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.