Comprehensive Optical Investigations of Charge Order in Organic Chain Compounds (TMTTF)₂X

Abstract: Charge ordering in the (TMTTF)₂X salts with centrosymmetric anions (X = PF‾₆ , AsF‾₆ , SbF‾₆ ) leads to a ferroelectric state around 100 K. For the first time and in great completeness, the intra- and intermolecular vibrational modes of (TMTTF)₂X have been investigated by infrared and Raman spectroscopy as a function of temperature and pressure for different polarizations. In this original paper, we explore the development and amount of charge disproportionation and the coupling of the electronic degrees of freedom to the counterions and the underlying lattice. The methyl groups undergo changes with temperature that are crucial for the anion cage formed by them. We find that the coupling of the TMTTF molecules to the hexafluorine anions changes upon cooling and especially at the charge-order transition, indicating a distortion of the anion. Additional features are identified that are caused by the anharmonic potential. The spin-Peierls transition entails additional modifications in the charge distribution. To complete the discussion, we also add the vibrational frequencies and eigenvectors based on ab-initio quantum-chemical calculations.