Raman signatures of the strong intramolecular and intermolecular charge oscillations in bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) κ-phase salts

First-principle calculations of the Raman intensities of a (BEDT-TTF)+ 2 centrosymmetric dimer lead to a full reconsideration of the assignment of the three C=C stretching phonons in K-phase BEDT-TTF salts. In contrast with previous interpretation, it is found that the presence of the out-of-phase coupling of the antisymmetric external C=C stretching mode has also to be taken into account. This mode, infrared-active for a single BEDT-TTF molecule, implies a strong intramolecular charge oscillation along BEDT-TTF long molecular axis. In the consequent reassignment of the C=C spectral region, a very broad band appearing in the cross polarized Raman spectra is interpreted as due to interdimer electron-molecular vibration (e-mv) coupling, to be contrasted with the well-known intradimer e-mv coupling that induces very strong infrared absorptions. Analysis of the data in terms of a properly developed e-mv scheme yields an evaluation of the Hubbard parameters relevant to the so-called effective dimer model often used to interpret the physical properties of K-BEDT-TTF salts. Finally, the Raman spectra of an asymmetric (BEDT-TTF)+0.6(BEDT-TTF)+0.4 dimer have been calculated in order to interpret the Raman spectra of the intriguing charge-ordered (CO), ferroelectric phase of K-(BEDT-TTF)2Hg(SCN)2Cl. It turns out that in the asymmetric dimer the strong intradimer the e-mv induced infrared C=C stretching absorption should appear with huge intensity also in Raman. The absence of such band in the spectra of K-(BEDT-TTF)2Hg(SCN)2Cl ferroelectric phase points to an alternative CO pattern with respect to the one generally accepted so far.