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

Abstract

We have investigated the infrared spectra of the quarter-filled charge-ordered insulators δ-(EDT-TTF-CONMe2)2X (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.

Relaxation dynamics in the one-dimensional organic charge-transfer salt δ−(EDT−TTF−CONMe2)2Br

“…Previous work on -Br has shown that it exhibits a charge-ordered Mott insulating state at room temperature and ambient pressure [26,27,28]. Upon cooling, at Ts = 190 K a structural orthorhombic-to-monoclinic phase transition occurs.…”

“…These stacks are adjacent along the b axis and separated by Br anions in the c direction, leading to the formation of planes of EDT-TTF-CONMe2 molecules perpendicular to the a axis. Within these planes, a CO pattern was also found along the b direction [26,28]. It should be noted, however, that the interstack distance is significantly larger than the intermolecular distance within a single stack making the material quasi-one-dimensional.…”

“…In the present work, we investigate the organic chargetransfer salt -(EDT-TTF-CONMe2)2Br (-Br), where EDT-TTF-CONMe2 [4,5-ethylenedithio-4'-(N,N-dimethylcarbamoyl)tetrathiafulvalene] represents the donor-molecule and Br is the counter anion. Since this compound exhibits CO [26,27,28], which exists already at room temperature, and as its space group was reported to be compatible with ferroelectricity [26], at first glance it may be considered a candidate for showing electronic ferroelectricity. However, -Br lacks dimerization [26] and, thus, the special ferroelectricity mechanism related to intra-dimer degrees of freedom that was considered for the mentioned compounds -Cl, -Hg, and -(ET)2I3 cannot play a decisive role in this material.…”

“…It should be noted, however, that the interstack distance is significantly larger than the intermolecular distance within a single stack making the material quasi-one-dimensional. A rather high charge disproportionation ratio for two neighboring molecules of about 9:1 was detected [26,27,28]. The lack of dimerization makes -Br a prototypical example of a one-dimensional Mott insulator with quarterfilled conduction band [26,27,28,37,41].…”

“…A rather high charge disproportionation ratio for two neighboring molecules of about 9:1 was detected [26,27,28]. The lack of dimerization makes -Br a prototypical example of a one-dimensional Mott insulator with quarterfilled conduction band [26,27,28,37,41].…”