Ferroelectricity of Chiral Organic Materials —What is New, and What is the Future?

Ferroelectricity is an old and traditional scientific topics but still a very active subject because it describes rich phenomena, such like phase transitions, bistable states, large dielectric constants, electro-mechanical coupling (piezoelectricity), magneto-electric coupling, etc. The microscopic origin of electric polarization is the atomic configuration (molecule) as well as its displacements, and therefore electro-mechanical coupling intrinsically appears. From the engineering viewpoint, a capacitor is still a very important device in an electrical circuit. Hot topics in the last two decades were so-called multiferroics, that is, ferroelectricity and magnetic order simultaneously appear in the material due to magneto-electric coupling. The search for organic ferroelectric compounds instead of traditional oxide ceramics has become very popular recently as the possibility of synthesizing new materials increases [1].

The first discovered ferroelectric compound was Rochelle salt [2,3] in 1920, which is contained in red wine and was investigated as a sonar device against a submarine during the First World War. The chemical formula is KNa [L-(+)-Tartrate] 4H₂O, and it contains the chiral organic molecule tartaric acid (C₄H₄O₆). Many people, including myself, paid no attention to the meaning of “chiral molecule” in a ferroelectric compound, until recently. The concept of “chirality” is not so simple. If an object for the term “chirality” is a molecule, “chiral molecules” exhibit handedness with left-hand and right-hand configuration enantiomers with regards to a symmetry operation like the mirror operation (the mirror operation is an inversion times a two-fold axis). If left-hand and right-hand chiral molecules coexist, the mixture is called racemic. The condition for homochiral molecules in the unit cell of the crystal lacking inversion, mirror, and rotary reflection symmetries results in 65 space groups. Among them, 20 possible ferroelectric space groups are included. On the other hand, if an object for the term “chirality” is a space group, “chiral space group” is limited to a specific 11 pairs of space groups (22 of the 65 above), in which left-hand and right-hand “chiral space groups” are connected, and 8 polar space groups are included. The space group of Rochelle salt is P2₁2₁2 for the paraelectric phase (PE) and P2₁11 for the ferroelectric phase (FE), and these are not “chiral space groups” even though they include homochiral molecules. A homochiral compound, lacking of mirror symmetry, is one of the interesting subject in a ferroelectric transition, but chiral ferroelectrics in purely organic systems are still rare. Examples are shown in the references of this article [4].

In this article, authors consider that homochiral molecules are easier to crystallize in ferroelectric structures and found a new member of the chiral organic ferroelectrics; [(R)-(+)-BINOL·2DMSO]/[(S)-(−)-BINOL·2DMSO], where the crystal contains two polar dimethylsulfoxide [DMSO: (CH₃)₂SO] molecules as guests (Fig. 1). Here, the alignments of the guest DMSO molecules are partially disordered in the chiral framework of BINOL. The dielectric properties of this compound come from the hydrogen bonding between the polar DMSO molecule and the chiral BINOL molecule. It was confirmed that the results of the dielectric measurements are the same for the right- and left-handed homochiral crystals, so the results of (R)-(+)-BINOL·2DMSO or (S)-(−)-BINOL·2DMSO could not be distinguished. From the measurements of the dielectric constant, specific heat capacity and sound velocity, two successive phase transitions were found at T_C1 = 190 K and T_C2 = 125 K. Below T_C1 and T_C2, they are ferroelectric phases because electrical polarization was observed as shown in Fig. 2. X-ray diffraction experiment confirmed the space group of each phases; P4₁2₁2(P4₃2₁2) above 190 K (PE), P4₁(P4₃) between 190 and 125 K (FE1), and P112₁ below 125 K (FE2). Interestingly, P4₁2₁2(P4₃2₁2) and P4₁(P4₃) are both “chiral space groups”. The transition at 125 K seems to be a structural or ferroelastic transition by electro-mechanical coupling.

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Fig. 1. Structure of the target compound. The figure was obtained from Fig. 1 of Ref. 4.

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Fig. 2. P–E hysteresis curve and polarization as a function of temperature. The figure was obtained from Fig. 5 of Ref. 4.

In the organic ferroelectrics field, the following is emphasized, which is in contrast to the ceramic ferroelectrics field. Molecular ferroelectrics exhibit intrinsically natural merits, such as flexibility, being lightweight, low-cost, and having an acoustic impedance matching that of the human body, and organic electronics are especially expected to be flexible thin-film smart devices. From a basic science viewpoint, electro-optic coupling with polarized light might be interesting due to the existence of chiral molecules. Further, electro-optic and electroresistive characteristics caused by switching the external electric field open the possibility of a new device for thin-film electronics in the future.

References

• [1] S. Horiuchi and Y. Tokura, Nat. Mater. 7, 357 (2008). 10.1038/nmat2137 Crossref, Google Scholar
• [2] J. Valasek, Phys. Rev. 17, 475 (1921). 10.1103/PhysRev.17.475 Crossref, Google Scholar
• [3] C. A. Beevers and W. Hughes, Nature 146, 96 (1940). 10.1038/146096a0 Crossref, Google Scholar
• [4] T. Nomura, T. Yajima, Z. Yang, R. Kurihara, Y. Ishii, M. Tokunaga, Y. H. Matsuda, Y. Kohama, K. Kimura, and T. Kimura, J. Phys. Soc. Jpn. 91, 064702 (2022). 10.7566/JPSJ.91.064702 Link, Google Scholar

Author Biographies