Raman and IR spectra of new organic metals (ET)8[Hg4X12(C6H5Y)2] (X, Y=Cl, Br)

Abstract

The Raman and IR reflectivity spectra of the new organic metals (ET)8[Hg4X12(C6H5Y)2] (X, Y=Cl, Br) based on the molecule bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET in abbreviated form) have been investigated. These metals differ from the previously studied compounds, particularly, in the type of ET molecular packing in the quasi-two-dimensional conducting layers. A high reflectivity and a plasma minimum observed in the IR reflectivity spectrum indicate the presence of quasi-free charge carriers (holes), as in other conducting ET salts. However, the Ag vibrational modes in the Raman spectrum are not activated in the IR spectrum, as is the case of conducting ET salts with other packing types. The Raman lines are assigned to the normal vibrations in the ET molecule, and their ionization shifts are determined. It is demonstrated that the frequencies of the most intense line ν3(Ag) show a linear dependence on the cation charge, which is characteristic of different ET salts. No correlations are revealed between the ν3(Ag) frequencies and the packing type. The strong background with a broad maximum at a Raman shift of about 3000 cm−1 is observed in the Raman spectra upon excitation with the 2.54-and 2.41-eV lines of an Ar+ laser. The assumption is made that such a background can be associated with the scattering by one-particle and collective electronic excitations.