Ji-Xing Gao

Dielectric and nonlinear optical dual switching in an organic–inorganic hybrid relaxor [(CH3)3PCH2OH][Cd(SCN)3]

A new organic–inorganic hybrid compound [(CH3)3PCH2OH][Cd(SCN)3] (1) has been synthesized, which exhibits a reversible phase transition at 248.5 K confirmed by differential scanning calorimetry. The phase transition in 1 is from a centrosymmetric space group Pmcn to a non-centrosymmetric space group P21, so that 1 exhibits a switchable second harmonic generation (SHG) effect between SHG-on and SHG-off states. This phase transition also displays switchable dielectric behaviors between high and low dielectric states accompanied by the remarkable dielectric relaxation described by the Cole–Cole equation. Variable-temperature single-crystal X-ray diffraction analyses reveal that the origin of the phase transition can be attributed to the motion or reorientation of the [(CH3)3PCH2OH]+ cations and the movement of (SCN)− ions in solid-state crystals. These superior physical properties suggest that 1 could be a potential switchable dielectric and NLO relaxor-type material, which provides a new approach to design novel multiple switch materials.

Notable Broad Dielectric Relaxation and Highly Efficient Red Photoluminescence in a Perovskite-Type Compound: (N-Methylpyrrolidinium)MnCl3

Multifunctional-material-integrated various properties have been an attractive research field. In spite of persistent explorations into such materials, the conditions of coexistence are still confused. Organic-inorganic hybrid compounds are appropratie for designing these materials because of both their rich properties and flexible compositions. Here, a perovskite-type organic-inorganic hybrid compound, (Hmpy)MnCl3 (1; Hmpy = N-methylpyrrolidinium), with temperature, light, and electric stimuli-response characteristics has been rationally designed and synthesized. This hybrid compound shows a dielectric anomaly, a broad dielectric dispersion of the temperature range from 296 to 400 K, and brilliant red fluorescence at 632 nm with a high quantum yield of 54.54% under UV excitation. The coexistence of temperature, optical, and electric multiple stimuli-response properties in 1 demonstrates that our finding has a profound influence on the further exploration of the novel multifunctional materials.

Unusual two-step sequential reversible phase transitions with coexisting switchable nonlinear optical and dielectric behaviors in [(CH3)3NCH2Cl]2[ZnCl4]

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.

High-temperature sequential structural transitions with distinct switchable dielectric behaviors in two organic ionic plastic crystals: [C4H11NBr] [ClO4] and [C4H11NBr] [BF4]

Two new organic ionic plastic crystals (OIPCs), [TMBM] [ClO4] (1) and [TMBM] [BF4] (2) (TMBM = trimethylbromomethyl ammonium), were successfully synthesized. Systematic characterization via differential scanning calorimetry (DSC), variable-temperature single-crystal X-ray diffraction, variable-temperature powder X-ray diffraction (PXRD) and dielectric measurements demonstrated that 1 and 2 undergo reversible high-temperature sequential phase transitions accompanied by distinct stepwise switchable dielectric anomalies (1 at 358 K and 388 K and 2 at 346 K and 379 K). The structural relatively ordered-disordered transitions of ClO4−, BF4− and [TMBM]+ contributed to the switching of the dielectric constant from the low-dielectric state to the high-dielectric state. Thus, OIPCs consisting of spherical ammonia cations and inorganic acid anions are proven to be promising in the search for high-temperature multi-step switchable dielectric materials.

Modulating molecular structures and dielectric transitions in organic‒inorganic hybrid crystals

In this paper, three new hybrid phase transition compounds, [Hmpy]CdBr3 (1, Hmpy = N-methylpyrrolidinuium cation), [Hmpy]2CdBr4 (2) and [Hmpy]3CdBr3·CdBr4 (3), were synthetized by means of regulating the ratio of reactants. Systematic characterizations consisting of variable temperature X-ray single crystal diffraction, differential scanning calorimetry (DSC) and dielectric measurements reveal that 1 with infinite one-dimensional (1D) [CdBr3]n− chains undergoes a phase transition around 278 K; 2 with isolated [CdBr4]2− tetrahedrons exhibits a high-temperature phase transition close to 367 K, accompanied by prominent switchable dielectric behavior. Interestingly, 1D [CdBr3]n− chains and isolated [CdBr4]2− tetrahedrons both exist in 3, associated with a phase transition at 320 K. The phase transitions in the three compounds are originated from the order-disorder transitions of the Hmpy cation. It is expected that our finding would promote the development of hybrid dielectric transition materials with adjustable properties.

A high-temperature molecular ferroelastic phase transition and switchable dielectric response in the trimethylbromomethylammonium salt [C4H11NBr] [PF6]

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.

Switchable dielectric phase transition behaviors in two organic-inorganic copper(II) halides with distinct coordination geometries

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.

Halogen Substitution Effects on Optical and Electrical Properties in 3D Molecular Perovskites

Both 3D organic-inorganic perovskites ([Et3P(CH2)2Cl][Cd(dca)3] (1) and [Et3P(CH2)2F][Cd(dca)3] (2) [dca = dicyanamide, N(CN)2-]) display two sequentially reversible high-temperature phase transitions and switchable dielectric properties. Through halogen substitution, 1 shows exceptional switching behaviour of second harmonic generation effects and remarkably 2 represents the first above-room-temperature 3D ferroelastic material characterized by two ferroelastic phases.