Abstract

The isostructural charge-transfer salts κ-[Et₄N](BEDT-TTF)₄M(CN)₆·3H₂O (M = Co^III, Fe^III, Cr^III) are organic semiconductors undergoing two intriguing phase transitions at T = 240 and 150 K. Polarized IR reflectance spectra (from 650 to 6500 cm⁻¹) of these compounds were studied at room temperature. An electronic dispersion in IR spectra was analyzed in terms of a Drude–Lorentz model and optical transport parameters were determined. IR reflectance spectra of Fe^III crystals were measured in the temperature range from 190 to 300 K and the influence of the phase transition at 240 K was studied. We report also FT-NIR Raman studies of Fe^III salt as a function of temperature from 110 to 300 K. The Raman line related to the ν₃(A_g) CC mode of BEDT-TTF splits below 240 K into three components providing evidence of charge redistribution in the BEDT-TTF layers.