Micro-Raman spectroscopy of single crystals of ET salts with mixed trihalide anions

We present a Raman study of isostructural organic conductors β-ET2X (ET is (bis(ethylenedithio)tetrathiafulvalene and X is I3, I2Br, IBr2 or nominally (I3)0.3(IBr2)0.7) grown by chemical oxidation. The low frequency region is studied down to 20 K for the 514 and 676 nm laser lines. In the case of β-ET2I3 single crystals we assign the 98 cm−1 band to the out-of-phase stretch of the symmetrical linear I3− anions. The band at 139 cm−1 in the Raman spectrum of β-ET2I2Br single crystals we assign to the in-phase stretch of the I2Br− anions, while the band at 120 cm−1 to the in-phase stretch of the I3− impurities. Domination of the band at 120 cm−1 we explain by high concentration of the I3− anions impurities for which Raman scattering occurs in resonant condition. The Raman spectra of β-ET2(I3)0.3(IBr2)0.7 single crystals, whose nominal stoichiometry was derived from X-ray diffraction, show that in fact the IBr2− and also the I2Br− anions dominate in the structure, while the I3− anions concentration is very small.