High-Temperature Metal-Free Molecular Ferroelectrics with Large Spontaneous Polarization

Abstract

In the past two decades, numerous molecular ferroelectrics have been reported. However, metal-free molecular ferroelectrics with high working temperatures and large spontaneous polarizations are still uncommon. Herein, we present two metal-free molecular ferroelectrics prepared from monoprotonated hexamethylenetetramine (HMTA), namely (HMTAH)Cl and (HMTAH)Br, which crystallize in a polar point group of 3m. In these crystals, the polar HMTAH+ organic cations can be reoriented 180° along the polar axis because of the quasispherical molecular geometry. As a result of the large shift of the positively charged protonated N atoms, these compounds demonstrate large spontaneous polarizations with values of 8.3 and 8.1 μC cm–2 and high working temperatures of 390 and 435 K, respectively. The ferroelectric property of these compounds is characterized with second-harmonic generation, ferroelectric hysteresis loop, and pyroelectric current measurements.

The Use of Semiconductor Quantum Dots with Large, Built-In Spontaneous Polarizations for the Electric Potential Stimulation of Biological Structures on the Nanoscale

“…The phenomenon of spontaneous polarization has been discussed in non-physiological applications [25,26]. In addition, spontaneous polarization has been discussed for a wide variety of materials, including CdS [27], ZnO [27,28], Fe-based compounds [29], metal-free molecular ferroelectrics [30], LiNbO 3 [31], BiFeO 3 [32], BN [33], and GaN [11].…”