Quantitative microscopic origin of ferroelectricity and of its nonlinear pressure response in hydrogen-bonded supramolecular co-crystals of phenazine and anilic acids

Co-crystals of the nominally nonpolar molecules phenazine (Phz) and the anilic acids (H2xa; chloranilic and bromanilic acid) were shown in 2005 to be insulating displacive-like organic ferroelectrics with large dielectric anomalies, switchable polarization of order 1 microcoulomb per square centimetre, and a Curie temperature that rises nonlinearly under hydrostatic pressure toward room temperature. The original report explicitly left open four issues: hydrogen-atom positions, the relative weight of hydrogen-bond versus pi-stacking electronic contributions to the polarization, the microscopic origin of the nonlinear pressure dependence of the Curie temperature, and general design rules for ambient-pressure room-temperature organic ferroelectrics. The subsequent literature has since settled the hydrogen positions (neutron and modulated-structure diffraction reveal partial, not absent, proton transfer), and several ambient-pressure organic ferroelectrics with Curie temperatures above 300 K have been realized. What remains open is a quantitative, experimentally verified account of the ground-state polarization (its decomposition into protonic, hydrogen-bond-electronic, and pi-electronic parts), a verified microscopic model of the nonlinear pressure behavior including the proposed neutral-ionic staging of the commensurate ground states, and transferable predictive design rules.

Background

A ferroelectric is a crystal that develops a spontaneously polar, switchable state below a Curie temperature $T_c$, usually detected as a sharp peak in the dielectric constant that follows Curie-Weiss behavior $\varepsilon = C/(T-\theta)$ on cooling. An antiferroelectric, by contrast, orders dipoles anti-parallel in adjacent sublattices. Before 2005, low-molecular-weight organic ferroelectrics were rare: they relied either on orientational ordering of already-polar molecules (modest dielectric response) or on neutral-ionic valence transitions in donor-acceptor charge-transfer complexes, which are too semiconducting for insulating device operation. Horiuchi, Ishii, Kumai, Okimoto, Tachibana, Nagaosa and Tokura then showed (Nature Materials 4, 163-166, 2005) that supramolecular co-crystallization - the self-assembly of two distinct neutral molecules into one ordered crystal held together by intermolecular interactions - of phenazine (Phz, a nonpolar diaza-aromatic base) with the 2,5-dihalo-3,6-dihydroxy-p-benzoquinones H2xa (x = ca, chloranilic acid; x = ba, bromanilic acid) produces genuine uniaxial ferroelectrics: Phz-H2ca with $T_c = 253$ K and Phz-H2ba with $T_c = 138$ K, dielectric constants peaking near $2\times10^3$-to-$3\times10^3$, remnant polarization $P_r$ of order $1\,\mathrm{\mu C\,cm^{-2}}$, resistivity above $10^8\,\mathrm{\Omega\,cm}$, and a symmetry lowering from centric $P2_1/n$ to polar $P2_1$ with only ~0.01 Angstrom displacements of molecular centers. Each O-H group of the acid donates an O-H...N hydrogen bond to a phenazine nitrogen, and the molecules stack into chains in which the ferroelectric dipolar coupling runs along the stacking axis ($b$) while the coupling between chains appears antiferroelectric; hydrostatic pressure raises $T_c$ nonlinearly, shifting the dielectric anomaly of Phz-H2ca to room temperature at about 6 kbar. The authors' own analysis gave the first hints of an unusual mechanism: a Berry-phase density-functional-theory calculation (the standard first-principles method for the polarization of periodic crystals) on the experimental ferroelectric structure yielded $P \approx 0.32\,\mathrm{\mu C\,cm^{-2}}$, several times larger than isolated-molecule dipole sums ($\approx 0.10$) or point-charge ionic estimates ($\approx 0.07$), while extended Hückel (a semi-empirical tight-binding electronic-structure method) overlap calculations indicated finite pi-pi electronic transfer along the stacks; X-ray data and infrared spectra indicated that the protons stayed on the acid oxygens, i.e. no full proton transfer to the base. Specialist terms used below: the maximum-entropy method (MEM) is a model-free reconstruction of the electron density from diffraction data; neutron diffraction is nucleus-sensitive and therefore the method of choice for locating hydrogen; a neutral-ionic (N-I) transition is a collective electron (and proton) transfer that converts a neutral molecular crystal into an ionic salt; an incommensurate phase has a modulation whose wavelength is not a rational multiple of the lattice, and a 'staged' or commensurate superlattice is its lock-in product; deuteration (H/D substitution) tests whether quantum proton motion matters. The 2005 paper closed by flagging four issues as future work.

Problem Statement

For the hydrogen-bonded supramolecular ferroelectric co-crystals of phenazine and the anilic acids (Phz-H2xa, x = ca, ba, and their deuterates), establish a quantitative, experimentally verified microscopic account of the ferroelectricity and its pressure response, consisting of three jointly supporting parts: (A) a decomposition of the spontaneous polarization of the ordered ground states (FE-I, the polar phase that sets in below $T_c$; the incommensurate ferroelectric phase; and the commensurate FE-II/FE-III superlattice phases, the lock-in products of the incommensurate modulation at lower temperature) into the protonic (proton displacement/partial transfer), the hydrogen-bond electronic, and the intermolecular pi-pi charge-transfer contributions, reconciling the currently divergent experimental and first-principles values (measured $P_s$ near $1\,\mathrm{\mu C\,cm^{-2}}$, $0.32\,\mathrm{\mu C\,cm^{-2}}$ computed for the neutral $P2_1$ structure, $4.5\,\mathrm{\mu C\,cm^{-2}}$ computed for the proton-transferred ground state, and a point-charge model of the low-temperature FE-II superstructure that already reproduces the measured value), verified against nucleus-sensitive and charge-sensitive experiment for the actual ground-state structures; (B) a verified microscopic theory of the nonlinear hydrostatic- and chemical-pressure dependence of $T_c$ - its initial rise (of order $+120\,\mathrm{K\,GPa^{-1}}$ for Phz-H2ca), saturation and eventual decrease above roughly $0.6$-$0.8\,\mathrm{GPa}$, and the isotope shifts - reproducing the measured nonlinear pressure dependence of $T_c$ together with the isotope shifts, including the respective roles of the ferroelectric intrastack and antiferroelectric interstack couplings, and a direct test of the proposed neutral-ionic staging of the commensurate ground states against experiment; and (C) composition/structure design rules, transferable beyond the Phz-H2xa family, for insulating organic ferroelectrics with $T_c \gtrsim 300\,\mathrm{K}$ at ambient pressure, stated precisely enough to be falsifiable and to predict at least one such insulating organic ferroelectric not already reported in the cited literature. The common scientific objective uniting these parts is a predictive microscopic understanding of hydrogen-bonded supramolecular ferroelectricity - which interactions produce the polarization, how pressure and chemical substitution tune the transition, and hence how such materials can be designed. The three parts are independently refutable sub-claims: establishing any one of them counts as a partial resolution, and a full resolution requires all three.

References

1. S. Horiuchi, F. Ishii, R. Kumai, Y. Okimoto, H. Tachibana, N. Nagaosa, Y. Tokura, 'Ferroelectricity near room temperature in co-crystals of nonpolar organic molecules', Nature Materials 4, 163-166 (2005). DOI: 10.1038/nmat1298, https://doi.org/10.1038/nmat1298
2. R. Kumai, S. Horiuchi, H. Sagayama, T.-h. Arima, M. Watanabe, Y. Noda, Y. Tokura, 'Structural Assignment of Polarization in Hydrogen-Bonded Supramolecular Ferroelectrics', Journal of the American Chemical Society 129, 12920-12921 (2007). DOI: 10.1021/ja075406r, https://doi.org/10.1021/ja075406r
3. K. Gotoh, T. Asaji, H. Ishida, 'Hydrogen bonding in two solid phases of phenazine-chloranilic acid (1/1) determined at 170 and 93 K', Acta Crystallographica Section C 63, o17-o20 (2006). DOI: 10.1107/S0108270106049468, https://doi.org/10.1107/s0108270106049468
4. T. Asaji, J. Seliger, V. Zagar, M. Sekiguchi, J. Watanabe, K. Gotoh, H. Ishida, S. Vrtnik, J. Dolinsek, 'Phase transition and temperature dependent electronic state of an organic ferroelectric, phenazine-chloranilic acid (1:1)', Journal of Physics: Condensed Matter 19, 226203 (2007). DOI: 10.1088/0953-8984/19/22/226203, https://doi.org/10.1088/0953-8984/19/22/226203
5. T. Asaji, J. Seliger, V. Zagar, H. Ishida, 'Correlation between proton transfer and 35Cl NQR frequency as well as molecular geometry of chloranilic acid in co-crystals with some organic bases', Magnetic Resonance in Chemistry 48, 531-536 (2010). DOI: 10.1002/mrc.2613, https://doi.org/10.1002/mrc.2613
6. F. Ishii, N. Nagaosa, Y. Tokura, K. Terakura, 'Covalent ferroelectricity in hydrogen-bonded organic molecular systems', Physical Review B 73, 212105 (2006). DOI: 10.1103/PhysRevB.73.212105, https://doi.org/10.1103/physrevb.73.212105
7. S. Horiuchi, R. Kumai, Y. Tokura, 'A Supramolecular Ferroelectric Realized by Collective Proton Transfer', Angewandte Chemie International Edition 46, 3497-3501 (2007). DOI: 10.1002/anie.200700407, https://doi.org/10.1002/anie.200700407
8. S. Horiuchi, Y. Tokura, 'Organic ferroelectrics', Nature Materials 7, 357-366 (2008). DOI: 10.1038/nmat2137, https://doi.org/10.1038/nmat2137
9. S. Horiuchi, R. Kumai, Y. Tokunaga, Y. Tokura, 'Proton Dynamics and Room-Temperature Ferroelectricity in Anilate Salts with a Proton Sponge', Journal of the American Chemical Society 130, 13382-13391 (2008). DOI: 10.1021/ja8032235, https://doi.org/10.1021/ja8032235
10. J. Fujioka, S. Horiuchi, N. Kida, R. Shimano, Y. Tokura, 'Anisotropic polarization π-molecular skeleton coupled dynamics in proton-displacive organic ferroelectrics', Physical Review B 80, 125134 (2009). DOI: 10.1103/PhysRevB.80.125134, https://doi.org/10.1103/physrevb.80.125134
11. S. Horiuchi, R. Kumai, Y. Tokura, 'Proton-displacive ferroelectricity in neutral cocrystals of anilic acids with phenazine', Journal of Materials Chemistry 19, 4421 (2009). DOI: 10.1039/b900987f, https://doi.org/10.1039/b900987f
12. K. Lee, B. Kolb, T. Thonhauser, D. Vanderbilt, D. C. Langreth, 'Structure and energetics of a ferroelectric organic crystal of phenazine and chloranilic acid', Physical Review B 86, 104102 (2012). DOI: 10.1103/PhysRevB.86.104102, https://doi.org/10.1103/physrevb.86.104102
13. F. Kagawa, K. Hatahara, S. Horiuchi, Y. Tokura, 'Domain-wall dynamics coupled to proton motion in a hydrogen-bonded organic ferroelectric', Physical Review B 85, 220101(R) (2012). DOI: 10.1103/PhysRevB.85.220101, https://doi.org/10.1103/physrevb.85.220101
14. R. Kumai, S. Horiuchi, J. Fujioka, Y. Tokura, 'Ferroelectricity and Pressure-Induced Phenomena Driven by Neutral Ionic Valence Instability of Acid-Base Supramolecules', Journal of the American Chemical Society 134, 1036-1046 (2012). DOI: 10.1021/ja208113p, https://doi.org/10.1021/ja208113p
15. S. Horiuchi, R. Kumai, Y. Tokura, 'High-Temperature and Pressure-Induced Ferroelectricity in Hydrogen-Bonded Supramolecular Crystals of Anilic Acids and 2,3-Di(2-pyridinyl)pyrazine', Journal of the American Chemical Society 135, 4492-4500 (2013). DOI: 10.1021/ja400318v, https://doi.org/10.1021/ja400318v
16. R. Kumai, F. Kagawa, S. Horiuchi, 'Successive Phase Transitions and Ferroelectric Domain State in Supramolecular Ferroelectric Phenazine-Chloranilic Acid', Nihon Kessho Gakkaishi (Journal of the Crystallographic Society of Japan) 55, 135-141 (2013). DOI: 10.5940/jcrsj.55.135, https://doi.org/10.5940/jcrsj.55.135
17. L. Noohinejad, S. Mondal, A. Wolfel, S. Ali, A. Schonleber, S. van Smaalen, 'Ferroelectricity of Phenazine-Chloranilic Acid at T = 100 K', Journal of Chemical Crystallography 44, 387-393 (2014). DOI: 10.1007/s10870-014-0527-1, https://doi.org/10.1007/s10870-014-0527-1
18. L. Noohinejad, S. Mondal, S. Ali, S. Dey, S. van Smaalen, A. Schonleber, 'Resonance-stabilized partial proton transfer in hydrogen bonds of incommensurate phenazine-chloranilic acid', Acta Crystallographica Section B 71, 228-234 (2015). DOI: 10.1107/S2052520615004084, https://doi.org/10.1107/s2052520615004084
19. T. Umanodan, S. Tanaka, S. Naruse, T. Ishikawa, K. Onda, S. Koshihara, S. Horiuchi, Y. Okimoto, 'Different Time-Scale Relaxation Dynamics in Organic Supramolecular Ferroelectrics Studied by Linear and Nonlinear Spectroscopy', Journal of the Physical Society of Japan 84, 073707 (2015). DOI: 10.7566/JPSJ.84.073707, https://doi.org/10.7566/JPSJ.84.073707
20. R. A. Wiscons, N. D. Goud, J. Damron, A. J. Matzger, 'Room-Temperature Ferroelectricity in an Organic Cocrystal', Angewandte Chemie International Edition 57, 9044-9047 (2018). DOI: 10.1002/anie.201805071, https://doi.org/10.1002/anie.201805071
21. W. Zhu, Y. Wang, C. Huang, L. Zhu, Y. Zhen, H. Dong, Z. Wei, D. Guo, W. Hu, 'Polymorph and anisotropic Raman spectroscopy of Phz-H2ca cocrystals', Science China Materials 64, 169-178 (2020). DOI: 10.1007/s40843-020-1372-6, https://doi.org/10.1007/s40843-020-1372-6
22. S. Horiuchi, S. Ishibashi, Y. Tokura, 'Hydrogen-bonded organic molecular ferroelectrics/antiferroelectrics', in Organic Ferroelectric Materials and Applications (Elsevier, 2022), chapter 13, pp. 47-84. DOI: 10.1016/B978-0-12-821551-7.00013-0, https://doi.org/10.1016/B978-0-12-821551-7.00013-0
23. Y. Okimoto, P. Xia, J. Itatani, H. Matsushima, T. Ishikawa, S. Koshihara, S. Horiuchi, 'Ultrafast opto-protonics in a hydrogen-bonded π-molecular ferroelectric crystal', APL Materials 10, 090702 (2022). DOI: 10.1063/5.0097571, https://doi.org/10.1063/5.0097571
24. Q. Pan, Z. Gu, R. Zhou, Z. Feng, Y. Xiong, T. Sha, Y.-M. You, R.-G. Xiong, 'The past 10 years of molecular ferroelectrics: structures, design, and properties', Chemical Society Reviews 53, 5781-5861 (2024). DOI: 10.1039/D3CS00262D, https://doi.org/10.1039/d3cs00262d
25. S. Seyedraoufi, O. Hewitt, S. J. Coles, G. M. Day, K. Berland, 'Engineering Small-Molecule Proton-Transfer Ferroelectrics by Crystal Structure Prediction: Design Limits at the Salt-Cocrystal Boundary', Chemistry of Materials 38, 7389-7401 (2026). DOI: 10.1021/acs.chemmater.6c01164, https://doi.org/10.1021/acs.chemmater.6c01164