Charge ordering and low-temperature lattice distortion in the β′-(BEDT-TTF)2CF3CF2SO3 dimer Mott insulator

We present single-crystal x-ray diffraction measurements, optical investigations, and electronic structure calculations for the organic charge-transfer salt β′-(BEDT-TTF)2CF3CF2SO3 synthesized by electrocrystallization. Electronic structure calculations confirm the quasi-one-dimensional behavior of the compound and optical conductivity measurements reveal the dimer-Mott insulating nature of the system. The splitting of the charge-sensitive ν2 mode in Raman spectra demonstrates the onset of an interlayer charge-ordered phase below 25 K, also suggested by the crystal structure considerations. This transition is accompanied by clear signatures of a lattice distortion in the BEDT-TTF donor layer, as shown by a splitting of the vibrational ν3 mode in infrared spectra. At the same time, the sharp redshift of the ν1 mode involving the BEDT-TTF ethylene groups strongly suggests a significant modification of the hydrogen-type bonding present between the BEDT-TTF donor layer and the CF3CF2SO3− anion layer. These observations point to a subtle interplay of charge and lattice degrees of freedom at the phase transition.