Optical study of the metal-insulator phase transition in β-(EDT-TTF-I2)2(Pb5/6□1/6I2)3 and β-(EDT-TTF-I2)2(Pb2/3+xAg1/3−2x□xI2)3 one-dimensional organic conductors

We report the polarized infrared reflectance and absorption spectra of the two radical cation salts β-(EDT-TTF-I2)2(Pb5/6□1/6I2)3 and β-(EDT-TTF-I2)2(Pb2/3+xAg1/3−2x□xI2)3, where x = 0.05 and □ - vacancy, from room temperature down to 10 K. Both materials display metallic response characteristic for quasi-one-dimensional organic conductors. The single crystal polarized absorption spectra display a number of vibrational features related to intramolecular modes of the EDT-TTF-I2 molecule. The 70 K metal-insulator phase transition has a modest influence on the infrared spectra of both materials. The temperature dependence of the electron-molecular vibration (EMV)-activated stretching CC mode of EDT-TTF-I2 suggests, that the charge density wave (CDW) fluctuations appear in both materials above the phase transition temperature. The temperature evolution of the reflectance spectra is analyzed using Drude model.