Infrared study on the formation of charge density waves in (TMTSF) 2X (X = Reo 4− and PF 6− at atmospheric pressure

Abstract

The temperature dependence of the infrared absorption spectra (4000–200 cm −) of (TMTSF) 2Reo 4 and (TMTSF) 2PF 6 are investigated with the aim at using the appearance of vibronic absorptions originated by electron-molecular vibration interactions to monitor the formation of charge density waves (CDW) on the donor stacks. The presence of vibronic bands at ≈1400, 436 and 264 cm −1 in (TMTSF) 2Re0 4 below 180 K shows that the anion ordering transition in this salt induces the formation of commensurate CDW. The absence of vibronic bands in the PF 6 − salt at 8K indicates that no CDW formation nor an underlying 2K f lattice distortion takes place at the metal-insulator (MI) phase transition. These results show that, at atmospheric pressure both salts do not undergo a conventional Peierls type MI transition.