Neutral-to-ionic transition in tetrathiafulvalene-p-chloranil as investigated by optical reflection spectra

Polarized reflection spectra and their temperature-dependence as well are investigated on tetrathiafulvalene-p-chloranil (TTFCA) single crystals over the temperature range from 2 K to room temperature. Abrupt changes are observed at T_c = 84.4 ± 0.5 K in the charge transfer and intramolecular excitation bands. These experimental results strongly indicate that the dominant part of the neutral-to-ionic transition in this crystal takes place at a sharply defined critical temperature, in contrary to the recent observation by Torrance et al. Phys. Rev. Lett.47, 1747 (1981)] that the transition occurs in a temperature range of about 30 K in width. A simple model is presented to account for the observed shift of the TTF⁺ intramolecular transition bands at T_c.