Lone-Pair-Electron-Driven Ionic Displacements in a Ferroelectric Metal–Organic Hybrid

Ferroelectric Organic-Inorganic Hybrid Ammonium Halogenobismuthate(III) for Piezoelectric Energy Harvesting

Halogenobismuthate(III) compounds are of recent interest because of their low toxicity and distinct electrical properties. The utility of these materials as ferroelectrics for piezoelectric energy harvesters is still in its early stages. Herein, we report a hybrid ammonium halogenobismuthate(III) [BPBrDMA]2·[BiBr5], crystallizing in a ferroelectrically active polar noncentrosymmetric Pna21 space group. Its noncentrosymmetric structure was confirmed by the detection of the second harmonic generation response. The ferroelectric P-E hysteresis loop measurements on the thin film sample of [BPBrDMA]2·[BiBr5] gave a saturation polarization (Ps) of 5.72 μC cm-2. The piezoresponse force microscopy analysis confirmed its ferroelectric and piezoelectric nature, showing characteristic domain structures and signature hysteresis and butterfly loops. The piezoelectric energy harvesting attributes of [BPBrDMA]2·[BiBr5] were further probed on its polylactic acid (PLA) composites. The 15 wt % [BPBrDMA]2·[BiBr5]-PLA polymer composite resulted in a high output voltage of 26.2 V and power density of 15.47 μW cm-2. The energy harvested from this device was further utilized for charging a 10 μF capacitor within 3 min.

Exceptional structural phase transition near room temperature in an organic-inorganic hybrid ferroelectric

An organic-inorganic hybrid ferroelectric, (C6H5CH2CH2NH3)2[HgI4], undergoes an exceptional structural phase transition near room temperature, triggered by a flip of half the organic cations and an order-disorder transition of the inorganic anions, and may be regarded as a displacive-type ferroelectric. This finding provides a new structural phase transition mechanism in molecule-based ferroelectrics.

Rational Design of Non-Centrosymmetric Hybrid Halide Perovskites

Structural non-centrosymmetry in semiconducting organic-inorganic hybrid halide perovskites can introduce functionalities like anomalous photovoltaics and nonlinear optical properties. Here we introduce a design principle to prepare Pb- and Bi-based two- and one-dimensional hybrid perovskites with polar non-centrosymmetric space groups. The design principle relies on creating dissimilar hydrogen and halogen bonding non-covalent interactions at the organic-inorganic interface. For example, in organic cations like I-(CH₂)₃-NH₂(CH₃)⁺ (MIPA), -CH₃ is substituted by -CH₂I at one end, and -NH₃⁺ is substituted by -NH₂(CH₃)⁺ at the other end. These substitutions of two -H atoms by -I and -CH₃ reduce the rotational symmetry of MIPA at both ends, compared to an unsubstituted cation, for example, H₃C-(CH₂)₃-NH₃⁺. Consequently, the dissimilar hydrogen-iodine and iodine-iodine interactions at the organic-inorganic interface of (MIPA)₂PbI₄ 2D perovskites break the local inversion symmetries of Pb-I octahedra. Owing to this non-centrosymmetry, (MIPA)₂PbI₄ displays visible to infrared tunable nonlinear optical properties with second and third harmonic generation susceptibility values of 5.73 pm V^-1 and 3.45 × 10^-18 m² V^-2, respectively. Also, the single crystal shows photocurrent on shining visible light at no external bias, exhibiting anomalous photovoltaic effect arising from the structural asymmetry. The design strategy was extended to synthesize four new non-centrosymmetric hybrid perovskite compounds. Among them, one-dimensional (H₃N-(CH₂)₃-NH(CH₃)₂)BiI₅ shows a second harmonic generation susceptibility of 7.3 pm V^-1 and a high anomalous photovoltaic open-circuit voltage of 22.6 V.

Dielectric Relaxation and Dielectric Switching Behaviors in (N,N-Diisopropylethylamine) Tetrachloroantimonate(III)

Dielectric switches have drawn renewed attention to the study of their many potential applications with the adjustable switch temperatures (T_s ). Herein, a novel antimony-based halide semiconductor, (N,N-diisopropylethylamine) tetrachloroantimonate ((DIPEA)SbCl₄ , DIPEA⁺ =N,N-diisopropylethylamine), with dielectric relaxation behavior and dielectric switches has been successfully synthesized. This compound, consisting of coordinated anion S b C l 4 ∞ - ${{\left[{{\rm S}{\rm b}{\rm C}{\rm l}}_{4}\right]}_{\infty }^{-}}$ chains and isolated DIPEA⁺ cations, undergoes an isostructural order-disorder phase transition and shows a step-like dielectric anomaly, which can function as a frequency-tuned dielectric switch with highly adjustable switch temperature (T_s ). Variable-temperature single-crystal structure analyses and first-principles molecular dynamics simulations give information about the general mechanisms of molecular dynamics. This work enriches the dielectric switch family and proves that hybrid metal halides are promising candidates for switchable physical or chemical properties.

Impact of alkaline-earth doping on electronic properties of the photovoltaic perovskite CsSnI3: insights from a DFT perspective

The oxidation of Sn(II) to the more stable Sn(IV) degrades the photovoltaic perovskite material CsSnI₃; however, this problem can be counteracted via alkaline-earth (AE) doping. In this work, the electronic properties of CsSn_1-xAE_xI₃, with x = 0 and 0.25, and AE = Mg and Ca, were investigated via Density Functional Theory. It is proven that the synthetic reactions of all these perovskites are thermodynamically viable. Besides, a slight strengthening in the metal-halide bonds is found in the Mg-doped perovskite; consequently, it exhibits the greatest bulk modulus. Nevertheless, the opposite occurrs with the Ca-doped perovskite, which has the smallest bulk modulus due to the weakening of its metal-halide bonds. The calculated bandgaps for CsSnI₃, Mg-doped and Ca-doped perovskites are 1.11, 1.32 and 1.55 eV, respectively, remaining remarkably close to the best photovoltaic-performing value for single-junction solar cells of 1.34 eV. Nevertheless, an indirect bandgap was predicted under Mg-doping. These results support the possibility of implementing AE-doped perovskites as absorber materials in single-junction solar cells, which can deliver higher output voltages than that using CsSnI₃. Finally, it was found that Sr or Ba doping could result in semiconductors with bandgaps close to 2.0 eV.

Accessing One-Dimensional Chains of Halogenoindates(III) in Organic-Inorganic Hybrids

Organic-inorganic hybrids of halogenoindates(III) are typically represented by one of the zero-dimensional units: InX₄^-, InX₅^2-, InX₆^3-, or In₂X₁₁^5-. Higher dimensional anionic forms, although not forbidden, have remained almost elusive. Here we report for the first time In³⁺-based organic-inorganic hybrids, (C₄H₅N₂S)₂InCl₅ and (C₄H₅N₂S)₂InBr₅, with 1D anionic chains of trans-halide-bridged InX₆ octahedra whose formation is guided by 2-mercaptopyrimidinium cations (C₄H₅N₂S⁺). The chains are characterized by the significant ease of deformation, which is reflected in the elongation of the bridging bonds or the displacement of In³⁺ ions. The materials show a robust band gap predominantly governed by C₄H₅N₂S⁺ cations. Dielectric relaxation processes in (C₄H₅N₂S)₂InBr₅ arise from the cations' dynamics and suggest the ability of the brominated system to accommodate even larger cations. Our work represents a successful attempt to expand the structural diversity of halogenoindates(III) and opens a pathway to reach multifunctional 1D In³⁺-based hybrids.

Inorganic-Organic Hybrid Molecular Materials: From Multiferroic to Magnetoelectric

Inorganic-organic hybrid molecular multiferroic and magnetoelectric materials, similar to multiferroic oxide compounds, have recently attracted increasing attention because they exhibit diverse architectures, a flexible framework, fascinating physics, and potential magnetoelectric functionalities in novel multifunctional devices such as energy transformation devices, sensors, and information storage systems. Herein, the classification of multiferroicity and magnetoelectricity is briefly outlined and then the recent advances in the multiferroicity and magnetoelectricity of inorganic-organic hybrid molecular materials, particularly magnetoelectricity and the relevant magnetoelectric mechanisms and their categories are summarized. In addition, a personal perspective and an outlook are provided.

Soft Ferroelectrics Enabling High-Performance Intelligent Photo Electronics

Soft ferroelectrics based on organic and organic-inorganic hybrid materials have gained much interest among researchers owing to their electrically programmable and remnant polarization. This allows for the development of numerous flexible, foldable, and stretchable nonvolatile memories, when combined with various crystal engineering approaches to optimize their performance. Soft ferroelectrics have been recently considered to have an important role in the emerging human-connected electronics that involve diverse photoelectronic elements, particularly those requiring precise programmable electric fields, such as tactile sensors, synaptic devices, displays, photodetectors, and solar cells for facile human-machine interaction, human safety, and sustainability. This paper provides a comprehensive review of the recent developments in soft ferroelectric materials with an emphasis on their ferroelectric switching principles and their potential application in human-connected intelligent electronics. Based on the origins of ferroelectric atomic and/or molecular switching, the soft ferroelectrics are categorized into seven subgroups. In this review, the efficiency of soft ferroelectrics with their distinct ferroelectric characteristics utilized in various human-connected electronic devices with programmable electric field is demonstrated. This review inspires further research to utilize the remarkable functionality of soft electronics.

Ferroelectric and Room-Temperature Ferromagnetic Semiconductors in the 2D MIMIIGe2X6 Family: First-Principles and Machine Learning Investigations

Inspired by experimentally discovering ferromagnetism and ferroelectricity in two-dimensional (2D) CrGeTe₃ and CuInP₂S₆ with similar geometric structures, respectively, we systematically investigated ferroic properties in a large family of 2D M_IM_IIGe₂X₆ (M_I and M_II = metal elements, X = S/Se/Te) by combining high-throughput first-principles calculations and the machine learning method. We identified 12 stable 2D multiferroics containing simultaneously ferromagnetic (FM) and ferroelectric (FE) properties and 35 2D ferromagnets without FE polarization. Particularly, the predicted FM Curie temperatures (T_C) of eight 2D FM+FE semiconductors are close to or above room temperature. The ferroelectricity originates from the spontaneous geometric symmetry breaking induced by the unexpected shift of Ge-Ge atomic pairs and the emergence of Ge lone pair electrons, which also strengthens the p-d orbital hybridization between X atoms and metal atoms, leading to enhanced super-super-exchange interactions and raising the FM T_C. Our findings not only enrich the family of 2D ferroic materials and present room-temperature FM semiconductors but also disclose the mechanism of the emerging ferroelectricity and enhanced ferromagnetism, which sheds light on the realization of high temperature multiferroics as well as FM semiconductors.

Towards ferroelectricity-inducing chains of halogenoantimonates(iii) and halogenobismuthates(iii)

In halogenoantimonate(iii) and halogenobismuthate(iii) organic–inorganic hybrids, chains of trans-connected octahedra, trans-[MX₅]_∞, are considered attractive anionic structures for inducing ferroelectricity. The latter is realized by displacing the bridging halogen atoms along the chain direction – the process that changes the polarity of the whole unit. Advances in the identification of such materials have been hindered, however, by substantial difficulty in obtaining such structures. Here we investigate structural and dielectric properties of three families of compounds based on 2-mercaptopyrimidinium, 2-aminopyrimidinium, and 2-amino-4-methylpyrimidinium cations in which 8 out of 12 compounds show trans-[MX₅]_∞ chains in their crystal structures. Two of the compounds adopt a polar P2₁ space group and are potentially ferroelectric. We perform a detailed structural analysis of all compounds with trans-[MX₅]_∞ chains discovered by far to understand the factors that lead to the chains' formation. We reveal that the size of a cation predominantly defines the accessibility of structures with this anionic form and we provide rules for designing hybrids with trans-[MX₅]_∞ chains to help guide future efforts to engineer materials with interesting non-linear electrical properties.

A discovered abundance of structures with rare and highly-desired anionic chains is examined to identify structural factors leading to the chains' formation.

Cooperative Nature of Ferroelectricity in Two-Dimensional Hybrid Organic-Inorganic Perovskites

Two-dimensional (2D) ferroelectric (FE) hybrid organic-inorganic perovskites (HOIPs) are promising for potential applications as miniaturized flexible ferroelectric/piezoelectric devices. Recently, several 2D HOIPs [e.g., Ruddlensden-Popper type HOIP BA₂PbCl₄ (BA = C₆H₅CH₂NH₃⁺)] were reported to possess room-temperature ferroelectricity. However, the underlying microscopic mechanisms for ferroelectricity in 2D HOIPs remain elusive. Here, by performing first-principles calculations and symmetry mode analysis, we demonstrate that there exists a cooperative coupling between A-site organic molecules and B-site inorganic Pb²⁺ ions that is essential to the ferroelectricity in 2D BA₂PbCl₄. The nonpolar ground state of the closely related compounds BA₂PbBr₄ and BA₂PbI₄ can also be explained in terms of the weakened cooperative coupling. We further predict that 2D BA₂PbF₄ displays in-plane ferroelectricity with a higher Curie temperature and larger electric polarization. Our work not only reveals the unusual FE mechanism in 2D HOIPs but also provides a solid theoretical basis for the rational design of 2D multifunctional materials.

A valuable strategy to improve ferroelectric performance significantly via metallic ion doping in the lattice nodes of metal-organic frameworks

Metal-organic frameworks (MOFs) with polar space groups in crystallography represent a class of potential molecular-based ferroelectrics; however, the rational design and enhancement of the performance of MOF-based ferroelectrics is a great challenge. In this work, a series of mixed-metal MOFs deriving from Mg2+ ions doped into the lattice nodes of the parent-MOF (Ni-MOF) are synthesized by an in situ solvothermal method. Taking advantage of Mg2+ ions doped in the Ni-MOF, the doped-MOFs (Mg/Ni-MOFs) appear to have a significant lattice distortion and noteworthy dipole asymmetry in the crystals. It is found that the obtained doped-MOFs show a significant enhancement of ferroelectricity compared with that of the parent-MOF. This study opens up a new landscape to explore possibilities for controlling ferroelectric performance in MOF-based ferroelectrics.

Crystal structure of bis(octahydrocyclopenta[c]pyrrolium)pentachlorobismuthate(III), (C7NH14)2BiCl5

(C7NH14)2BiCl5, monoclinic, P21/n (no. 14), a = 13.6087(3) Å, b = 7.4645(2) Å, c = 21.7967(6) Å, β = 104.3220(9)°, V = 2145.34(10) Å3, Z = 4, R gt(F) = 0.0205, wR ref(F 2) = 0.0528, T = 293(2) K.

Ferroelectricity in Ethylammonium Bismuth-Based Organic-Inorganic Hybrid: (C2H5NH3)2[BiBr5]

The (C₂H₅NH₃)₂[BiBr₅] (EBB) crystals adopt the one-dimensional (1D) polymeric anionic form [BiBr₅]_∞^2-, which is preferred by halobismuthates(III) exhibiting polar properties and realized in R₂MX₅ stoichiometry. Differential scanning calorimetry and dilatometric measurements reveal reversible structural phase transitions: at 160 K (phase I → phase II) and 120 K (phase II → phase III). The resolved crystal structures of EBB show the centrosymmetric space group in phase I (Aeam), polar (Pca2₁) in phase II, and polar (Aea2) in phase III. The presence of dielectric hysteresis loops in phases II and III evidence ferroelectric properties. The dielectric response [ε*(ω,T)] of EBB close to 160 K is characteristic of ferroelectrics with a critical slowing down process. The molecular mechanism of a paraelectric-ferroelectric phase transition at 160 K is explained as "order-disorder" (assigned to the dynamics of the ethylammonium cations) and dominating "displacive" (related to strong distortion of the 1D anionic network). The optical band gap obtained from UV-vis measurements is about 2.6 eV. The conduction band minimum is formed by the hybridized Bi 6p and Br 4p states. An analysis of the CSD results for haloantimonates(III) and halobismuthates(III) ferroelectrics characterized by [MX₄]^-, [M₂X₉]^3-, [MX₅]^2-, and [M₂X₁₁]^5- anions is given.

Correlation between crystal structures and polar (ferroelectric) properties of hybrids of haloantimonates(iii) and halobismuthates(iii)

Halogenoantimonates(iii) and halogenobismuthates(iii) are a highly versatile class of organic–inorganic hybrid materials, applicable in optoelectronics and switchable dielectric devices.

Hydrogen-bonded organo-amino phosphonium halides: dielectric, piezoelectric and possible ferroelectric properties

Molecular ferroelectric materials are an exciting class of materials for potential applications in energy and electronics. Herein, we report examples of hydrogen-bonded binary salts of diphenyl diisopropylamino phosphonium halides [Ph2(iPrNH)2P]·X [DPDP·X, X = Cl, Br, I] which show dielectric, piezoelectric and NLO properties and some potentially ferroelectric attributes at room temperature. The phosphonium bromide salt was prepared by bromination of the phosphine precursor Ph2PCl and its subsequent treatment with isopropyl amine. The chloride and iodide salts were synthesized by the halogen exchange reaction of the bromide salt. The variable temperature single crystal X-ray analysis indicates the retention of the polar non-centrosymmetric phase of these materials for a wide range of temperatures from 100 to 400 K and above. All these assemblies were shown to exhibit 1D H-bonded chain structures along the crystallographic b-axis. The P-E loop measurements of these salts gave curves similar to those of non-linear leaky dielectric materials. However, the vertical piezoresponse force microscopy (V-PFM) analyses showed the existence of polarizable domain inversions indicating the possibility of ferroelectric behaviour in these materials. The temperature dependent dielectric measurements on these salts support the absence of phase transition temperatures in these assemblies. Also, bias-dependent PFM studies reveal their piezoelectric nature as the obtained converse piezoelectric coefficients are consistent with the d33 values obtained by the direct quasi-static methods.

Synthesis and characterization of a new organic–inorganic hybrid ferroelectric: (C4H10N)6[InBr6][InBr4]3·H2O

Compound (C₄H₁₀N)₆[InBr₆][InBr₄]₃·H₂O undergoes a paraelectric–ferroelectric phase transition at 232 K, which triggered by the disorder–order transition of Br atoms in [InBr₄]⁻ anions.

Tuning the structure and properties of a multiferroic metal–organic-framework via growing under high magnetic fields

High magnetic field-induced synthesis has been demonstrated to tune the structure and properties of the multiferroic metal–organic framework [(CH₃)₂NH₂][Mn(HCOO)₃].

The lone-pair-electron-driven phase transition and order-disorder processes in thermochromic (2-MIm)SbI4 organic-inorganic hybrid

The easy to prepare and stable in air (2-methylimidazolium) tetraiodoantimoniate(iii) single-crystals with optical band gap of 2.17(1) eV at room temperature have been synthesized. The crystal structure features one-dimensional [SbI₄]^-_n anionic chains, which are intercepted with stacks of 2MIm⁺ ions. At 294/295 K, it undergoes a structural phase transition to an incommensurately modulated phase as a result of subtle, lone-pair-electron-driven distortions of the anions. Separately from the anion displacements, the ordering of 2MIm⁺ countercations takes place over a wide temperature range of the modulated phase. The disorder changes from dynamic to static around 200 K, which affects the crystal structure leading to discontinuities and step-like contraction of the lattice parameters. The material is thermochromic with prominent color changes, from raspberry to yellow at low temperatures. The calculated electronic structures and observed optical properties signify its semiconducting character.

Ferroelectricity in bis(ethylammonium) pentachlorobismuthate(iii): synthesis, structure, polar and spectroscopic properties

A brief description of the thermal, structural and dielectric properties of bis(ethylammonium) pentachlorobismuthate(iii) ferroelectric with P_s that equals to 1.4 μC cm⁻² at 180 K is presented.