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Switchable nonlinear optical (NLO) materials have aroused broad interest on account of their captivating optical and electronic properties. We demonstrate a novel perovskite-type crystal with exceptional hydrogen bond interactions that are associated with the onset of reorientational motions of organic cations and thus induce the occurrence of two successive phase transitions to be a two-step NLO switch. This finding affords an alternative approach for the design and assembly of switchable NLO materials.

Organic–inorganic hybrid materials with a high-temperature phase transition have been widely applied as a kind of smart material. In this work, a new organic–inorganic 2H-perovskite compound, namely [Me3NCH2CH3]CdCl3 (1), is reported with a high temperature dielectric transition showing excellent reversibility and sustainability. 1 has an ABX3 hybrid perovskite structure, where A is a [Me3NCH2CH3]+ cation, B is a divalent metal atom, and X is a halogen atom. Differential scanning calorimetry (DSC) and dielectric measurements were applied for further characterization. Variable temperature single-crystal X-ray diffraction revealed that 1 crystallizes in the orthorhombic Pbca space group (No. 61) at 293 K and hexagonal P63/mmc space group (No. 194) at 363 K. The structural phase transition is primarily attributed to an ordered–disordered type resulting from the [Me3NCH2CH3]+ cations. Organic–inorganic perovskite compounds, as a result of a long exploration history and forward-looking prospects, can deepen research into molecular materials and their physical properties can be effectively optimized in the next-generation of temperature-responsive electrical switches.

In this paper, three zero-dimensional organic–inorganic hybrid compounds [(CH3)3S]2[CdBr4] (1), [(CH3)3S]2[MnBr4] (2) and [(CH3)3S]2[ZnBr4] (3) were synthesized. The phase transition behavior of 1, 2 and 3 was well characterized by differential scanning calorimetry (DSC) and variable temperature single crystal diffraction measurements. The phase transition temperature of 1, 2 and 3 was at ca. 315 K in the heating process. The vigorous ordered–disordered reorientation and displacement motion of [(Me3)3S]+ and [MBr4]2− of 1, 2 and 3 induce the structural phase transition from the centrosymmetric (CS) space group Pnma to the non-centrosymmetric (NCS) space group P212121. The apparent second-harmonic generation (SHG) switching responses further confirm this CS to NCS symmetry breaking. Moreover, dielectric studies illustrate that 1, 2 and 3 display distinctly switchable dielectric behavior, revealing their potential application in dielectric switches. This finding suggests that sulfonium-based organic–inorganic hybrids can be used to build phase transition materials, broadening the way for exploring dielectric and nonlinear optical (NLO) switching materials.

A novel organic-inorganic ABX3 perovskite-type material with specific hydrogen bonding interactions, N,N-dimethylethanolammonium trichlorocadmate ([DMEA]CdCl3), has been synthesized as a phase transition material. It is notable that the DMEA cations are arranged to form one-dimensional chains connected by hydrogen bonds at room temperature, which are very sparse in other perovskite-type compounds. The strong intermolecular interactions have made the phase transition temperature of the material reach up to 429 K, as confirmed by differential scanning calorimetry measurements, variable-temperature structural analyses, and dielectric measurements. The origin of the symmetry-breaking phase transition is associated with the motion or reorientation of the DMEA cations, accompanied by the crystal structures from orthorhombic Pnma to monoclinic P21/c with the temperature decreases. The finding of [DMEA]CdCl3 with unprecedented hydrogen bonding interactions has opened a new avenue to design novel phase transition materials with higher transition temperatures.

By introducing halogens to 16 symmetric tetramethylammonium, one new molecular ferroelastic compound [TMBMA] [PF6] (1) (TMBMA = trimethylbromomethylammonium) was successfully synthesized. Differential scanning calorimetry (DSC) measurements demonstrated that 1 underwent a typical first-order phase transition at ca. 380 K with a large thermal hysteresis of 30 K. Single crystal X-ray diffraction data, the variable-temperature powder X-ray diffraction (PXRD) patterns and the complex domain-wall orientations suggested that the phase transition from monoclinic (P21/c) to cubic (Pmm) belongs to a ferroelastic phase transition with an Aizu notation of mmF2/m. Dielectric studies show that 1 exhibits distinct switchable dielectric behavior and satisfactory thermal stability, revealing its potential application in dielectric switches. As expected, the TMBMA cations bearing an ordered–disordered motion turn out to be crucial in the generation of the ferroelastic phase transition. Therefore, this molecular design strategy of introducing halogens to tetraalkylammoniums can not only reduce the symmetry of the tetraalkylammonium molecules at room temperature, but can also maintain the highly disordered characteristics at high temperatures. This provides an efficient route for us to find new molecular ferroelastics and dielectric switches.

Bistable optical-electrical duplex switches represent a class of highly desirable intelligent materials because of their potential applications in the fields of next-generation flexible devices. However, controllable photoelectric switchable materials with high-performance dielectric-switching and optical-switching properties are still scarce, with triplex bistable switches being rarely reported. Herein, a novel optoelectronic triple-functional organic-inorganic material, 5-azonia-spiro[4,4]nonane tetrabromomanganese (1, [ASN]2[MnBr4], ASN = (CH2)4N(CH2)4), which undergoes a reversible solid-state phase transition around 327 K and exhibits a recognizable second harmonic generation (SHG) effect between SHG-on and SHG-off states, has been successfully synthesized and grown as block crystals. Intriguingly, the bromine-doped crystal can exhibit intense green luminescence with a high quantum yield of 13.07% under UV excitation, extending its application in the field of photoelectric seamless integration and/or flexible multifunctional devices.

Two new organic–inorganic chlorocuprates(II), [(ClCH2)(CH3)3N]2CuCl4 (1), which contains a discrete [CuCl4]2− tetrahedron, and [(ClCH2)(CH3)3N]CuCl3 (2), which comprises one-dimensional [CuCl3]− chains of an edge-sharing CuCl5 pyramid, have been synthesized. Compound 1 exhibits a symmetry-breaking phase transition at 335.2 K, while 2 undergoes an isostructural phase transition at 333.8 K, as disclosed by differential scanning calorimetry measurements and variable-temperature X-ray diffraction analyses. The dielectric measurements reveal that both compounds 1 and 2 show prominent switchable dielectric activities between high and low dielectric states in the vicinity of the phase transition temperature. In addition, the contrast between the two dielectric states in 1 is much higher than that in 2, which is related to the more remarkable dynamic change of the [(ClCH2)(CH3)3N]+ cation in 1 during the phase transition process. It is believed that this work will pave a new avenue for the design of multifunctional phase transition materials by exploring the area of Cu2+-based organic–inorganic metal halides.

Two isostructural ionic cobalt(ii) halides, trimethylchloromethyl ammonium tetrachlorocobalt(ii) (1, [Me3NCH2Cl]2[CoCl4]) and trimethylbromomethyl ammonium tetrabromocobalt(ii) (2, [Me3NCH2Br]2[CoBr4]), have been discovered as new high-temperature phase transition materials. 1 exhibits two successive structural phase transitions from P212121 to P21/c at 252 K and then to Pnma at 335 K, accompanied by step-like dielectric anomalies. The two-step sequential reversible phase transitions of 1 derive from the order-disorder transformation of CoCl4 anions and organic Me3NCH2Cl cations, respectively. Its analogue 2 also crystallizes in P21/c at 293 K by changing the halogen atoms of both cations and anions of 1. Unlike 1, 2 displays only one structural phase transition with a much higher transition temperature at 387 K, which is mainly influenced by intermolecular interactions and more energy is required to provide the freedom of the motion. All the findings in the present work indicate that the phase transition temperature and properties of materials can be tuned by using halogens in this series of compounds, which could open a new way to design and assemble novel high-temperature phase transition materials.

A novel material with coexisting switchable nonlinear optical (NLO) and dielectric properties, [trimethylchloromethyl ammonium]2[ZnCl4] [(TMCM)2-ZnCl4], which undergoes two reversible solid state phase transitions at 337 K (Tc1) and 258 K (Tc2), respectively, has been successfully synthesized and grown as block crystals. Exceptionally, variable-temperature X-ray single crystal structural analyses of (TMCM)2-ZnCl4 reveal that the unusual space group changes from P212121 at 223 K to P21/c between Tc2 and Tc1 to Pmcn above Tc1. As expected, the dielectric value of (TMCM)2-ZnCl4 exhibits two remarkable step-like enlargements around Tc1 and Tc2, indicating that it exhibits a low dielectric state, an intermediate dielectric state and a high dielectric state. In particular, the second harmonic generation (SHG) efficiency shows a sharp step-like increase from almost zero to 0.47, and its SHG activities in a high-NLO state can be recovered after several cycles without any attenuation. The transitions of (TMCM)2-ZnCl4 could be intrinsically ascribed to the sequential order–disorder transformations of both the cations and anions. All these results demonstrate its potential application as a switchable and tunable molecular dielectric and NLO material.

Following our recent findings on dielectric materials, we synthesized a new host–guest supramolecular metal–halide compound, [(2-AMPD)(18-crown-6)]CuCl4 (1, 2-AMPD = 2-aminomethylpiperidinium). Systematic characterization techniques such as variable-temperature crystal structure analyses, differential scanning calorimetry (DSC) measurements, temperature-dependent dielectric measurements and powder X-ray diffraction (PXRD) measurements demonstrate that 1 undergoes a reversible phase transition at room temperature, accompanied by switchable dielectric responses and remarkable anisotropy along three different crystallographic axes. The structural phase transition mechanism is triggered by the order–disorder transition of the 18-crown-6 molecules. We believe that these findings might further promote the application of a host–guest inclusion compound in the field of switchable dielectric materials.