Infrared and Raman study of the charge-ordered state of θ-(ET)2Cu2CN[N(CN)2]2

The polarized Raman and infrared spectra of theta-(ET)(2)CU2CN[N(CN)(2)](2) [ET = bis(ethylenedithio)tetrathiafulvalene] were measured at various temperatures below 300K. We investigated the three C=C stretching modes, V-2, V-3 and V-27. The spectral pattern is compatible with the horizontally charge-ordered state, where the site-charge distributions are +0.1(7) and +0.8(3). We have found that the vibronic V-3 bands correlate to the dihedral angles, psi, in the theta-ET salts. This relation is well reproduced by the numerical calculation based on the cluster model with C-2 symmetry. The spectral features suggest the dynamical fluctuation of nearly localized charges even above its phase transition temperature, similar to220 K.