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

Abstract

Charge ordering in the (TMTTF) 2 X 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) 2 X 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.

Light-Induced Current Oscillations in the Charge-Ordered State of (TMTTF)2SbF6

“…Due to charge order, the dielectric constant exhibits a maximum at this temperature (see panel (c)), more or less following Curie's law [39]. As presented in Figure 2, the charge-sensitive vibrational modes split due to charge disproportionation [7]; similar conclusions are drawn from the NMR spectra [40] plotted in Figure 1…”

“…During the last years several comprehensive investigations have been conducted on the optical [5][6][7][8][9] and transport properties [10], the lattice [11,12] and electronic structure [13][14][15] as well as on the magnetic properties [16][17][18][19][20][21]. However, not much is known about the non-linear properties at the charge-order transition taking place at T CO = 157 K and transient conduction phenomena.…”

“…No significant change is observed in the magnetic susceptibility at T CO [17] plotted in Figure 1 TaF − 6 as a function of temperature T recorded along the c axis. The charge imbalance 2δ is estimated from the difference of the two resonance frequencies by 2δ = ∆ν/(80 cm −1 /e) [7,41]. Figure 3.…”

Infrared Investigations of the Neutral-Ionic Phase Transition in TTF-CA and Its Dynamics

“…Below the transition, it saturates at a fixed value below 20 K. This is originally caused by the thermal contraction of the unit cell enhancing the charge transfer. It is interesting to note that this behavior is in strong contrast to the behavior of the TMTTF-salts [48] where the transition of the charge ordered state evolves continuously indicated by the charge imbalance. In this way, it resembles the behavior observed at the charge-order phase transition of α-(BEDT-TTF) 2 I 3 where a strong involvement of the lattice was shown recently [61][62][63][64].…”

“…In the neutral state, the structure of TTF is bent, i.e., the C-H bonds point out of the molecule plane; similar observations are made for other common charge-transfer salts, such as TMTTF and BEDT-TTF [48]. For the calculation, we assumed therefore that the point group of TTF is C 2v .…”