Spectroscopic evidence of the charge ordering in κ-[Et4N](BEDT-TTF)4M(CN)6·3H2O (M = CoIII, FeIII, CrIII)

Polarised IR reflectance spectra of κ-[Et4N](BEDT-TTF)4M(CN)6·3H2O (M=CoIII, FeIII, CrIII) single crystal and FT-NIR Raman spectra of powdered samples were measured as a function of temperature. The salts undergo two phase transitions at about T=240 and 150 K. From Raman spectra results that on decreasing temperature below 240 K a charge redistribution is gradually developed. The electronic dispersion in IR spectra was analysed with Drude–Lorentz model to determine the optical transport parameters. The phase transition at T=150 K has an influence on both electronic and vibrational spectra, in particular, a new energy gap is formed (charge gap) and a new band at 1342 cm−1, assigned to BEDT-TTF+ cations, is observed.