Experimental study of the role of intramolecular vibrations in the Peierls transition of organic conductors

The temperature dependence of the infrared spectra (4000-200 cm -1 ) of powder samples of the highly conducting mixed valence TTF compounds (TTF)Cl 0.68 , (TTF)Br 0.76 , (TTF)I 0.71 , (TTF)(SCN) 0.58 and (TTF)(SeCN) 0.58 is investigated. At temperatures below the metal-insulator phase transitions induced by three-dimensional ordering of charge density waves (CDW) the spectra show additional strong absorptions and antiresonance dips. The plots of their intensity against temperature display abrupt changes of slope in correspondence with the transition temperatures and, for the case of (TTF)(SCN) 0.58 , a close resemblance to that reported for the X-ray superlattice intensity. These results are interpreted by applying the phase phonon model to a CDW system displaying a three-dimensional ordering transition. The additional absorptions are assigned to the excitation of phase phonons originating in the coupling between electrons and some intramolecular vibrations of TTF. The antiresonance dips are attributed to Fano-like effects. The relative intensities of the phase phonon absorptions related to totally symmetric vibrational modes compare well with the corresponding electron-molecular-vibration (EMV) coupling constants calculated a priori. The infrared spectra suggest also that the EMV interaction is not limited to totally symmetric modes.