Microreflectance Infrared Study of Bis(Ethylenedithio)-Tetrathiafulvalene (BEDT-TTF or ET) Salts

For several β- and κ-phase salts of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or simply ET) and its deuterium analog d8-ET, microreflectance infrared spectra were obtained by employing polarized and unpolarized light, and their vibronic regions were examined. These salts exhibit a strong vibronic absorption under polarized light. The vibronic absorption of the β-phase salts has a much stronger polarization-dependency than that found for the κ-phase salts. For the β-phase salts, the optimum vibronic absorption occurs when the polarized light vector is parallel to their donor-molecule stacking direction. Among the ET salts without structural disorder, the highest C-C-H bending frequency for the superconductors is lower than ∼1320 cm−1 while that for the non-superconductors is higher than ∼1320 cm−1.