Spectroscopic studies of the phase transition from the mott insulator state to the charge-ordering state of κ-(ET)_4[M(CN) _6][N(C_2H_5)_4]·2H_2O (M = Co~(III) and Fe~(III)) salts

摘要

Polarized reflectivity spectra versus temperature of two isostructural charge-transfer salts κ-(ET)_4[M(CN) _6][N(C_2H_5)_4]·2H_2O (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.