Electron-molecular vibration coupling in 2-D organic conductors : high and low temperature phases of α-(BEDT-TTF)2I3

The polarized infrared (400-4 000 cm-1) absorption spectra of single crystals of the α form of (BEDT-TTF)2I3 are reported for the high temperature conducting phase at T = 300 K and for the low temperature semiconducting one at T = 20 K. For comparison and to supplement the already available data, the conductivity spectra obtained by Kramers-Kronig transformation of polarized reflectance data (80-6000 cm -1, T = 300 K) are also reported. The structures superimposed on the broad absorptions due to electronic intra- and interband transitions are extensively discussed and attributed to the coupling of conduction electrons to totally symmetric intramolecular modes of BEDT-TTF involving the stretching of C=C and C-S bonds. Such an assignment is based on Raman data and a preliminary normal coordinate analysis of neutral BEDT-TTF. The guidelines for the development of a quantitative microscopic model of vibrationally induced electron charge oscillations in 2-D organic conductors are presented. A qualitative discussion of the model allows us to account for the observed dichroism of the vibronic structures in room temperature α-(BEDT-TIF) 2I3. Comparison of the 300 K and 20 K spectra and a study of the temperature dependence of powder absorption spectra allow us to identify the spectral changes induced by the 135 K metal-insulator (M-I) transition. They are discussed in relation to possible structural changes accompanying the M-I transition. It is found that displacements of the BEDT-TTF molecules in the organic sheets are needed to account for the observed spectral changes.