One-Dimensional Fluctuating Nanodomains in the Charge-Transfer Molecular System TTF-CA and their First-Order Crystallization

Abstract

We report on the direct observation by x-ray diffuse scattering measurements of thermally induced one-dimensional nanoscale ordered fluctuations in the mixed-stack charge-transfer molecular system tetrathiafulvalene-p chloranil (TTF-CA), the prototype for the neutral-ionic phase transition. The unusual physical properties of this compound are considered to be driven by such one-dimensional excitations. The results are discussed in relation to previous experimental and theoretical experiments both at thermal equilibrium and under light irradiation.

Phenomenology of the Neutral-Ionic Valence Instability in Mixed Stack Charge-Transfer Crystals

“…Evidence of the elusive LR-CT in the proximity of NIT has been sought by a very careful analysis of the diffuse X-ray scattering as a function of temperature [40,41]. The sharp Lorentzian-shaped diffuse X-ray signal collected along one spatial direction (Figure 12) implies the lack of long-range correlation order.…”

Back to the Structural and Dynamical Properties of Neutral-Ionic Phase Transitions

“…A number of theoretical works supports this remarkable manifestation of electronic ferroelectricity [14,66,67]. On the other hand, it is also important to underline that the pre-transitional fluctuations which develop in the N phase on approaching the N-I instability are strongly 1D [10,11]. This is particularly true in TTF-CA, for which no inter-stack correlations are observed even close to the N-I transition.…”

“…Thus, 1D correlated objects manifest via diffuse planes in the reciprocal space, and its spatial extension, the correlation length, can be extracted from the inverse of the width of these diffuse planes. For TTF-CA, diffuse planes perpendicular to the stacking axis a are observed above T NI [11]. These planes are localized around integer values of the Miller index h, except for h = 0 (Figure 8).…”

“…The high degree of covalency, with respect to conventional ferroelectric materials, makes these mixed stack charge transfer crystals highly electronically susceptible, and so drives the most spectacular manifestation of electronic ferroelectricity [9]; • The strong 1D fluctuations in the neutral phase [10,11] enhances the effect of electron-phonon interaction. It may give rise to a pronounced Peierls-type anomaly in lattice phonon spectrum and/or the generation of unconventional non-linear excitations along DA stacks, with a puzzling interplay between quantum and thermal effects; •…”

X-ray diffraction for material science

“…It is more or less spread out within the reciprocal space, depending on the size of these local deviations. Lets consider the prototype quasi-1D molecular solid TTF-CA [9,10]. The crystal structure is made of a mixed-stack sequence of alternating TTF donor (D) and CA acceptor (A) molecules.…”