Ferroelectric hybrid organic-inorganic perovskites and their structural and functional diversity

Orientational Disorder and Molecular Correlations in Hybrid Organic-Inorganic Perovskites: From Fundamental Insights to Technological Applications

Hybrid organic-inorganic perovskites (HOIP) have emerged in recent years as highly promising semiconducting materials for a wide range of optoelectronic and energy applications. Nevertheless, the rotational dynamics of the organic components and many-molecule interdependencies, which may strongly impact the functional properties of HOIP, are not yet fully understood. In this study, we quantitatively analyze the orientational disorder and molecular correlations in archetypal perovskite CH3NH3PbI3 (MAPI) by performing comprehensive molecular dynamics simulations and entropy calculations. We found that, in addition to the usual vibrational and orientational contributions, rigid molecular rotations around the C-N axis and correlations between neighboring molecules noticeably contribute to the entropy increment associated with the temperature-induced order-disorder phase transition, ΔSt. Molecular conformational changes are equally infrequent in the low-T ordered and high-T disordered phases and have a null effect on ΔSt. Conversely, the couplings between the angular and vibrational degrees of freedom are substantially reinforced in the high-T disordered phase and significantly counteract the phase-transition entropy increase resulting from other factors. Furthermore, the tendency for neighboring molecules to be orientationally ordered is markedly local, consequently inhibiting the formation of extensive polar nanodomains at both low and high temperatures. This theoretical investigation not only advances the fundamental knowledge of HOIP but also establishes physically insightful connections with contemporary technological applications like photovoltaics, solid-state cooling, and energy storage.

Flexible mechano-optical dual-responsive perovskite molecular ferroelectric composites for advanced anticounterfeiting and encryption

Hybrid organic-inorganic molecular ferroelectrics have emerged as promising materials for multifunctional piezoelectric devices. However, they present challenges in practical applications because of their inherent brittleness and poor ductility. Herein, we present a flexible mechano-optical dual-responsive molecular ferroelectric composite by incorporating trimethylchloromethyl ammonium (TMCM)-MnCl3 into styrene ethylene butylene styrene (SEBS) matrix. The SEBS/TMCM-MnCl3 exhibits excellent stretchable mechanical properties (tensile strain >1300%, thickness of 30 μm), piezoelectricity, and photoluminescence, enabling advanced visual-tactile-fused anticounterfeiting and encryption applications. Anticounterfeiting and antitampering tags are developed to judge whether the valued items are true or tampered with based on pattern recognition and piezoelectric response, respectively. Additionally, high-security password keyboards featuring triple-layer encryption are designed, offering more password combinations (524,288 times greater than those of traditional password devices relying solely on digital encryption) and enhanced security reliability against cracking attempts. This work can inspire designs of multifunctional optoelectronic materials and enable visual-tactile-fused intelligent applications in human-machine interfaces, information security, and advanced robotics.

Molecular Engineering Regulation Achieving Out-of-Plane Polarization in Rare-Earth Hybrid Double Perovskites for Ferroelectrics and Circularly Polarized Luminescence

Out-of-plane polarization is a highly desired property of two-dimensional (2D) ferroelectrics for application in vertical sandwich-type photoferroelectric devices, especially in ultrathin ferroelectronic devices. Nevertheless, despite great advances that have been made in recent years, out-of-plane polarization remains unrealized in the 2D hybrid double perovskite ferroelectric family. Here, from our previous work 2D hybrid double perovskite HQERN ((S3HQ)4EuRb(NO3)8, S3HQ=S-3-hydroxylquinuclidinium), we designed a molecular strategy of F-substitution on organic component to successfully obtain FQERN ((S3FQ)4EuRb(NO3)8, S3FQ=S-3-fluoroquinuclidinium) showing circularly polarized luminescence (CPL) response. Remarkably, compared to the monopolar axis ferroelectric HQERN, FQERN not only shows multiferroicity with the coexistence of multipolar axis ferroelectricity and ferroelasticity but also realizes out-of-plane ferroelectric polarization and a dramatic enhancement of Curie temperature of 94 K. This is mainly due to the introduction of F-substituted organic cations, which leads to a change in orientation and a reduction in crystal lattice void occupancy. Our study demonstrates that F-substitution is an efficient strategy to realize and optimize ferroelectric functional characteristics, giving more possibility of 2D ferroelectric materials for applications in micro-nano optoelectronic devices.

The H/F substitution strategy can achieve large spontaneous polarization in 1D hybrid perovskite ferroelectrics

Hybrid organic-inorganic perovskite (HOIP) ferroelectrics exhibit polarization reversibility and have a wide range of applications in the fields of smart switches, memorizers, sensors, etc. However, the inherent limitations of small spontaneous polarization (P s) and large coercive field (E c) in ferroelectrics have impeded their broader utilization in electronics and data storage. Molecular ferroelectrics, as a powerful supplement to inorganic ferroelectrics, have shown great potential in the new generation of flexible wearable electronic devices. The important research responsibility is to greatly improve progressiveness and overcome the above limitations. Here, a novel one-dimensional (1D) HOIP ferroelectric, (3-F-BTAB)PbBr3 (3-F-BTAB = 3-fluorobenzyltrimethylammonium), was successfully synthesized by employing the H/F substitution strategy to modify parent compound (BTAB)PbBr3 (BTAB = benzyltrimethylammonium), which undergoes a ferroelectric phase transition with Aizu notation 2/mF2 at 420 K. Notably, (3-F-BTAB)PbBr3 demonstrates exceptional ferroelectric properties with a large P s of 7.18 μC cm-2 and a low E c of 1.78 kV cm-1. As far as we know, (3-F-BTAB)PbBr3 features the largest P s among those reported for 1D lead-based HOIP ferroelectrics. This work enriches the 1D lead-based ferroelectric family and provides guidance for applying ferroelectrics in low-voltage polar memories.

Designing Zero-Dimensional Cadmium-Based Organic-Inorganic Hybrids: The Role of Halogen Doping in Modulating Multifunctional Properties

Advances in materials science are increasingly dependent on the development of multifunctional materials capable of improving system efficiency and reducing the environmental impact. In this study, two zero-dimensional (0D) cadmium-based organic-inorganic hybrid materials (BEMPD)2CdBr4 (BEMPD-Br, 1) and (BEMPD)2CdBr2Cl2 (BEMPD-ClBr, 2) (BEMPD = 1-(2-bromoethyl)-1-methylpiperidine) were prepared by halogen doping. Compound 2 is a mixed halide in which there are two halogen sites, Cl and Br, and in a disordered state, which has a regulatory effect on the structural distortion and properties of the compound. The Curie temperatures of compounds 1 and 2 are 348 and 390 K, respectively, and the UV-vis absorption spectra showed that the direct band gaps of compounds 1 and 2 were 4.68 and 4.8 eV, respectively. In addition, room-temperature photoluminescence experiments show broadband emission peaks at 717 and 683 nm for compounds 1 and 2, respectively, with fluorescence lifetimes of 2.414 and 3.915 μs. These 0D hybrids provide an avenue for the development of smart materials and optoelectronic devices, and also provide positive clues for manipulating the properties of organic-inorganic hybrid compounds.

The Halogenation Effect Induces a Variety of Switchable Phase Transition and Second-Harmonic-Generation Materials

Halogen engineering offers a means of enhancing the physical properties of materials by fine-tuning the rotational energy barrier and dipole moment, which proved to be effective in achieving switchable phase transitions and optical responses in materials. In this work, by substituting the methyl group in ligand N-ethyl-1,5-diazabicyclo[3.3.0]octane (CH3CH2-3.3.0-Dabco) with halogen atoms X (Cl or Br) and then contining to react it with FeBr3 in a HBr aqueous solution, we successfully synthesized three kinds of organic-inorganic hybrid switchable phase-change materials, [CH3CH2-3.3.0-Dabco]FeBr4 (1), [ClCH2-3.3.0-Dabco]FeBr4 (2), and [BrCH2-3.3.0-Dabco]FeBr4 (3), which were fully characterized by single-crystal X-ray diffraction and variable-temperature powder X-ray diffraction. Compared to compound 1, compounds 2 and 3 show two pairs of reversible phase transitions, dielectric anomalies, and a second-harmonic-generation effect, which are successfully induced due to the halogen substitution. This study offers an effective molecular design strategy for the exploration and construction of iron halide organic-inorganic hybrid materials with temperature-adjustable physical properties.

Electrically Induced Crystal Field Distortion in a Ferroelectric Perovskite Revealed by Electron Paramagnetic Resonance

The magnetoelectric material has attracted multidisciplinary interest in the past decade for its potential to accommodate various functions. Especially, the external electric field can drive the quantum behaviors of such materials via the spin-electric coupling effect, with the advantages of high spatial resolution and low energy cost. In this work, the spin-electric coupling effect of Mn2+-doped ferroelectric organic-inorganic hybrid perovskite [(CH3)3NCH2Cl]CdCl3 with a large piezoelectric effect was investigated. The electric field manipulation efficiency for the allowed transitions was determined by the pulsed electron paramagnetic resonance. The orientation-included Hamiltonian of the spin-electric coupling effect was obtained via simulating the angle-dependent electric field modulated continuous-wave electron paramagnetic resonance. The results demonstrate that the applied electric field affects not only the principal values of the zero-field splitting tensor but also its principal axis directions. This work proposes and exemplifies a route to understand the spin-electric coupling effect originating from the crystal field imposed on a spin ion being modified by the applied electric field, which may guide the rational screening and designing of hybrid perovskite ferroelectrics that satisfy the efficiency requirement of electric field manipulation of spins in quantum information applications.

Highly Stable MOF-Type Lead Halide Luminescent Ferroelectrics

Lead halide molecular ferroelectrics represent an important class of luminescent ferroelectrics, distinguished by their high chemical and structural tunability, excellent processability and distinctive luminescent characteristics. However, their inherent instability, prone to decomposition upon exposure to moisture and light, hinders their broader ferroelectric applications. Herein, for the first time, we present a series of isoreticular metal-organic framework (MOF)-type lead halide luminescent ferroelectrics, demonstrating exceptional robustness under ambient conditions for at least 15 months and even when subjected to aqueous boiling conditions. Unlike conventional metal-oxo secondary building units (SBUs) in MOFs adopting highly centrosymmetric structure with limited structural distortion, our lead halide-based MOFs occupy structurally deformable [Pb2X]+ (X=Cl-/Br-/I-) SBUs that facilitate a c-axis-biased displacement of Pb2+ centers and substantially contribute to thermoinducible structural transformation. Importantly, this class of MOF-type lead halide ferroelectrics undergo ferroelectric-to-paraelectric phase transitions with remarkably high Curie temperature of up to 505 K, superior to most of molecular ferroelectrics. Moreover, the covalent bonding between phosphorescent organic component and the light-harvesting inorganic component achieves efficient spin-orbit coupling and intersystem crossing, resulting in long-lived afterglow emission. The compelling combination of high stability, ferroelectricity and afterglow emission exhibited by lead halide MOFs opens up many potential opportunities in energy-conversion applications.

Giant Anisotropic Thermal Expansion Phase Transition of Silver Iodide Anionic Organic-Inorganic Hybrid

Hybrid metal halide materials with charming phase transition behaviors have attracted considerable attention. In former works, much attention has been focused on the phase transition triggered by the order-disorder or displacement motions of the organic component. However, manipulating the variation of the inorganic component to achieve the phase transition has rarely been reported. Herein, two novel organic-inorganic hybrid materials, [THPM]n[AgX2]n (THPM = 3,4,5,6-tetrahydropyrimidin-1-ium, X = I for 1 and Br for 2) with the [AgX2]nn- anionic chain structure, were synthesized. At 293 K, the [AgX2]nn- chains in 1 were constructed by the tetramer units of Ag atoms, while that in 2 was assembled by the dimer structure. Upon heating to 355 K, owing to the variation of the metallophilic interaction between adjacent Ag atoms, a unique transformation process from tetramer to dimer in [AgI2]nn- chains of 1 can be detected and endow 1 with a giant anisotropic thermal expansion with linear strain of ∼7% and shear strain of ∼20%, which can be used as a mechanical actuator for switching. Alternatively, for 2, no phase transition process can be observed upon the temperature variation. This work provides an effective approach to design phase transition materials triggered by the inorganic part.

Emerging Spintronic Materials and Functionalities

The explosive growth of the information era has put forward urgent requirements for ultrahigh-speed and extremely efficient computations. In direct contrary to charge-based computations, spintronics aims to use spins as information carriers for data storage, transmission, and decoding, to help fully realize electronic device miniaturization and high integration for next-generation computing technologies. Currently, many novel spintronic materials have been developed with unique properties and multifunctionalities, including organic semiconductors (OSCs), organic-inorganic hybrid perovskites (OIHPs), and 2D materials (2DMs). These materials are useful to fulfill the demand for developing diverse and advanced spintronic devices. Herein, these promising materials are systematically reviewed for advanced spintronic applications. Due to the distinct chemical and physical structures of OSCs, OIHPs, and 2DMs, their spintronic properties (spin transport and spin manipulation) are discussed separately. In addition, some multifunctionalities due to photoelectric and chiral-induced spin selectivity (CISS) are overviewed, including the spin-filter effect, spin-photovoltaics, spin-light emitting devices, and spin-transistor functions. Subsequently, challenges and future perspectives of using these multifunctional materials for the development of advanced spintronics are presented.

Crystal Structure and Optical Properties Characterization in Quasi-0D Lead-Free Bromide Crystals (C6H14N)3Bi2Br9·H2O and (C6H14N)3Sb3Br12

Low dimensional organic inorganic metal halide materials have shown broadband emission and large Stokes shift, making them widely used in various fields and a promising candidate material. Here, the zero-dimensional lead-free bromide single crystals (C6H14N)3Bi2Br9·H2O (1) and (C6H14N)3Sb3Br12 (2) were synthesized. They crystallized in the monoclinic crystal system with the space group of P21 and P21/n, respectively. Through ultraviolet-visible-near-infrared (UV-vis-NIR) absorption analysis, the band gaps of (C6H14N)3Bi2Br9·H2O and (C6H14N)3Sb3Br12 are found to be 2.75 and 2.83 eV, respectively. Upon photoexcitation, (C6H14N)3Bi2Br9·H2O exhibit broad-band red emission peaking at 640 nm with a large Stokes shift of 180 nm and a lifetime of 2.94 ns, and the emission spectrum of (C6H14N)3Sb3Br12 are similar to those of (C6H14N)3Bi2Br9·H2O. This exclusive red emission is ascribed to the self-trapping exciton transition caused by lattice distortion, which is confirmed through both experiments and first-principles calculations. In addition, due to the polar space group structure and the large spin-orbit coupling (SOC) associated with the heavy elements of Bi and Br of crystal 1, an obvious Rashba effect was observed. The discovery of organic inorganic metal bromide material provides a critical foundation for uncovering the connection between 0D metal halide materials' structures and properties.

Chiral-Achiral Cations Intercalation Induced Lead-Free Chiral-Polar Hybrid Perovskites Enable Self-Powered X-Ray and Ultraviolet-Visible-Near-Infrared Photo Detection

Lead halide hybrid perovskites have made great progress in direct X-ray detection and broadband photodetection, but the existence of toxic Pb and the demand for external operating voltage have severely limited their further applications and operational stability improvements. Therefore, exploring "green" lead-free hybrid perovskite that can both achieve X-ray detection and broadband photodetection without external voltage is of great importance, but remains severely challenging. Herein, using centrosymmetric (BZA)3BiI6 (1, BZA = benzylamine) as a template, a pair of chiral-polar lead-free perovskites, (BZA)2(R/S-PPA)BiI6 (2-R/S, R/S-PPA = (R/S)-1-Phenylpropylamine) are successfully obtained by introducing chiral aryl cations of (R/S)-1-Phenylpropylamine. Compared to 1, chiral-polar 2-R presents a significant irradiation-responsive bulk photovoltaic effect (BPVE) with an open circuit photovoltage of 0.4 V, which enables it with self-powered X-ray, UV-vis-NIR broadband photodetection. Specifically, 2-R device exhibits an ultralow detection limit of 18.5 nGy s-1 and excellent operational stability. Furthermore, 2-R as the first lead-free perovskite achieves significant broad-spectrum (377-940 nm) photodetection via light-induced pyroelectric effect. This work sheds light on the rational crystal reconstruction engineering and design of "green" hybrid perovskite toward high-demanded self-powered radiation detection and broadband photodetection.

Large Enhancement of Polarization in a Layered Hybrid Perovskite Ferroelectric Semiconductor via Molecular Engineering

Switchable spontaneous polarization is the vital property of ferroelectrics, which leads to other key physical properties such as piezoelectricity, pyroelectricity, and nonlinear optical effects, etc. Recently, organic-inorganic hybrid perovskites with 2D layered structure have become an emerging branch of ferroelectric materials. However, most of the 2D hybrid ferroelectrics own relatively low polarizations (<15 µC cm-2 ). Here, a strategy to enhance the polarization of these hybrid perovskites by using ortho-, meta-, para-halogen substitution is developed. Based on (benzylammonium)2 PbCl4 (BZACL), the para-chlorine substituted (4-chlorobenzylammonium)2 PbCl4 (4-CBZACL) ferroelectric semiconductor shows a large spontaneous polarization (23.3 µC cm-2 ), which is 79% larger than the polarization of BZACL. This large enhancement of polarization is successfully explained via ab initio calculations. The study provides a convenient and efficient strategy to promote the ferroelectric property in the hybrid perovskite family.

Halogen-Regulated Tc and X-ray Radiation Detection in 2D Hybrid Perovskite Ferroelastic Semiconductor

Organic-inorganic hybrid perovskites (OIHPs) have received particular attention due to their characteristic structural tunability and flexibility. These features make OIHPs behave with excellent modifications on macroscopic properties, such as ferroicity or semiconductor performances, etc. Herein, we report two 2D hybrid stibium-based halide perovskite (C3H7N)3Sb2X9 (X = Br, 1; Cl, 2) ferroelastic semiconductor possessing dual switching properties of dielectric and second harmonic generation (SHG). Notably, these two hybrids exhibit halogen-regulated ferroelasticity and semiconductor properties. There is a significant difference in Curie temperature (Tc) and X-ray radiation detection sensitivity (S), i.e., the ΔTc and ΔS are 38 K and 87 μC Gyair-1 cm-2, respectively. Meanwhile, crystals 1 and 2 do not show dark current drift in cyclic measurements of different radiation doses with stable switching ratios of 30 and 10, separately. Meanwhile, these results were proven by scientific experimental results and density functional theory (DFT) calculations. Our work presents a facile and practical method to regulate macroproperties on the molecular level, providing a new vision to develop hybrid perovskite ferroic-photoelectric materials.

Deciphering the in situ phonon evolution of potassium sodium niobate under varying temperature and electric fields

The application of in situ Raman spectroscopy under multiple fields is widely recognized as an effective approach for investigating the physical mechanism of phase transitions in ferroelectrics, because it can directly provide the detailed information about the vibration evolution of various phonon modes within lattices, such as bond stretching and rotation. Based on this technique, our work aims to thoroughly probe the dynamics of phase transitions in traditional ferroelectric potassium sodium niobate [(K,Na)NbO3, KNN] under external fields, by analyzing the in situ dependence of wavenumber and intensity of phonon modes under the varying temperature and electric fields. The results indicate that different vibration modes respectively relating to the A-site ions and NbO6 octahedra in KNN exhibit distinct and abrupt distortion behavior during the orthorhombic-tetragonal and tetragonal-cubic transitions. Moreover, a certain degree of distortion can still be observed in the cubic phase above the Curie temperature. With an applied electric field, KNN presents quite different electrostriction in orthorhombic and tetragonal phases. Particularly, more than one kind of phonon mode undergoes non-linear variations under the varying electric fields, accompanied by the mutations at some fixed fields. These findings will be conducive to further understanding the phase transition mechanism in KNN from the perspective of phonon evolution. Simultaneously, it will also give crucial guidance for the design and development of KNN-based ferroelectrics as well as functional devices.

Ferroelectricity and Related Properties of Nitratecadmate(II) Hybrid with Metal-Vacancy

All crystals are not ideal, and many of their properties are often determined not by the regular arrangement of atoms, but by the irregular arrangement of crystal defects. Many properties of materials can be controlled effectively by proper use of solid defects. By substitution of NH4 + ion of a hexagonal perovskite structure (H2 dabco)(NH4 )(NO3 )3 (dabco=1,4-diazabicyclo[2.2.2]octane, 1) with Cd2+ ion, we obtained a new metal-vacancy compound (H2 dabco)2 Cd(H2 O)2 (NO3 )6 (2). It exhibits a ferroelectric-paraelectric phase transition at 261 K. A comparison of the various-temperature single-crystal structures indicates that the coordination twist of Cd2+ ion leads to instability of the lattices and excellent ferroelectricity. These findings reveal that the vacancy can be utilized as an element to produce ferroelectricity and may start the chemistry of metal-vacancy coordination compounds. These findings reveals that the vacancy can be utilized as an effective means to tune the symmetry and produce ferroelectricity.

Volume-Confined Fabrication of Large-Scale Single-Crystalline Molecular Ferroelectric Thin Films and Their Applications in 2D Materials

With outstanding advantages of chemical synthesis, structural diversity, and mechanical flexibility, molecular ferroelectrics have attracted increasing attention, demonstrating themselves as promising candidates for next-generation wearable electronics and flexible devices in the film form. However, it remains a challenge to grow high-quality thin films of molecular ferroelectrics. To address the above issue, a volume-confined method is utilized to achieve ultrasmooth single-crystal molecular ferroelectric thin films at the sub-centimeter scale, with the thickness controlled in the range of 100-1000 nm. More importantly, the preparation method is applicable to most molecular ferroelectrics and has no dependency on substrates, showing excellent reproducibility and universality. To demonstrate the application potential, two-dimensional (2D) transitional metal dichalcogenide semiconductor/molecular ferroelectric heterostructures are prepared and investigated by optical spectroscopic method, proving the possibility of integrating molecular ferroelectrics with 2D layered materials. These results may unlock the potential for preparing and developing high-performance devices based on molecular ferroelectric thin films.

Photophysical Behavior of Triethylmethylammonium Tetrabromoferrate(III) under High Pressure

The pressure-induced properties of hybrid organic-inorganic ferroelectrics (HOIFs) with tunable structures and selectable organic and inorganic components are important for device fabrication. However, given the structural complexity of polycrystalline HOIFs and the limited resolution of pressure data, resolving the structure-property puzzle has so far been the exception rather than the rule. With this in mind, we present a collection of in situ high-pressure data measured for triethylmethylammonium tetrabromoferrate(III), ([N(C2H5)3CH3][FeBr4]) (EMAFB) by unraveling its flexible physical and photophysical behavior up to 80 GPa. Pressure-driven X-ray diffraction and Raman spectroscopy disclose its soft and reversible structural distortion, creating room for delicate band gap modulation. During compression, orange turns dark red at ∼2 GPa, and further compression results in piezochromism, leading to opaque black, while decompressed EMAFB appears in an orange hue. Assuming that the mechanical softness of EMAFB is the basis for reversible piezochromic control, we present alternations in the electronic landscape leading to a 1.22 eV band narrowing at 20.3 GPa while maintaining the semiconducting character at 72 GPa. EMAFB exhibits an emission enhancement, manifested by an increase of photoluminescence up to 17.3 GPa, correlating with the onsets of structural distortion and amorphization. The stimuli-responsive behavior of EMAFB, exhibiting stress-activated modification of the electronic structure, can enrich the physical library of HOIFs suitable for pressure-sensing technologies.

H/F Substitution Achieved Enantiomeric Organic Inorganic Hybrid Perovskites and Trigonal Structure [DMFP]3(CdBr3)(CdBr4)

Organic-inorganic hybrid perovskites (OIHPs) have been emerging as a hot research topic due to their potential applications in energy storage, semiconductors, and electronic devices. Herein, we systematically investigated the synthesis and phase transition behaviors of the enantiomeric OIHPs, (R) and (S)-N,N-dimethyl-3-fluoropyrrolidinium cadmium bromide ([DMFP][CdBr3]), and the hybrid trigonal structure [DMFP]3 (CdBr3)(CdBr4). The enantiomers have a mirror-symmetric structure and enhanced solid-state phase transition points of 417 and 443 K, in contrast to the nonfluorinated parent compound, N,N-dimethyl-pyrrolidinium cadmium bromide ([DMP][CdBr3], 385 K). Moreover, racemic H/F substitution on the pyrrolidinium cations leads to the formation of a trigonal compound, showing above-room-temperature structural phase transition and dominant ferroelasticity. This work discovers chiral enantiomeric OIHPs through H/F substitution, demonstrating a useful chemical synthesis strategy for exploring novel phase transition materials.

The halogen substitution strategy of inorganic skeletons triggers dielectric and band gap regulation of hybrid perovskites

Organic-inorganic hybrid perovskites (OIHPs) with dielectric switching functions have aroused comprehensive scientific interest, benefitting from their promising applications in sensors and information storage. However, to date, most of these materials discovered thus far possess a single function and are limited in their applicability, failing to meet the requirements of diverse applications. Moreover, the discovery of these materials has been largely serendipitous. Building multifunctional OIHPs with dielectric switching and semiconductors remains a daunting task. In this context, by introducing [C7H16N]+ as cations and in combination with lead halide with semiconducting properties, two OIHPs [C7H16N]PbI3 (1) and [C7H16N]PbBr3 (2) ([C7H16N]+ = (cyclopropylmethyl) trimethylammonium) have been successfully designed. They have dielectric switching properties close to 253 and 279 K and semiconducting behavior with band gaps of 2.67 and 3.22 eV. The phase transition temperature increased by 26 K through halogen substitution. In summary, our findings in this study provide insights into the application of the halogen substitution regulation strategy and open up new possibilities for designing perovskite semiconductors with dielectric switching functionality.

[Ba4 (S2 )][ZnGa4 S10 ]: Design of an Unprecedented Infrared Nonlinear Salt-Inclusion Chalcogenide with Disulfide-Bonds

Salt-inclusion chalcogenides (SICs) have been receiving widespread attention due to their large second harmonic generation (SHG) responses and wide bandgaps, however most of them suffer from small birefringence limiting their technical application. Herein, by introducing the π-conjugated (S2 )2- units in the ionic guest of salt-inclusion structure, the first disulfide-bond-containing SIC, [Ba4 (S2 )][ZnGa4 S10 ] has been synthesized. It exhibits the widest bandgap up to 3.39 eV among polychalcogenides and strong SHG response as large as that of AgGaS2 (AGS). Importantly, its birefringence reaches a max value of 0.053@1064 nm among AGS-like SICs, indicating it is a promising IR nonlinear optical (NLO) material. Theoretical calculations reveal that the π-conjugated (S2 )2- units and covalent GaS layers favor the enhanced birefringence and large SHG response. This work provides not only a new type of SIC for the first time, but also new lights on the design of IR NLO materials.

Centimeter-Size Single Crystals of Halide Perovskite Photoferroelectric Solid Solution with Ultrahigh Pyroelectricity Boosted Photodetection

Photoferroelectrics, especially emerging halide perovskite ferroelectrics, have motivated tremendous interests owing to their fascinating bulk photovoltaic effect (BPVE) and cross-coupled functionalities. However, solid solutions of halide perovskite photoferroelectrics with controllable structure and enhanced performance are scarcely explored. Herein, through mixing cage cation, a homogeneous halide perovskite photoferroelectric PA2 FAx MA1-x Pb2 Br7 solid solution (PA, FA and MA are CH3 CH2 CH2 NH3 + , NH2 CHNH2 + and CH3 NH3 + , 0≤x≤1) has been developed, which demonstrates tunable Curie temperature in a wide range of 263-323 K and excellent optoelectrical features. As the component adjusted to x=0.7, the bulk crystal demonstrates ultrahigh pyroelectric coefficient up to 1.48 μC cm-2  K-1 around room temperature. Strikingly, benefiting from the light-induced pyroelectricity and remarkable BPVE, a self-powered and sensitive photodetector based solid solution crystals with boosted responsivity and detectivity over than 1300 % has been achieved. This pioneering work sheds light on the exploration of photoferroelectric solid solutions towards high-performance photoelectronic devices.

Targeted regulation and optimization of multifunctional phase transition materials by novel void occupancy engineering

As an innovative form of stimulus-response materials, organic-inorganic hybrid phase transition materials have become a wonderful contender in the field of functional electronic equipment due to their versatile structure, intensive functions and straightforward preparation. However, the targeted regulation and optimization of the electrical/optical response, along with the establishment of regular structure-performance relationships, pose significant challenges in meeting the diverse demands of practical applications over an extended period. Herein, we conducted a systematic investigation into the role of lattice void occupancy in regulating phase transition temperature (Tp) and related optical/electrical bistability. By taking hybrid material [TMEA][Cd(SCN)3] featuring a flexible ammonium cation [TMEA]+ (TMEA = ethyltrimethylammonium) as the prototype, we successfully synthesized three phase transition materials, namely [DEDMA][Cd(SCN)3], [TEMA][Cd(SCN)3] and [TEA][Cd(SCN)3] (DEDMA = diethyldimethylammonium, TEMA = triethylmethylammonium, and TEA = tetraethylammonium), and the excellent regulation of the physical properties of these compounds was achieved through subtle engineering of void occupancy. More strikingly, [TEA][Cd(SCN)3] exhibits remarkable bistable properties in terms of dielectric and nonlinear optical responses (with second-harmonic generation intensity reaching 2.5 times that of KDP). This work provides a feasible avenue to reasonably customise organic-inorganic hybrid phase transition materials and finely adjust their intriguing functionalities.

Crystal Sponge Behavior in a Two-Dimensional Rare-Earth Hybrid Coordinate Polymer

Stimuli-responsive multifunctional materials (SRMMs) have attracted tremendous attention due to their dynamic responses to external stimuli. However, it remains challenging to simultaneously achieve solvent-induced single-crystal to single-crystal (SCSC) transformation and structural phase transition after desolvation. Here, we report a two-dimensional (2D) rare-earth organic-inorganic hybrid coordinate polymer [(CH3)3NCH2Cl]2[Eu·H2O]2[CH2(SO3)2]4·2H2O (1) that exhibits a reversible SCSC transformation by changing to 2 ([(CH3)3NCH2Cl][Eu·H2O][CH2(SO3)2]2). Impressively, the SCSC transformation process couples with large changes in quantum efficiency dropped from 33.68% of 1 to 20.07% that of 2. Furthermore, polymer 2 shows an isomorphic structural phase transition associated with switching dielectric. Notably, the distance of the 2D layers shows reversible change during the two successive transition processes displaying a crystal sponge behavior. This work reveals the potential of rare-earth 2D hybrid coordination polymers in the design of multifunctional responsive materials and opens a new prospect to explore the construction of novel SRMMs.

Near-90° Switch in the Polar Axis of Dion-Jacobson Perovskites by Halide Substitution

Ferroelectricity in two-dimensional hybrid (2D) organic-inorganic perovskites (HOIPs) can be engineered by tuning the chemical composition of the organic or inorganic components to lower the structural symmetry and order-disorder phase change. Less efforts are made toward understanding how the direction of the polar axis is affected by the chemical structure, which directly impacts the anisotropic charge order and nonlinear optical response. To date, the reported ferroelectric 2D Dion-Jacobson (DJ) [PbI₄]^2- perovskites exhibit exclusively out-of-plane polarization. Here, we discover that the polar axis in ferroelectric 2D Dion-Jacobson (DJ) perovskites can be tuned from the out-of-plane (OOP) to the in-plane (IP) direction by substituting the iodide with bromide in the lead halide layer. The spatial symmetry of the nonlinear optical response in bromide and iodide DJ perovskites was probed by polarized second harmonic generation (SHG). Density functional theory calculations revealed that the switching of the polar axis, synonymous with the change in the orientation of the sum of the dipole moments (DMs) of organic cations, is caused by the conformation change of organic cations induced by halide substitution.

One-Dimensional Chiral Copper Iodide Chain-Like Structure Cu4 I4 (R/S-3-quinuclidinol)3 with Near-Unity Photoluminescence Quantum Yield and Efficient Circularly Polarized Luminescence

Chiral organic-inorganic hybrid metal halide materials have shown great potential for circularly polarized luminescence (CPL) related applications for their tunable structures and efficient emissions. Here, this work combines the highly emissive Cu₄ I₄ cubane cluster with chiral organic ligand R/S-3-quinuclidinol, to construct a new type of 1D Cu-I chains, namely Cu₄ I₄ (R/S-3-quinuclidinol)₃ , crystallizing in noncentrosymmetric monoclinic P2₁ space group. These enantiomorphic hybrids exhibit long-term stability and show bright yellow emission with a photoluminescence quantum yield (PLQY) close to 100%. Due to the successful chirality transfer from the chiral ligands to the inorganic backbone, the enantiomers show intriguing chiroptical properties, such as circular dichroism (CD) and CPL. The CPL dissymmetry factor (g_lum ) is measured to be ≈4 × 10^-3 . Time-resolved photoluminescence (PL) measurements show long averaged decay lifetime up to 10 µs. The structural details within the Cu₄ I₄ reveal the chiral nature of these basic building units, which are significantly different than in the achiral case. This discovery provides new structural insights for the design of high performance CPL materials and their applications in light emitting devices.

Nonlinear Optical Glass-Ceramic From a New Polar Phase-Transition Organic-Inorganic Hybrid Crystal

Melt-quenched glasses of organic-inorganic hybrid crystals, i.e., hybrid glasses, have attracted increasing attention as an emerging class of hybrid materials with beneficial processability and formability in the past years. Herein, we present a new hybrid crystal, (Ph₃ PEt)₃ [Ni(NCS)₅ ] (1, Ph₃ PEt⁺ =ethyl(triphenyl)phosphonium), crystallizing in a polar space group P1 and exhibiting thermal-induced reversible crystal-liquid-glass-crystal transitions with relatively low melting temperature of 132 °C, glass-transition temperature of 40 °C, and recrystallization on-set temperature of 78 °C, respectively. Taking advantage of such mild conditions, we fabricated an unprecedented hybrid glass-ceramic thin film, i.e., a thin glass uniformly embedding inner polar micro-crystals, which exhibits a much enhanced intrinsic second-order nonlinear optical effect, being ca. 25.6 and 3.1 times those of poly-crystalline 1 and KH₂ PO₄ , respectively, without any poling treatments.

A nickel(ii)-based one-dimensional organic-inorganic halide perovskite ferroelectric with the highest Curie temperature

Organic-inorganic halide perovskites (OIHPs) are very eye-catching due to their chemical tunability and rich physical properties such as ferroelectricity, magnetism, photovoltaic properties and photoluminescence. However, no nickel-based OIHP ferroelectrics have been reported so far. Here, we designed an ABX3 OIHP ferroelectric (3-pyrrolinium)NiCl3, where the 3-pyrrolinium cations are located on the voids surrounded by one-dimensional chains composed of NiCl6-face-sharing octahedra via hydrogen bonding interactions. Such a unique structure enables the (3-pyrrolinium)NiCl3 with a high spontaneous polarization (P s) of 5.8 μC cm-2 and a high Curie temperature (T c) of 428 K, realizing dramatic enhancement of 112 and 52 K compared to its isostructural (3-pyrrolinium)MCl3 (M = Cd, Mn). To our knowledge, remarkably, (3-pyrrolinium)NiCl3 should be the first case of nickel(ii)-based OIHP ferroelectric to date, and its T c of 428 K (35 K above that of BaTiO3) is the highest among all reported one-dimensional OIHP ferroelectrics. This work offers a new structural building block for enriching the family of OIHP structures and will inspire the further exploration of new nickel(ii)-based OIHP ferroelectrics.

Halogen engineering of organic-inorganic hybrid perovskites displaying nonlinear optical, fluorescence properties and phase transition

In order to meet the needs of social development, increasing research attention has been paid to multifunctional molecular-based phase-transition materials. The traditional phase-transition materials with a single functional property can be transformed into magnificent ones by adding additional functional properties-for instance photoluminescence and magnetic order- because having two or more functional properties simultaneously greatly broadens the fields of their applications. At present, there are very few multifunctional phase-transition materials showing excellent performance, and the crystal structure design and performance optimization of materials still need to be studied in depth. Herein, we report the development of two organic-inorganic hybrid materials: (MBA)₂ZnI₄ (1, MBA = 4-methoxybenzylammonium) with switchable dielectricity and a high phase-transition temperature (T_c = 359.55 K), and (MBA)₂ZnBr₄ (2) with green luminescence (λ_exc = 314 nm) and nonlinear optical properties (0.75× KDP). A two-dimensional (2D) fingerprint analysis of the Hirshfeld surface plots revealed a significant difference between the hydrogen-bonding interaction before the phase transition and that afterwards. The two compounds were further verified, from energy band structure calculations, to be direct-band-gap semiconductors. In conclusion, this work has provided a viable strategy, involving the application of chemical modifications, for designing various functional materials.