Raman signatures of the strong intra- and inter-molecular charge oscillations in bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) \k{appa}-phase salts

TL;DR

This paper uses first-principles calculations to reinterpret Raman spectra of BEDT-TTF salts, revealing new insights into charge oscillations, phonon modes, and charge ordering phenomena in these complex materials.

Contribution

It provides a revised assignment of Raman-active phonons, introduces a detailed electron-molecular vibration coupling scheme, and explores charge ordering effects in asymmetric dimers.

Findings

Reassignment of C=C stretching phonons in BEDT-TTF salts.

Identification of inter-dimer electron-molecular vibration coupling.

Insights into charge ordering and ferroelectric phases.

Abstract

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{appa}-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 intra-molecular 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 inter -dimer electron-molecular vibration (e-mv) coupling, to be contrasted with the well known intra-dimer e-mv coupling that induces very strong infrared absorptions. Analysis of the data in term 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{appa}-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{appa}-(BEDT-TTF)_2 Hg(SCN)_2 Cl. It turns out that in the asymmetric dimer the strong intra-dimer 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{appa}-(BEDT-TTF)_2 Hg(SCN)_2 Cl ferroelectric phase gives clear alternative indications about the possible CO pattern in such a phase.