Unusual High-Temperature Reversible Phase-Transition Behavior, Structures, and Dielectric–Ferroelectric Properties of Two New Crown Ether Clathrates

Molecular
ferroelectrics with high-temperature reversible phase-transition
behaviors are very rare and have currently become one of the hotspots
in the field of ferroelectric materials. Herein we display two new
crown ether clathrates possessing unusual high-temperature ferroelectric
phase-transition behaviors, cyclohexyl ammonium 18-crown-6 tetrafluoroborate
(or perchlorate), [Hcha-(18-crown-6)]+ [BF4]− (1) and [Hcha-(18-crown-6)]+[ClO4]− (2) (Hcha = protonated
cyclohexyl ammonium). We have proven their reversible structural phase
transitions by variable-temperature PXRD measurements and temperature
evolutions of Raman bands. Both clathrates exhibit clear ferroelectric
phase transitions at about 397 and 390 K, respectively, revealed by
the thermal anomalies of differential scanning calorimetry (DSC) measurements,
together with abrupt dielectric anomalies in the heating and cooling
processes. The measurements on ferroelectric properties using the
single crystals showed optimized spontaneous polarization (Ps) of ca. 3.27 μC cm–2 for 1 and 3.78 μC cm–2 for 2.