Room-Temperature Ferroelectricity in an Organic Cocrystal

Ferroelectric materials exhibit switchable remanent polarization due to reversible symmetry breaking under an applied electric field. Previous research has leveraged temperature-induced neutral-ionic transitions in charge-transfer (CT) cocrystals to access ferroelectrics that operate through displacement of molecules under an applied field. However, displacive ferroelectric behavior is rare in organic CT cocrystals and achieving a Curie temperature (T-C) above ambient has been elusive. Here a cocrystal between acenaphthene and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane is presented that shows switchable remanent polarization at room temperature (T-C=68 degrees C). Raman spectroscopy, X-ray diffraction, and solid-state NMR spectroscopy indicate the ferroelectric behavior is facilitated by acenaphthene (AN) rotation, deviating from conventional design strategies for CT ferroelectrics. These findings highlight the relevance of non-CT interactions in the design of displacive ferroelectric cocrystals.