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.

Multisequential reversible phase transitions based on molecular materials have important applications in ferroelastic materials, ferroeletric materials, switchable dielectric materials, and temperature-controlling materials. Here, we report that a new compound, [Hcpa-(18-crown-6)] + [ClO 4 ] − (1) (where Hcpa represents protonated cyclopentylamine cations) displays unusual multisequential reversible phase transitions accompanied by switchable dielectric behaviors. The stepwise synergistic disordering of Hcpa cations and ClO 4 − anions leads to the sequential reversible phase transitions and symmetry breaking. These unusual reversible phase transitions were further confirmed by the variable-temperature powder X-ray diffractometry (PXRD), thermal anomalies of differential scanning calorimetry (DSC) measurements, and abrupt dielectric anomalies in the heating and cooling processes.

Low-dimensional hybrid organic−inorganic perovskites (HOIPs) possess more localized electronic states and narrower conduction and valence bands to promote self-trapping of excitons and stronger exciton emission; therefore, they are widely used as building blocks for various applications in the fields of optoelectronics, photovoltaics, light-emitting diodes, luminescence, fluorescence, and so forth. Despite the past decades of intensive study, the discovered low-dimensional chiral HOIPs are rare as of the 1D chiral HOIP single crystals reported in 2003, as well as the low-dimensional chiral HOIP ferroelectrics are particularly scarce since the first chiral two-dimensional (2D) and/or one-dimensional (1D) HOIP ferroelectrics reported. Herein, two new low-dimensional HOIPs with the same conformational formula [R-MPA] 2 CdCl 4 (R-MPA + = (R)-(−)-1methyl-3-phenylpropylamine) were successfully synthetized by means of regulating the stoichiometric proportion of R-MPA and CdCl 2 in two ways of 1:1 (1) and 2:1 (2). By combining single-crystal X-ray diffraction, circular dichroism (CD) spectroscopy, differential scanning calorimetry, temperature-dependent dielectric constant, temperature-dependent second-harmonic generation (SHG) effect, polarization-dependent SHG response, and P−E hysteresis loop, we reveal that 1 is a 1D nonchiral molecular ferroelectric and 2 is the first zero-dimensional (0D) chiral ferroelectric with distinct CD signals; meanwhile, 2 exhibits increased properties of high-T c , large dielectric constant, SHG isotropy, and ferroelectricity than that of 1. These results not only shed light on the high tunability of the low-dimensional HOIP ferroelectrics but also open up an avenue to explore multifunctional chiral ferroelectrics.

Three homologous 2D organic–inorganic perovskites exhibit regulative magnitudes of phase transition temperature up to 185 K and band gap up to 1.79 eV as halogen atoms vary from Cl, Br to I.