concluded that the π electrons form a low dimensional spin system, as a result of charge separation and Mott localization. Characteristics of the 4ƒ electrons appears as remarkable anisotropy and temperature dependence of the magnetic susceptibility of the Ho, Er and Yb compounds.