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

We present a Raman study of isostructural organic conductors beta-ET2X (ET is (bis(ethylenedithio)tetrathiafulvalene and X is I-3, I2Br, IBr2 or nominally (I-3)(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 beta-ET2I3 single crystals we assign the 98 cm(-1) band to the out-of-phase stretch of the symmetrical linear I-3(-) anions. The band at 139 cm(-1) in the Raman spectrum of beta-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 I-3(-) impurities. Domination of the band at 120 cm(-1) we explain by high concentration of the I-3(-) anions impurities for which Raman scattering occurs in resonant condition. The Raman spectra of beta-ET2(I-3)(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 I-3(-) anions concentration is very small.