Anion arrangement effects on electronic states of κ-type BEDT-TTF compounds

We investigate the anion arrangement effect on the electronic states of various bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) compounds composed of BEDT-TTF layers and counter-anion networks via Raman scattering measurements. The anion vibrational modes and charge-sensitive BEDT-TTF vibrational modes of disordered-anion compounds κ-(BEDT-TTF)₂Cu₂(CN)₃, κ-(BEDT-TTF)₂Ag₂(CN)₃, and (BEDT-TTF)Ag₄(CN)₅, indicate splitting, contrary to the case of non-disordered-anion compounds κ-(BEDT-TTF)₂Ag(CN)[N(CN)₂] and κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl. The splitting behaviors indicate that several possible arrangements of CN bonds exist in the anion networks. These results suggest that the disorder in the anion structures is not directly due to the recently reported charge disproportionation or relaxor-like dielectric response in some κ-type BEDT-TTF compounds. Additionally, in κ-(BEDT-TTF)₂Cu₂(CN)₃, the frequency of the anion mode and the dielectric constant have local maxima at the same temperature. This result suggests that the anion modes are affected by the dielectric responses of the counter cations. (author)