No abstract

To predict or identify ferroelectricity is essential for extending the family of molecular ferroelectrics and thereby promoting their practical applications in nonvolatile memories, capacitors, piezoelectric sensors and nonlinear optical devices. In this respect, symmetry breaking is of particular importance, since the paraelectric phase adopting any of the 32 crystallographic point groups is always broken into one of the 10 ferroelectric point groups, i.e. C1, C2, C1h, C2v, C4, C4v, C3, C3v, C6 and C6v.1 It is the Curie symmetry principle that determines the group-subgroup relationship between paraelectric and ferroelectric phases, and thus 88 species of potential ferroelectric phase transitions are deduced. However, in some cases such as croconic acid and triglycine sulfate (TGS), the existence of pseudo center of symmetry makes it difficult to accurately recognize the ferroelectric phase. Then inspired by the Neumann's principle, which states that the symmetry of any physical property of a crystal must include the symmetry elements of the point group of the crystal, the temperature-dependent SHG effect and dielectric property become useful for detecting symmetry breaking and ferroelectricity. Consequently, in the light of the Curie symmetry principle and Neumann's principle, ferroelectrics can be effectively distinguished from innumerable compounds with various crystal structures collected in the Cambridge Structural Database. Taking advantage of such strategy and combining with the measurements of ferroelectric hysteresis loops and ferroelectric domains, we have successfully discovered a series of low-temperature and high-temperature molecular ferroelectrics with high performance.2-6 This study does help to avoid blindly searching for molecular ferroelectrics.

A family of three-dimensional chiral metal-formate frameworks of [NH(4)][M(HCOO)(3)] (M = Mn, Fe, Co, Ni, and Zn) displays paraelectric to ferroelectric phase transitions between 191 and 254 K, triggered by disorder-order transitions of NH(4)(+) cations and their displacement within the framework channels, combined with spin-canted antiferromagnetic ordering within 8-30 K for the magnetic members, providing a new class of metal-organic frameworks showing the coexistence of magnetic and electric orderings.

Tetrazole compounds have been studied for more than one hundred years and applied in various areas. Several years ago Sharpless and his co-workers reported an environmentally friendly process for the preparation of 5-substituted 1H-tetrazoles in water with zinc salt as catalysts. To reveal the exact role of the zinc salt in this reaction, a series of hydrothermal reactions aimed at trapping and characterizing the solid intermediates were investigated. This study allowed us to obtain a myriad interesting metal-organic coordination polymers that not only partially showed the role of the metal species in the synthesis of tetrazole compounds but also provided a class of complexes displaying interesting chemical and physical properties such as second harmonic generation (SHG), fluorescence, ferroelectric and dielectric behaviors. In this tutorial review, we will mainly focus on tetrazole coordination compounds synthesized by in situ hydrothermal methods. First, we will discuss the synthesis and crystal structures of these compounds. Their various properties will be mentioned and we will show the applications of tetrazole coordination compounds in organic synthesis. Finally, we will outline some expectations in this area of chemistry. The direct coordination chemistry of tetrazoles to metal ions and in situ synthesis of tetrazole through cycloaddition between organotin azide and organic cyano group will be not discussed in this review.

Piezoelectric materials produce electricity when strained, making them ideal for different types of sensing applications. The most effective piezoelectric materials are ceramic solid solutions in which the piezoelectric effect is optimized at what are termed morphotropic phase boundaries (MPBs). Ceramics are not ideal for a variety of applications owing to some of their mechanical properties. We synthesized piezoelectric materials from a molecular perovskite (TMFM)x(TMCM)1–xCdCl3 solid solution (TMFM, trimethylfluoromethyl ammonium; TMCM, trimethylchloromethyl ammonium, 0 ≤ x ≤ 1), in which the MPB exists between monoclinic and hexagonal phases. We found a composition for which the piezoelectric coefficient d33 is ~1540 picocoulombs per newton, comparable to high-performance piezoelectric ceramics. The material has potential applications for wearable piezoelectric devices.

Acentric three-dimensional coordination polymers bis(isonicotinato)zinc (1) and bis(4-pyridylacrylato)cadmium⋅H O (2) were synthesized under hydro(solvo)thermal conditions; they exhibit a threefold (see picture) and fivefold diamondoid structure, respectively. Both 1 and 2 are active for second harmonic generation and exhibit remarkable thermal stability.

A ferroelectric with a high phase‐transition temperature (Tc) is an indispensable condition for practical applications. Over the past decades, both strain engineering and the isotope effect have been found to effectively improve the Tc within ferroelectric material systems. However, the former strategy seems to prefer working in inorganic ferroelectric thin films, while the latter is also limited to some certain systems, such as hydrogen‐bonded ferroelectrics. It is noted that a mono‐fluorinated molecule is geometrically very similar to its parent molecule and the substitution of H by an F atom can introduce a chiral center on the molecule to template or stabilize polar structures. Significantly, the barrier of rotation of the fluorinated organic molecules is raised, resulting in a remarkable increase in Tc. Herein, by applying the molecular design strategy of H/F substitution to the organic–inorganic perovskite ferroelectric (pyrrolidinium)CdCl3 with a low Tc of 240 K, two high‐Tc chiral perovskite ferroelectrics, (R)‐ and (S)‐3‐F‐(pyrrolidinium)CdCl3 are successfully synthesized, for which the Tc reaches 303 K. The significant enhancement of 63 K in Tc extends the ferroelectric working temperature range to room temperature. This finding provides a new effective way to regulate the Tc in ferroelectrics and to design high‐Tc molecular ferroelectrics.

Semiconducting ferroelectricity is realized in hybrid perovskite-type compounds (cyclohexylammonium)2 PbBr4-4 x I4 x (x = 0-1). By adjusting the composition x, the bandgap is successfully tuned from previously reported 3.65 eV to as low as 2.74 eV, and the excellent ferroelectricity was kept intact. This finding may contribute to improving the photoelectronic and/or photovoltaic performance of hybrid perovskite-type compounds.