Spectroscopic studies of the phase transition from the mott insulator state to the charge-ordering state of κ-(ET)4[M(CN)6][N(C2H5)4]·2H2O (M = Co(III) and Fe(III)) salts.

Polarized reflectivity spectra versus temperature of two isostructural charge-transfer salts κ-(ET)4[M(CN)6][N(C2H5)4]·2H2O (M = Co(III) and Fe(III)) (ET = bis(ethylenedithio)tetrathiafulvalene) were studied. The electronic and vibrational spectra exhibit a drastic change at around 150 K. On the basis of the spectral analysis, we deduced the nature of the phase transition. The phase transition at 150 K is due to a charge ordering; above this temperature, strong charge fluctuations are observed.