Electronic Ferroelectricity in a Molecular Crystal with Large Polarization Directing Antiparallel to Ionic Displacement

Ferroelectric polarization of $6.3\mu \mathrm{C}{\mathrm{cm}}^{-2}$ is induced by the neutral-to-ionic transition, upon which nonpolar molecules of electron donor tetrathiafulvalene (TTF) and acceptor $p$-chloranil (CA) are incompletely ionized to $\pm 0.60e$ and dimerized along the molecular stacking chain. We find that the ferroelectric properties are governed by intermolecular charge transfer rather than simple displacement of static point charge on molecules. The observed polarization and poling effect on the absolute structural configuration can be interpreted in terms of electronic ferroelectricity, which not only exhibits antiparallel polarity to the ionic displacement but also enhances the polarization more than 20 times that of the point-charge model.

Figure 1

Schematic illustrations of the electric polarization (solid arrows) and displacement direction (small open arrows) of ions in the conventional displacive-type ferroelectrics (a) and the TTF-CA crystal (b). The positively charged TTF and negatively charged CA acceptor molecules are displaced against the electric field. Bent arrows denote the electric charge-transfer process from TTF to CA. (c) Molecular structures of TTF, CA, and ${\mathrm{QBrCl}}_3$. (d) Crystal structure of TTF-CA and molecular displacement directions under electric field.

Figure 2

Temperature-dependent properties of TTF-CA. (a) Change of ionicity probed by the $\mathrm{C}=\mathrm{O}$ stretch mode frequency of the CA molecule. (b) Relative permittivity measured along the crystal $a$ direction. The inset shows the temperature dependence of inverse permittivity ($f=300\mathrm{kHz}$) with a fit (straight line) to the Curie-Weiss law.

Figure 3

The polarization measured with triangular ac electric field ($E\Vert a$). (a) $P\mathrm{\text{−}}E$ hysteresis loops of $\mathrm{TTF}\mathrm{\text{−}}{\mathrm{QBrCl}}_3$ at various temperatures. (b) Hysteresis loops of TTF-CA with various frequencies at 51 K. (c) The $P\mathrm{\text{−}}E$ curves of TTF-CA using the positive-up-negative-down procedure with a double triangular waveform voltage ($f=500\mathrm{Hz}$, $T=59\mathrm{K}$) [illustrated on (d)]. The purely hysteresis component (solid curve) was obtained by subtracting the nonhysteresis contributions (up and down runs) from the total ones (positive and negative runs).

Figure 4

Degree of bulk polarization under electric field $E(\Vert a)$ probed by the anomalous x-ray scattering effect. (a) Temperature dependence of normalized integrated intensity $I_{+}\equiv I(101)/\{I(101)+I(\overline{1}0\overline{1})\}$ and $I_{-}\equiv I(\overline{1}0\overline{1})/\{I(101)+I(\overline{1}0\overline{1})\}$ of Bijvoet pair reflections 101 and $\overline{1}0\overline{1}$ at constant $E=+4\mathrm{kV}{\mathrm{cm}}^{-1}$. Open symbols are intensity data measured at 32 min after switching off to $E=0$ at $T=50\mathrm{K}$ and represent a polarization decay toward the multidomain state. (b) Normalized intensity $I_{+}$ and $I_{-}$ under $E$ increasing from $-4\mathrm{kV}{\mathrm{cm}}^{-1}$ to $+4\mathrm{kV}{\mathrm{cm}}^{-1}$ at 61 K. Switching of the polarization interchanges their magnitudes with each other at $E\sim 1\mathrm{kV}{\mathrm{cm}}^{-1}$.