Optical study of the metal–insulator phase transition in β-(EDT-TTF-I 2) 2(Pb 5/6□ 1/6I 2) 3 and β-(EDT-TTF-I 2) 2(Pb 2/3+ xAg 1/3−2 x□ xI 2) 3 one-dimensional organic conductors

Abstract

We report the polarized infrared reflectance and absorption spectra of the two radical cation salts β-(EDT-TTF-I 2) 2(Pb 5/6□ 1/6I 2) 3 and β-(EDT-TTF-I 2) 2(Pb 2/3+ x Ag 1/3−2 x □ x I 2) 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-I 2 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 C C mode of EDT-TTF-I 2 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.