Preparation, structures and physical properties of κ-type two-dimensional conductors based on unsymmetrical extended tetrathiafulvalene:: 2-cyclopentanylidene-1,3-dithiolo[4,5-d]-4,5-ethylenedithiotetrathiafulvalene (CPDTET)

Several cation radical salts of an unsymmetrical donor, 2-cyclopentanylidene-1,3-dithiolo [4,5-d]-4',5'-ethylenedithiotetrathiafulvalene (CPDTET) have been prepared. Most salts with octahedral (AsF6-, SbF6-, NbF6- and TaF6-) and linear (I-3(-)) anions showed high conductivity (sigma(rt) = 10(0)-10(2) S cm(-1)) and metallic conductive behaviour around room temperature. Among them, the AsF6- salt displayed metallic temperature dependence down to 4.2 K, while the SbF6- and TaF6- salts exhibited metal-to-semiconductor transitions at 200 and 140 K, respectively. The AsF6- and SbF6- salts have a kappa-type donor arrangement in which two donor molecules are strongly dimerized in a 'head-to-head' manner. 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 almost half-filled band structure at room temperature and a small energy gap opens at low temperature. The measurements of magnetic susceptibility indicate that these two salts exhibited Pauli-paramagnetic temperature independence characteristic of metallic materials, though they also showed weak antiferromagnetic interaction. The differences between AsF6- and SbF6- salts are also discussed and the origin of the metal-to-semiconductor transition is clarified.