Characterization of the quasi-one-dimensional compounds delta-(EDT-TTF-CONMe2)(2)X, X=AsF6 and Br by vibrational spectroscopy and density functional theory calculations

We have investigated the infrared spectra of the quarter-filled charge-ordered insulators delta-(EDT-TTF-CONMe2)(2)X (X=AsF6, Br) along all three crystallographic directions in the temperature range from 300 to 10 K. DFT-assisted normal mode analysis of the neutral and ionic EDT-TTF-CONMe2 molecule allows us to assign the experimentally observed intramolecular modes and to obtain relevant information on the charge ordering and intramolecular interactions. From frequencies of charge-sensitive vibrations we deduce that the charge-ordered state is already present at room temperature and does not change on cooling, in agreement with previous NMR measurements. The spectra taken along the stacking direction clearly show features of vibrational overtones excited due to the anharmonic electronic molecule potential caused by the large charge disproportionation between the molecular sites. The shift of certain vibrational modes indicates the onset of the structural transition below 200 K. (C) 2014 AIP Publishing LLC.