Anomalous valence fluctuation near a ferroelectric transition in an organic charge-transfer complex

Dielectric susceptibility, X-ray diffraction and infrared molecular vibrational spectra have been investigated for a charge-transfer complex, tetrathiafulvalene (TTF)-2-bromo-3,5,6-trichloro-p-benzoquinone (QBrCl(3)) to probe a ferroelectric valence transition at T-c = 66 K in comparison with the typical first-order neutral-ionic transition in the isostructural analog, TTF-p-chloranii (QCl(4)). Around T-c, the anomalously rapid fluctuation of molecular valence between the quasineutral and quasi-ionic state is evidenced by the temperature variation of infrared spectra, signaling the quantum motion of the neutral-ionic domain walls on the one-dimensional molecular stacks.