Evidence for a soft mode in the temperature induced neutral-ionic transition of TTF-CA

We report a parallel infrared and Raman investigation of the temperature induced neutral-ionic phase transition (NIT) of the mixed-stack charge-transfer (CT) crystal tetrathiafulvalene-chloranil (TTF-CA). We show that before the phase transition the vibronic bands present in the infrared spectra are due to sum and difference combinations, involving the lattice mode which gives rise to the Peierls distortion at the transition. We can then follow the temperature evolution of this Peierls mode, which shows a clear softening (from 70 to 20 cm(-1)) before the first order transition to the ionic ferroelectric state takes place. (C) 2003 Elsevier Science B.V. All rights reserved.