FT-IR absorption spectroscopy of BEDT-TTF radical salts: charge transfer and donor-anion interaction

Abstract

IR absorption spectra from various K-phases of BEDT·TfF radical cation salts as well as from a-(BEDT·TIFhI, arc presented. From tbese spectra one special mode l'n representing the asymmetric ring C-C stretching vibration is assigned and the dependence of its frequency from the average charge on the donor molecule is given. The investigation of the metal-insulator phase transition of a-(BEDT·TrF)2I, at 135 K with respect to this vibrational model lin gives strong indication of a charge localization in at least one of the two crystallographic different stacks. Furthennore, we correlate the frequencies of the Cfh stretching modes of a variety of BEDT·TIF radical salts witb tbe superconducting phase transition temperatures. These frequencies are evidently not charge dependent, but significanUy reOect the strength of the interaction of the donor molecule with the respecti'Ye anion. It could be shown that phases with a higher Tc of their superconducting transition exhibit a smaller red shift of these frequencies, i.e., a less attractive donor-anion interaction.

Field and frequency dependent transport in the two-dimensional organic conductor α-(BEDT-TTF)2I3

“…The Fermi surface consists of small pockets on the Brillouin zone boundary, which are two-dimensional in nature, but created from a onedimensional Fermi surface crossing the Brillouin boundary. In agreement with this, recent MIR absorption Illeasurements [33,34] indicate a localization of charge in one of the two different stacks of the donor molecules. This would imply that the crystals lose their .quasi two-dimensional electronic properties below the phase transition and exhibit more quasI onedimensional properties, since the coupling between neighboring stacks is reduced or lost.…”

“…They are responsible for a change in the charge distribution within the molecule, the dimerization, and the interaction between neighboring BEDT-TTF molecules. The result of the IR reflectivity and absorption [33,34] point in this direction, as do thermal expansion measurements [29].…”

Infrared and Raman Studies of Charge Ordering in Organic Conductors, BEDT-TTF Salts with Quarter-Filled Bands

“…Later, Chow reported a similar charge order for (TMTTF) 2 X (X = PF 6 , AsF 6 ) using 13 C-NMR [11]. In the case of a two-dimensional organic compound, Moldenhauer et al, reported that localization of charge occurs in one of the stacks of α-(BEDT-TTF) 2 I 3 (BEDT-TTF corresponds to bis(ethylenedithio) tetrathiafulvalene) [12]. After the theoretical prediction of the CO ground state for α-(BEDT-TTF) 2 I 3 by Kino and Fukuyama [13], CO in two-dimensional organic compounds was suggested in α-(BEDT-TTF) 2 I 3 [14], and more clearly shown in θ-(BEDT-TTF) 2 RbZn(SCN) 4 [15].…”

“…Kino and Fukuyama examined the effect of on-site Coulomb interaction within the Hartree-Fock approximation, and they predicted that a charge localization with a vertical stripe is the cause of the MI transition [13,126]. Before the theoretical prediction, Moldenhauer et al, reported that the localization of electrons occurs at the B and C sites whereas the electrons at the A and A′ sites are delocalized below T MI [12]. The 13 C-NMR study suggested the existence of two differently charged molecules below T MI [119,127].…”

“…In addition, they did not consider the effect of electron-molecular vibration interaction, which will be discussed in the next subsection. The linear relationship for the infrared-active ν 27 was first utilized by Moldenhauer et al, for estimating the iconicity of BEDT-TTF [12]. However, this relationship is given based on the incorrect assignment of BEDT-TTF + [46].…”