Can molecular ferroelectrics challenge pure inorganic ones?

Abstract

Molecular materials have attracted much attention due to their advantages of easy modification and fabrication, environmentally friendly and less energy-cost processing, transparent, light weight, mechanical flexibility and possible multi-functionalities, compared to their counter parts of pure inorganic or metallic ones, and many functionalities and properties have been discovered and realized inmolecule-basedmaterials [1–4]. Ferroelectrics, as an important class of multifunctional electroactive materials possessing electrically or mechanically switchable, temperature-dependent polarization and/or tunable dielectric responses and nonlinear electro-optic effects, have many applications in temperature sensing, data storage, mechanical actuation, energy harvesting, electromagnetic wave manipulation, and so on [5–8]. The first ferroelectrics, Rochelle salt, which was discovered in 1920–1, is in fact a molecular material, and later a few other molecular systems, such as the well-known potassium dihydrogen phosphate and tri-glycine sulfate, were discovered and developed. However, the rapid development of ferroelectrics took place only after the discovery of pure inorganic ferroelectrics, perovskite barium titanate (BTO) and lead zirconate titanate, because of their large spontaneous polarization (PS), high Curie temperature (TC), large dielectric constant (e′), and low dielectric loss (e′′ or tanδ), which are usually unavailable for molecular ferroelectrics [5,6]. These shortcomings for molecular ferroelectrics have been gradually overcome [9–13]; for example, an organic ferroelectrics, croconic acid, has been reported to possess aPS of 21μCcm−2 at room temperature, comparable to BTO [9]. Very recently, a research team led by Professor Ren-Gen Xiong in Southeast University, Nanjing, China, has reported [14] that a molecular ferroelectric crystal of diisopropylammonium bromide salt (DIPAB, Figs 1 and 2) showed a PS of 23 μC cm−2, high TC of 426 K, large e′ up to 103, and low tanδ of ∼0.4%, all close to or beyond the pure inorganic BTO, representing a breakthrough in the research of molecular ferroelectrics.