Charge ordering in the κ-phase BEDT-TTF salts with Co(CN)6 and Fe(CN)6 anions studied by infrared and Raman spectroscopies

We studied the temperature dependence of the Raman scattering spectra of two semiconducting, isostructural charge transfer salts kappa-[Et4N](BEDT-TTF)(4)M(CN)(6).3 H2O (M = Co-III, Fe-III) within the region of C=C stretching vibrations (1200-1700 cm(-1)). Moreover, polarised reflectance spectra (600-10000 cm(-1)) of the Co-III. salt were recorded as a function of temperature and compared with previous infrared studies of the Fe-III salt. Both salts undergo a phase transition at T = 150 K related to a charge ordering inside the conducting BEDT-TTF layers. Due to the charge ordering, new vibrational bands corresponding to BEDT-TTF+1 cations are recorded both in Raman and infrared spectra. Infrared electronic bands also experience strong modifications at 150 K: a new energy gap is formed and a new charge-transfer band in the near-infrared region is observed. The most important difference is that the charge redistribution in Fe-III. salt is developed gradually below about 240 K, whereas in Co-III salt this is an abrupt process related to the phase transition at 150 K.