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.
Highlights
The physics of the one-dimensional organic compounds (TMTC F)2X is an active research topic for already three decades [1,2,3,4,5,6,7,8]
We would like to point out that our local probe of the C=C bonds is a direct measure of the charge per temperature-dependent resistivity of (TMTTF) molecule; the plots in Figure 8 and Figure 12 give the temperature dependence of the charge-order amplitude, which can be considered as the order parameter of this phase
First let us analyze the symmetry of the unit cell and vibrational modes of the (TMTTF)2X salts crystallized in the triclinic P1 space
Summary
Except (TMTTF)2ClO4, all Fabre salts develop a charge-ordered phase below T < TCO, as can be seen by a kink in ρ(T ) [Figure 2b] [7,37,38], nuclear magnetic resonance (NMR) [10,11,15,39], electron spin resonance (ESR) [35,40], dielectric [18,20,22], and optical measurements [41,42,43,44,45]. Charge order is supposed to be driven by the effective Coulomb repulsion V between neighboring molecules with respect to the bandwidth W = 2t Since both parameters depend on the intersite distance, the charge-order transition temperature TCO can be tuned by applying physical [13,49,50,51,52,53] as well as by chemical pressure [7,37]. We observe the low-frequency intermolecular vibrational modes of the TMTTF molecules that are seen in the Raman spectra and become infrared active due to charge disproportionation
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