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 -chloranil (QCl 4 ). Around T c , the anomalously rapid fluctuation of molecular valence between the quasi-neutral 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.