Anisotropic polarizationπ-molecular skeleton coupled dynamics in proton-displacive organic ferroelectrics

Abstract

We have investigated the polarization -molecular skeleton coupled dynamics for the proton-displacive organic ferroelectrics, cocrystal of phenazine with the 2,5-dihalo-3,6-dihydroxy-p-benzoquinones by measurements of the terahertz/infrared spectroscopy. In the course of the ferroelectric-to-paraelectric transition, the ferroelectric soft phonon mode originating from the intermolecular dynamical displacement is observed in the imaginary part of dielectric spectra ⑀ 2 , when the electric field of the light ͑E͒ is parallel to the spontaneous polarization ͑P͒. The soft phonon mode is isolated from the intramolecular vibrational mode and hence the intramolecular skeleton dynamics is almost decoupled from the polarization fluctuation. In the spectra for E parallel to the hydrogen-bonded supramolecular chain, by contrast, the vibrational mode mainly originating from the oxygen atom motion within the -molecular plane is anomalously blurred and amalgamated into the polarization relaxation mode concomitantly with the dynamical proton disorder. This indicates that the dynamical disorder of the intramolecular skeleton structure, specifically that of oxygen atom, is strongly enhanced by the proton fluctuation and is significantly coupled to the polarization fluctuation along the hydrogen-bonded supramolecular chain. The results are discussed in terms of the proton-mediated anisotropic polarization -molecular skeleton interaction, which characterizes these emerging proton-displacive ferroelectrics.

Terahertz Radiation Imaging of Ferroelectric Domain Topography in Room-Temperature Hydrogen-Bonded Supramolecular Ferroelectrics

“…In the present work, we successfully visualized ferroelectric domains and DWs in the inside and surface regions of as-grown crystals by using the anisotropy of the optical properties in the terahertz frequency regions. In organic molecular ferroelectrics, collective modes show up along the direction of the ferroelectric polarization [21,34], resulting in the large optical anisotropy in the terahertz frequency region. Thus, the present method is not restricted to [D-55DMBP][Dia] but can be applied to a variety of organic ferroelectrics with optical anisotropy in the terahertz frequency region.…”

Structure and energetics of a ferroelectric organic crystal of phenazine and chloranilic acid

“…2 The cocrystal of phenazine (Phz) and chloranilic acid (H 2 ca) is one of several recently discovered hydrogen-bonded organic ferroelectrics that have superior crystallinity and properties compared to conventional ferroelectric polymers. [2][3][4][5][6][7][8][9][10][11][12][13] The crystal structure of Phz-H 2 ca has been determined by X-ray 3,7 and neutron 8 diffraction experiments. The centrosymmetric paraelectric structure (monoclinic P 2 1 /n, T > T c = 253 K) is shown in Fig.…”

Correlation between proton transfer and 35Cl NQR frequency as well as molecular geometry of chloranilic acid in co‐crystals with some organic bases

“…In Fig. 5, the data points were plotted on the line for e 2 Qq/h at the position where a consistent theoretical value was obtained for η as compared with the observed one. From the abscissa for a set of data (e 2 Qq/h, η), the electron donor orbital occupancy σ was determined as listed in Table 2.…”

“…In the organic ferroelectric phenazine–chloranilic acid (1:1),1, 2 H 2 ca molecule seems to be in the electrically neutral state both in the paraelectric phase I and ferroelectric phase II. For the low‐temperature ferroelectric phase IV below 137 K,2 however, the 35 Cl and 14 N nuclear quadrupole resonance (NQR) study3 suggested the existence of Hca − and protonated phenazine on the basis of the report by Ikeda et al ,4 which states that the 35 Cl NQR frequency decreases when the valence state of chloranilic molecule changes from neutral (0) to monovalent (1−) and further to divalent (2−). On the other hand, it is also recognized that the bond distances of the chloranilic acid molecule show a systematic change depending on the valence state 5.…”