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Two organic–inorganic hybrid compounds [(CH3)3CN(CH3)2CH2F]2[MBr4] (M = Cd and Zn) undergo ferroelastic phase transitions. The substitution of metal center Cd with Zn of inorganic anions regulate the ferroelastic phase transitions.

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.

Nonlinear optical (NLO) switches and piezoelectric materials have drawn widespread attention benefiting from the enormous potential applications in photonic technology devices and smart sensors. Herein, we report two lead-free organic–inorganic hybrid materials, [(R)-3-hydroxy-pyrrolidinium]2SbBr5 (1) and [(R)-3-hydroxy-pyrrolidinium]5Sb4Br17 (2), in a ratio of 3 : 2 and 1 : 1 of (R)-pyrrolidin-3-ol and SbBr3, respectively. The introduction of homochiral cations ensures that 1 and 2 crystallize in non-centrosymmetric and chiral space groups, realizing the combination of piezoelectricity and second harmonic generation (SHG) properties. Moreover, the different proportions of reagents endow the two compounds with diverse structures and physical properties. Remarkably, 1 presents a novel bi-step SHG switch and demonstrates semiconductor properties with a band gap of 2.49 eV. 2 displays a SHG switch and a band gap of 3.00 eV, together with a piezoelectric response (d22 = 22 pC N−1). These findings offer a valuable way to design organic–inorganic hybrid materials combining multiple splendid physical properties and stimulate further exploration of the mutual relations between structures and physical properties.

A one-dimensional coordination polymer [(C2H4OH)C4H9NS]CdCl3 (1) with flexible N-(2-hydroxyethyl)thiomorpholinium cations coordinating to central metal Cd crystallized in a chiral space group P212121 and showed a reversible phase transition at 174.4/162.8 K, which corresponded to the second harmonic generation (SHG) activity and dielectric switching. The introduction of a thioether group enables 1 to adsorb Pd(II) metal ions and the SHG signal and dielectric switching disappeared after adsorption; hence, Pd(II) adsorption can be monitored by the measurements of SHG and dielectric switching. This study emphasizes the advantages of introducing functional groups into the phase transition materials and presents a new approach to design multifunctional materials.

Smart stimulus-responsive materials, which can automatically respond to external stimuli (thermal/optical/electrical) and rapidly switch between high/low states (switch ON/OFF), have been an important feature in high-tech fields such as smart electronic devices, the Internet of things, robots and so on. However, the effective design and development of these materials with simple fabrication methods, multiple response modes and logical control features are still facing enormous challenges. Herein, the star family, organic–inorganic hybrid perovskites, embraces a new member, [C5H14N]2·[CdCl4] (1), which simultaneously displays rapid dielectric and fluorescence response under the stimulus of heat. Specifically, 1 exhibits a phase transition below room temperature (Tc = 239 K, and ΔT < 8 K) including a thermal anomaly, dielectric transition, and symmetry breaking. And the Mn-doped crystal could exhibit temperature-dependent red luminescence under UV excitation with a high signal-to-noise ratio of 3. Besides, the dielectric/fluorescent dual response triggered by heat exhibits a rapid response and high fatigue resistance. This finding will be a further step to the practical application of stimulus-responsive materials based on two-dimensional perovskites in smart electronic devices.

A semiconductor material [(CH2)3NH2S]2BiCl5 (1) exhibits dielectric switching and Pd uptake functions. The thioether group in 1 provides an opportunity for Pd uptake. After 1 adsorbs Pd, the dielectric switching disappears, so that Pd adsorption can be monitored by dielectric measurements. Moreover, the material becomes an insulator after adsorbing Pd.

The research on multifunctional response materials has been rapidly increasing with the boom of the Information Age, due to their attractive applications in various smart devices. However, the design and regulation of multifunctional response materials by structural adjustments are still facing enormous challenges. Here, we report two organic–inorganic hybrids (IPTMA)2CdBr4 and (IPTMA)2MnBr4 (IPTMA = isopropyl-trimethylammonium), which exhibit excellent switchable dielectric response upon exposure to a thermal stimulus. It is considered that the changes in the metal skeletons by replacing Cd with Mn result in different functional characteristics for the two hybrids. Moreover, (IPTMA)2MnBr4 shows highly efficient green-light emission with a high quantum yield of 52.6%, and further shows another, optical response, which makes it a potential candidate for application in smart response devices. This work would offer a new perspective on the effective design of multifunctional response materials.

Multifunctional materials enable the integration of multiple physical signals in a single device for seamless integration, offering new opportunities in areas such as modern smart devices. For the needs of flexible devices, molecular functional materials are highly preferred due to their structural tunability, environmental friendliness and easy film formation. However, designing such multifunctional materials suitable for thin film devices has always been a huge challenge, especially semiconducting switchable dielectrics. Considering the unique structural properties and mechanical flexibility of plastic crystals, we have successfully designed an excellent multifunctional material, (N,N-dimethylpiperidinium)3Bi2Cl9, which shows outstanding dielectric switching and semiconducting behavior comparable to ZnO. The plastic phase transition below room temperature and multiaxial characteristics caused by structural variation give its thin film preferred structural flexibility, good uniformity and stability. These attributes make it a potential candidate in flexible multifunctional devices, realizing the high-performance integration of semiconducting and switchable properties.

Sensors and memory switches promise to optimize the extent of, and access to, switchable dielectric materials. Multiaxial time-delayed dielectric switches holding extraordinary thermal hysteresis exhibit extensive potential in delay sensing and time-delayed triggering devices. Nonetheless, efficient evoking is still scarce. Since [(CH3)3NCH2Cl]MnCl31 was discovered by us, this cation rotation pattern with motor-type motion structure has further attracted our attention. Here, organic ionic plastic crystal (OIPC) [trimethylallyl ammonium]3[Bi2Cl9] [(TMAA)3-Bi2Cl9], a high-temperature multiaxial precision time-delayed dielectric switch crystal triggered by linear/propeller/ball three-form motion, mainly stems from the order–disorder transitions of [TMAA]+ cations, which are evidenced by heat anomalies of the temperature-dependent dielectric and differential scanning calorimetry (DSC) measurements, and the variable-temperature powder X-ray diffractometry (PXRD). The long time needed for a molecule to transition from rotational motion to a static condition can be ascribed to the exceptional linear-propeller-global ball-like motion, inducing the large thermal hysteresis. Intriguingly, the motion-stationary mode is reversible, that is to say, the thermal hysteresis temperature holds constant in each time, which is highly conducive to the precision time-delayed sensing. More importantly, our work will prompt the generation and evolution of novel delay sensors and switches, which makes it a new upsurge in high-performance dielectric response materials with multiaxial natures.

Three new organic molecule-based compounds, ethyl-trimethyl-phosphonium picrate (1, [ETtmp][picrate]), hydroxymethyl-trimethyl-phosphonium picrate (2, [HMtmp][picrate]), and cyclopentyl-trimethyl-phosphonium picrate (3, [CPtmp][picrate]), have been corroborated as high-temperature phase transition materials possessing switchable dielectric behaviors. Compounds 1, 2 and 3 undergo dielectric anomalies which could be tuned in two pronounced dielectric states and switched by reversible phase transitions at 320.8 K, 393.9 K and 398.1 K, respectively. For compounds 1, 2 and 3, not only the phase transition temperatures but also the magnitudes of the dielectric anomalies presented variations by regulating the guest cations. The respective dielectric constants in the high dielectric states are 1.6, 2.7 and 3.6 times those in the low dielectric states for compounds 1, 2 and 3. And considering the big entropy changes (ΔS) in the three title compounds, we predicted the sensitivities of the phase transition temperatures (Tc) to the applied pressure. And the estimated barocaloric coefficients (δTc/δP) in the three title compounds indicate that they potentially perform barocaloric effects of interest for cooling applications under adequate pressure.

An organic-inorganic perovskite compound [(CH3)4P][Cd(SCN)3] (1) and its fluorine-substituted product [(CH3)3PCH2F][Cd(SCN)3] (2) exhibit ferroelastic phase transitions above room temperature. The very close van der Waals radii of H and F atoms ensure isomorphism of the crystal structures. However, the higher phase transition temperature, stronger ferroelastic spontaneous strain value and dielectric properties of 2 can possibly be explained by differences in the electronegativity between F and H atoms.

Phase transition materials are a class of smart materials with special optoelectronic properties. In particular, high symmetry has proven to be the most advantageous feature for thin film devices. However, phase transition design, particularly with high symmetry, is a huge challenge. Herein, based on the advanced molecule reported by us, [(CH3)3N(CH2)2Cl]2[ZnCl4] (1) was verified to be a phase transition dielectric crystal with an infrequent large leap symmetry breaking from P21/c to I41/a at 333 K. The dielectric value exhibited a remarkable step-like enlargement around TC, suggesting a standard and visual low-high interchange of dielectric switching. The existence of dielectric switching properties triggered by a thermal stimulus is intrinsically induced by the anionic and cationic order–disorder motion. It creates a little anisotropy along all axial directions in numerical values and cardinal numbers, which is very beneficial for the cultivation of large-size crystals and the fabrication of corresponding devices.

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.

Molecular bistable dielectric switches represent a class of highly desirable intelligent materials due to their sensitive switchable responses, simple and environmentally friendly processing, light weights, and mechanical flexibility. However, most of these switches can only work at a very low temperature, extremely limiting their potential applications. Herein, three layered organic-inorganic hybrid perovskite-type compounds of the general formula A2BX4, [NH3(CH2)2Cl]2[CdCl3Br] (1), [NH3(CH2)2Cl]2[CdCl4] (2) and [NH3(CH2)2Cl]2[CdBr4] (3), which display sensitive dielectric switching reversibility and remarkable switching anti-fatigue, have been successfully designed. Differential scanning calorimetry and dielectric measurements for 1 confirm its reversible phase transition at around 166 K. Through anion modulation, the phase transition temperatures of 2 and 3 can be greatly improved up to 237 K and 254 K, respectively. Structural analysis discloses that the phase transition temperature's shifts may result from the differences among the inorganic frameworks. Moreover, due to the significant order-disorder transitions of ammonium cations, the permittivities of 1, 2 and 3 change abruptly at around the phase transition points, making them excellent stimuli-responsive electrical switches. Such an anion-modulated method will open up new possibilities of highly efficiently tuning the phase transition temperature of molecular bistable dielectric switches.

Three organic–inorganic hybrid analogues, ([Et3(n-Pr)P][Mn(dca)3]) 1, ([Et3(CH2CHCH2)P][Mn(dca)3]) 2, ([Et3(CH2OCH3)P][Mn(dca)3]) 3, [dca = dicyanamide, N(CN)2−], show similar three-dimensional perovskite frameworks, in which the guest phosphonium cations occupy the cavities. Compounds 1 and 3 belong to the orthorhombic noncentrosymmetric space group P212121 at room temperature, while compound 2 crystallizes in the monoclinic centrosymmetric space group P21/c. Differential scanning calorimetry (DSC) and dielectric measurements confirmed the phase transitions in compounds 1, 2 and 3, where subtle structural distinctions of guest cations affect the crystal lattices, phase transition temperatures and physical properties. Interestingly, compounds 1 and 3 are SHG active at room temperature and can be used as NLO switches tuned by guest cations and triggered by temperature.

A bistable optical–electrical duple switch belongs to a class of highly satisfying intelligent materials that can transform optical and electrical responses simultaneously in one device. A perovskite-type high-temperature phase transition compound with one-dimensional chain-like crystal structure, ([(CH3)3PCH2OCH3][PbBr3], 1), displays remarkable bistable photoluminescent-dielectric duple switching behaviors. The noteworthy order–disorder transition of the phosphonium cation and the motions of anions contribute to the phase transition, leading to the space group P21/c at a low temperature phase to C2/c at a high temperature phase. 1 exhibits a prominent step-like dielectric anomaly at 401.0 K and demonstrates novel optical properties with a band gap of 3.54 eV. The photoluminescence intensity suddenly declines from 398 K to 408 K, which may be attributed to the occurrence of phase transition. The electron cloud distributions of the frontier orbital in compound 1 have been calculated using a DFT program.

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.

A new organic–inorganic perovskite-type compound [(CH3)3PCH2OH][CdBr3] exhibits a ferroelastic phase transition at 339 K. Domain structures were observed and analyzed. The origin of the phase transition can be attributed to the motion or reorientation of the [(CH3)3PCH2OH]+ cations and the displacement of Cd2+ and Br− ions in solid-state crystals.

Three analogue N-heterocyclic complexes, 1-propyl-1-methylpiperidinium perchlorate (1, [PMpip][ClO4]), 1-cyanomethyl-1-methylpiperidinium perchlorate (2, [CMpip][ClO4]), and 1-cyanomethyl-1-methylmorpholinium perchlorate (3, [CMmor][ClO4]) are identified as phase transition materials displaying switchable dielectric behaviors. Despite the common [ClO4]− anion and the closely related cations, compound 1 crystallizes in the orthorhombic space group P212121, but compounds 2 and 3 belong to the monoclinic space group P21/n with distinct cell dimensions. Compounds 1, 2 and 3 undergo reversible phase transitions around 199, 387 and 416 K, respectively, accompanied by notable step-like dielectric anomalies which could be switched by the phase transition and be tuned in distinct dielectric states. The respective dielectric constants in the high dielectric states are 1.2, 2.2 and 3.2 times that in the low dielectric states for compounds 1, 2 and 3. Generally, these differences in the phase transitions and dielectric properties are caused by the distinct molecular structures and hydrogen-bonding conformations resulting from the structural variations in the side-chain and the ring structure.

The hydrogen-bonding supramolecule with 4-ethoxyanilinium and [18]crown-6 is introduced to [Ni(dmit)2]− salt. The arrangement of supramolecular cations formed a three-dimensional structure with the one-dimensional channel filled with [Ni(dmit)2]− anions. The temperature-dependent structural analyses and DSC measurement disclose the first-order phase transition occurred around 285 K, where the lattice parameters show an abrupt change without a space group change. The title compound crystallizes in No. 1 space group P-1 and is piezoelectrically active with d33 value of 4.8 pC N−1. The frequency- and temperature-dependent dielectric constants and potential energy calculation are consistent with the forward–backward motion of the ethoxyl group in the cation.