Infrared and Raman studies of the phase transition in the organic conductor (TTM-TTP)I3

We report the polarized IR reflectance as well as Raman scattering investigations of the organic charge transfer salt (TTM-TTP)I3 as a function of temperature, below and above the metal-insulator phase transition at T = 160 K. The IR reflectance was measured in the frequency region from 600 to 10,000 cm−1, for the electrical vector of the polarized light parallel and perpendicular to the TTM-TTP stacking axis. For the polarization parallel to stacks the IR spectra are typical for semiconducting charge transfer salts. The electronic part of IR spectra was analysed in terms of a Lorentz model and temperature dependence of the optical transport parameters was determined. For the polarization perpendicular to the stacks we observed two electronic bands at about 5000 and 8000 cm−1. The phase transition at 160 K has nearly no influence on the IR spectrum. The Raman scattering for different excitations (λ = 514.5, 632.6 and 785 nm) was mainly studied within the region of CC stretching vibrations. In this frequency range, three Raman lines at 1426, 1453 and 1486 cm−1 attributed to TTM-TTP molecules are observed. Below 160 K a splitting of the band 1486 cm−1 into two peaks at about 1488 and 1498 cm−1 is found. The intensity and temperature behaviour of the split band at 1498 cm−1 is strongly dependent on sample. The observed spectral modifications are related to an asymmetric deformation of TTM-TTP. Taking into account temperature dependence of bands attributed to the CH stretching and SCH3 bending vibrations, we suggest that the TTM-TTP deformation can exist also above the phase transition temperature. Above 160 K molecules fluctuate between distorted and symmetrical state forming non-stable domains (pre-transitional effects), but below 160 K the molecular distortion and domains are stable. The existence of electronic band at 5000 cm−1 for the polarization perpendicular to TTM-TTP supports this picture.