Hydrogen Bond Driven Ferroelectricity in a Croconic Acid-Based Two-Component Organic Salt

Organic ferroelectrics have emerged as a promising class of functional materials owing to their lightweight nature, structural flexibility, low processing temperatures, and environmentally benign composition. In this study, we report the synthesis and comprehensive characterization of guanidinium croconate, a new two-component organic salt obtained via acid-base neutralization of croconic acid and guanidine. Single-crystal x-ray diffraction reveals a polar crystal structure stabilized by an extensive three-dimensional N─H···O hydrogen-bonded network that supports long-range polar order. Macroscopic ferroelectricity is confirmed through polarization-electric field (P-E) hysteresis loops and positive-up-negative-down (PUND) measurements, yielding a remnant polarization of 3.5 µC/cm². Piezoresponse force microscopy (PFM) demonstrates clear ferroelectric domain structures, reversible 180° phase switching, and a converse piezoelectric coefficient (d₃₃) of 10-13.5 pm/V. Kurtz-Perry second-harmonic generation (SHG) measurements show an efficiency of 0.11 relative to KDP at room temperature. Variable-temperature SHG and differential scanning calorimetry reveal a strongly hysteretic ferroelectric-to-paraelectric phase transition at 510 K. Density functional theory calculations indicate a large molecular dipole moment of 12.56 D, arising from enhanced ionic charge separation. These results highlight ionic assembly as an effective strategy for designing hydrogen-bond-driven organic ferroelectrics.