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)

Abstract

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 (I3) anions showed high conductivity (sigma;rt=100-102 Scm1) and metallic conductive behaviour around room temperature. Among them, the AsF6 salt displayed metallic temperature dependence down to 4.2K, while the SbF6 and TaF6 salts exhibited metal-to-semiconductor transitions at 200 and 140K, respectively. The AsF6 and SbF6 salts have a kappa;-type donor arrangement in which two donor molecules are strongly dimerized in a -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.