Quantum Neutral–Ionic Phase Transition as Investigated by Raman Scattering and X-ray Diffraction

Abstract

The neutral–ionic (N–I) phase transitions of the molecular valence state have been investigated by Raman spectroscopy and x-ray diffraction for a series of isostructural charge-transfer complexes, 4,4′-dimethyltetrathiafulvalene-p-chloranil (DMTTF-QCl₄) and Br-substituted analogs DMTTF-QBr_nCl_4−n, in which the N–I transition temperature can be critically controlled with n. The anomalously broadened Raman spectrum of the valence-probing molecular vibration as well as the diffuse x-ray scattering of the ionic superlattice is observed ubiquitously around the N–I phase boundary, including the vicinity of the quantum critical (0 K-transition) point. The results quantify the quantum fluctuation between the two states of matter, van der Waals and ionic crystals.