Ferroelectricity and Uniaxial Negative Thermal Expansion in a Purely Organic Multifunctional Material

The coexistence of ferroelectricity and negative thermal expansion (NTE) in a material is rare and has yet to be observed in a purely organic single-component system. Here, we report the bifunctional characteristics of 2-(4-(trifluoromethyl)phenyl)-1H-phenanthro [9,10-d] imidazole, a single-component system, exhibiting uniaxial ferroelectricity and anisotropic NTE properties in its various crystal forms. This purely organic material exists in trimorphic and hydrated forms. While one of the two polar forms retains its ferroelectric phase up to ∼497 K, the other exhibits a reversible large NTE below room temperature (RT), and a third centric form divests these properties. Further, the noncentric hydrated form also experiences NTE below RT. The mechanisms of ferroelectricity and NTE in this material are elucidated based on the analyses of their single-crystal X-ray structures. The interconversion of the trimorphs is demonstrated via thermal analyses. The experimental value of the in-crystal spontaneous polarization (4.6 μC/cm2) agrees well with that estimated from the periodic theoretical calculations. Given its promising figures of merit for ferroelectricity including a low coercive field (5.8 kV/cm), unusual thermal behavior, switching from NTE to positive thermal expansion, and a minimal volumetric change, this lightweight material is expected to find potential applications in nonexpansive organic electronics.