We present single-crystal x-ray diffraction measurements, optical investigations, and electronic structure calculations for the organic charge-transfer salt ${\beta }^{\prime }$-(BEDT-TTF)${}_2{\mathrm{CF}}_3{\mathrm{CF}}_2{\mathrm{SO}}_3$ 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 ${\nu }_2$ 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 ${\nu }_3$ mode in infrared spectra. At the same time, the sharp redshift of the ${\nu }_1$ 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 ${\mathrm{CF}}_3{\mathrm{CF}}_2{\mathrm{SO}}_3^{-}$ anion layer. These observations point to a subtle interplay of charge and lattice degrees of freedom at the phase transition.

• Received 20 August 2019
• Revised 13 December 2019

DOI:https://doi.org/10.1103/PhysRevB.101.035150