Multidirectional Colossal Polarization in a Centrosymmetric Crystal via Long-Range Ion Displacement

Ferroelectrics exhibit spontaneous polarization and are highly functional materials that are widely used in various devices. However, the design of conventional ferroelectric compounds is largely limited to noncentrosymmetric, polar structures. Herein, we demonstrate that colossal polarization occurs by long-range ion displacement even in the nonpolar proton conductor Na2MnN(CN)4·3.25H2O (1·H2O). This compound crystallized in the nonpolar I2/a space group but exhibited extremely large polarization owing to conducting protons, as confirmed by ferroelectric hysteresis and thermally stimulated depolarization current measurements. A remanent polarization of 438 mC cm-2 was achieved along the [010] direction of single crystals at 298 K under 80% relative humidity at 0.01 Hz. The polarization values are associated with proton conduction, largely depending on the frequency, temperature, and humidity. Single-crystal characterization revealed that polarization emerges in multiple directions and that the polarization magnitude and frequency-response vary depending on the crystallographic axis. These results indicate that the polarization phenomena, driven by long-range ion displacements, are strongly governed by the anisotropic one-dimensional framework of 1·H2O. Moreover, the functionality of this system is underpinned by hydrogen bonds between the inorganic skeleton and water, as confirmed by experiments employing H2O and D2O. This study demonstrates that ionic conductors can exhibit polar functionalities without breaking the inversion symmetry.