PREPARATION, STRUCTURES AND PHYSICAL-PROPERTIES OF KAPPA-TYPE 2-DIMENSIONAL CONDUCTORS BASED ON UNSYMMETRICAL EXTENDED TETRATHIAFULVALENE -THIOLO[4,5-D]-4,5-ETHYLENEDITHIOTETRATHIAFULVALENE (CPDTET)

Several cation radical salts of an unsymmetrical donor, 2-cyclopentany lidene-1,3-dithiolo [4,5-d]-4',5'-ethylenedithiotetrathiafulvalene (CP DTET) have been prepared. Most salts with octahedral (AsF6-, SbF6-, Nb F6- and TaF6-) and linear (I-3(-)) anions showed high conductivity (si gma(rt) = 10(0)-10(2) S cm(-1)) and metallic conductive behaviour arou nd room temperature. Among them, the AsF6- salt displayed metallic tem perature dependence down to 4.2 K, while the SbF6- and TaF6- salts exh ibited metal-to-semiconductor transitions at 200 and 140 K, respective ly. The AsF6- and SbF6- salts have a kappa-type donor arrangement in w hich two donor molecules are strongly dimerized in a 'head-to-head' ma nner. The calculated Fermi surfaces of these salts are two-dimensional folded circles. Measurement of the thermoelectric power suggests that the AsF6- salt is a normal metal, however, the SbF6- salt has an almo st half-filled band structure at room temperature and a small energy g ap opens at low temperature. The measurements of magnetic susceptibili ty indicate that these two salts exhibited Pauli-paramagnetic temperat ure independence characteristic of metallic materials, though they als o showed weak antiferromagnetic interaction. The differences between A sF6- and SbF6- salts are also discussed and the origin of the metal-to -semiconductor transition is clarified.