Optical characterization of 2kF bond-charge-density wave in quasi-one-dimensional 3/4-filled (EDO-TTF)2X (X=PF6 and AsF6) -: art. no. 075107

We present the electronic and vibrational spectra of quasi-one-dimensional 3/4-filled 34(EDO-TTF)(2)X (EDO-TTF=ethylenedioxy-tetrathiafulvalene, X=PF6 and AsF6) above and below the metal-insulator phase transition (T-MI=280 K for the PF6 salt and 268 K for the AsF6 salt). For the low-temperature insulating phase, the pattern of both bond and charge order was identified. Almost all charge density is localized on the strongly bound central pair of 0110 tetramer giving rise to a characteristic spectrum of electronic excitations. Infrared spectra along the stacking axis show evidence of strong electron-molecular vibration coupling between the charge transfer band within the pair and some specific intramolecular vibrations. This charge order is assisted by a molecular deformation.