Abstract

We present the polarized reflection spectra of (BEDT-ATD)₂X(solvent) (X = AsF₆, PF₆, BF₄; solvent = THF, DHF, DO), where THF is tetrahydrofuran, DHF 2,5-dihydrofuran, and DO 1,3-dioxolane. BEDT-ATD⁺ has an intramolecular excitation in a very low-energy region (4000–5000 cm⁻¹). These isostructural compounds are characterized as strongly correlated quasi-1D metals. The optical spectra in all these compounds suggest that the screw-axis symmetry is broken below the metal–insulator transition temperature. This broken symmetry is confirmed by the X-ray diffraction experiment in (BEDT-ATD)₂BF₄(THF) and (BEDT-ATD)₂PF₆(DHF). The low-temperature structure has the 4k_F-modulated lattice (dimerized structure). The magnetic susceptibility supports this 4k_F modulation as well. We present the low-temperature crystal structures of (BEDT-ATD)₂BF₄(THF) and (BEDT-ATD)₂PF₆(DHF). BF₄⁻ and THF are fully ordered in (BEDT-ATD)₂BF₄(THF) and DHF is almost ordered in (BEDT-ATD)₂PF₆(DHF). The vibronic modes are observed in the reflection spectra of all these compounds even above the metal–insulator transition temperature, which suggests that the screw-axis symmetry is locally broken.