Raman study of charge disproportionation in α-(BEDT-TTF)2I3

Raman studies of alpha-(BEDT-TTF)(2)I-3 [BEDT-TTF is bis-ethylenedithiolo(tetrathiafulvalene)] have been performed to investigate the charge distribution between BEDT-TTFs below and above the metal-insulator phase transition at T-M.(I)=135 K. Following the crystal-symmetry lowering at T-M.(I), clear +/-0.3e deviation from average 0.5e charge has been found in the nonmagnetic insulating phase. In the paramagnetic semi-metal-like phase, the split of one of the Raman active mode suggests charge nonequivalence between donors.