Deciphering Divergent Ferroelectric Behaviour in Hydrogen-bonded Organic Polymorphs

Abstract

Polymorphism is widely studied for tuning material properties, but is rarely exploited in organic ferroelectrics, because of the requirement for polar point-group symmetry. Moreover, their strategic design demands a fundamental understanding of how molecular packing dictates polarization. In this study, we investigate two polymorphic polar forms, tetragonal and orthorhombic, of an organic enamine-imine compound using a combination of quantum crystallography and first-principles calculations. Through multipolar modeling of highresolution X-ray and neutron diffraction data, complemented by dynamic theoretical structure factors, we confirm the presence of resonance-assisted hydrogen bonds along the proton tautomeric pathways in both forms. Our first-principles calculations indicate near-identical energy barriers to proton transfer in both cases, yet their macroscopic ferroelectric properties differ significantly. Sublattice polarization analysis demonstrates that the negligible net polarization of the orthorhombic form stems from an antiparallel arrangement of proton tautomeric pathways. These findings highlight how molecular packing and hydrogen-bond networks can be leveraged to tune ferroelectric performance, providing a blueprint for the design of next-generation organic electronic materials.