Ferroelectricity, Piezoelectricity, and Unprecedented Starry Ferroelastic Patterns in Organic-Inorganic (CH3C(NH2)2)3[Sb2X9] (X = Cl/Br/I) Hybrids

In this study, we present a novel class of lead-free hybrid antimony halides incorporating the acetamidinium cation, with the chemical compositions: (CH3C(NH2)2)3[Sb2Cl9] (ACA), (CH3C(NH2)2)3[Sb2Br9] (ABA), and (CH3C(NH2)2)3[Sb2I9] (AIA) . Despite their seemingly analogous chemical formulations, these compounds exhibit diverse physical characteristics, predominantly dictated by the differences in their metal-halide architectures. Indeed, the anionic frameworks of ACA and AIA are reminiscent of well-known ferroelectric materials, with ACA distinguished by its piezoelectric and ferroelastic characteristics, underpinned by a buckled honeycomb two-dimensional (2D) layers of antimony chloride. Conversely, AIA is characterized by its ferroelectric attribute, with discrete bioctahedral units forming a zero-dimensional (0D) structure. A surprising structural deviation constitutes the anionic sublattice of ABA, which amalgamates features from both ACA and AIA, yielding an unprecedented hybrid two-component (0D + 2D) anionic architecture. The ferroelectric properties of AIA have been demonstrated through pyroelectric current measurements and hysteresis loop analyses. Additionally, the noncentrosymmetric nature of ACA and AIA has been corroborated by second harmonic generation experiments. The piezoelectricity of ACA was confirmed using piezoresponse force microscopy (PFM). Furthermore, observations under a polarizing microscope revealed distinct ferroelastic properties in both ACA and ABA, characterized by well-defined and abundant star patterns previously observed only in simple oxides and alloys.

Novel lead-free bismuth-based perovskite-like (BrC5H13N)3Bi2Br9: synthesis, structural investigations and optoelectronic properties

Lead-free organic-inorganic hybrid perovskites have attracted increasing attention owing to their advantages of reduced toxicity, photo-detectability, switchable dielectric device application, ferroelectric properties and distinctive optical characteristics. Despite their promising features, the chemical engineering of hybrid perovskites remains a challenge, as identifying the appropriate strategies is essential to achieve the desired properties such as controlled bandgap energy and phase transition behaviours. Numerous approaches have been explored to optimize these characteristics. In this study, we employed halogenation of the organic component as a targeted strategy to enhance the stability and performance of hybrid perovskite materials. This approach enabled the successful synthesis of a non-centrosymmetric halobismuthate (BrC5H13N)3Bi2Br9 compound (BrC5H13N+: (2-bromoethyl)trimethylammonium), which exhibited excellent optic and electric properties and crystallized in the non-polar P212121 space group. The inorganic framework was precisely arranged with [Bi2Br9]3- polyhedra that were face-shared and separated by organic cations, resulting in an A3B2X9-type structure. Additionally, the compound (BrC5H13N)3Bi2Br9 possessed an indirect band gap of 2.58 eV, which suggests this material's semiconductor character. Photoluminescence (PL) studies revealed that the compound exhibited a broad band emission at about 730 nm. The electrical properties as a function of frequencies and temperatures showed the contribution of the grain and grain boundary to conduction, and AC conductivity confirmed the semiconductor behaviour. The activation energy suggested the combination of ionic and electronic conduction. These findings enrich the understanding on the behaviour of A3B2X9-type low-dimensional hybrids and holds promise in extending the application of lead-free hybrids to the field of ferroelectric, electric and optic materials.

Understanding the Relation between the Structure and Emission Properties in 0D (2-ABI)3Bi1-xSbxCl6·H2O: Impact of Structural Distortion and Stereoactivity of ns2 Lone Pairs

Zero-dimensional (0D) organic-inorganic metal halides (OIMHs) have emerged as a fascinating class of materials, offering unprecedented opportunities for the development of low-cost solid-state lighting devices. Herein, we report two novel 0D Bi/Sb-based OIMHs, (2-ABI)3MCl6·H2O (M = Bi, Sb), crystallizing in the monoclinic P21/n space group. Under 365 nm UV light irradiation, the Sb-analogue displays a bright and broad yellow emission band at 580 nm with a photoluminescence (PL) quantum yield of 34.6%, which may be attributed to the distortion-induced self-trapped excitons (STEs). Theoretical study of (2-ABI)3SbCl6·H2O in the ground state and excited state shows specific bonds are distorted significantly indicating the role of the stereoactivity of the 5s2 lone pair. To understand the effect of gradual Sb substitution on the structure and the optical properties of (2-ABI)3BiCl6·H2O, a series of mixed compounds, (2-ABI)3Bi1-xSbxCl6·H2O (refined x = 0, 0.11, 0.40, 0.53, 0.87, 1) was prepared. This work provides important insight into the correlation between the PL properties and local structure distortion in the ground and excited states induced by the expression of 5s2 lone-pair electrons of Sb3+, thus providing an efficient tool for exploring highly emissive 0D OIMHs and further modulating their structures to enhance the luminescent properties for practical optoelectronic applications.

Ferroelectricity and piezoelectric energy harvesting of an A3M2X9-type 0D bromobismuthate hybrid with a bulky organic quaternary amine

Organic-inorganic hybrid ferroelectric compounds of the halobismuthate family have emerged as a focal point of research owing to their reduced toxicity and distinctive optical characteristics. This study presents a novel ammonium hybrid perovskite, [BPMBDMA]·[Bi2Br9], which exhibits both ferro- and piezoelectric properties and crystallizes in the polar noncentrosymmetric Pca21 space group. The nonlinear optical (NLO) activity of [BPMBDMA]·[Bi2Br9] was corroborated through second harmonic generation measurements evidencing its noncentrosymmetric structure, which was further substantiated by piezoresponse force microscopy analyses. Ferroelectric P-E hysteresis loop investigations conducted on a thin film sample of [BPMBDMA]·[Bi2Br9] revealed a saturation polarization (Ps) as much as 11.30 μC cm-2 at ambient temperature. To explore the piezoelectric energy harvesting capabilities of [BPMBDMA]·[Bi2Br9], composite materials were fabricated using polylactic acid (PLA) as a matrix. Notably, a device comprising 10 wt% [BPMBDMA]·[Bi2Br9] in PLA demonstrated a remarkable output voltage of 24.6 V and a peak power density of 13.65 μW cm-2. The practical applicability of this device's output performance was further evaluated through a capacitor charging experiment, wherein a 10 μF capacitor was charged within 160 seconds.

Ferroelectric, Switchable Dielectric and Nonlinear Optical Properties in Inorganic-Organic Lead-Free 1D Hybrids Based on Bi(III) and Azetidine: (C3NH8)2[BiCl5], (C3NH8)2[BiBr5]

This study investigates lead-free organic-inorganic hybrids (C3NH8)2[BiCl5] (ABC) and (C3NH8)2[BiBr5] (ABB), focusing on their structural, dielectric, ferroelectric, and optical properties. Both compounds exhibit paraelectric (I) to ferroelectric (II) phase transitions (PTs) at 230/233 K and 228/229 K, respectively, transitioning from orthorhombic (Pnma) to monoclinic (P21) phases, with distorted [BiX6]3- octahedra forming 1D chains. Quasielastic neutron scattering and solid-state 1H NMR studies reveal the localized motion of azetidinium cations. Dielectric measurements of ABC and ABB show step-like permittivity changes by 11-12 units post-transition I → II, demonstrating reversible switching behavior. Absorption studies reveal band gaps of 3.24 eV for ABC and 2.76 eV for ABB, classifying them as insulators. Luminescence spectra at 77K display 578 nm (ABC) and 708 nm (ABB) emissions, attributed to 3P1,0 → 1S0 transitions. Both compounds demonstrate stable second harmonic generation (SHG) switching of the on-off type. The switching performance is evaluated over multiple thermal cycles using the treq metric, which decreases with increasing temperature change rate and indicates that ABB's SHG switching is approximately 30% faster than that of ABC.

Phase Transitions and Dynamics in Mixed Three- and Low-Dimensional Lead Halide Perovskites

Lead halide perovskites are extensively investigated as efficient solution-processable materials for photovoltaic applications. The greatest stability and performance of these compounds are achieved by mixing different ions at all three sites of the APbX3 structure. Despite the extensive use of mixed lead halide perovskites in photovoltaic devices, a detailed and systematic understanding of the mixing-induced effects on the structural and dynamic aspects of these materials is still lacking. The goal of this review is to summarize the current state of knowledge on mixing effects on the structural phase transitions, crystal symmetry, cation and lattice dynamics, and phase diagrams of three- and low-dimensional lead halide perovskites. This review analyzes different mixing recipes and ingredients providing a comprehensive picture of mixing effects and their relation to the attractive properties of these materials.

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.

Temperature symmetry breaking and properties of lead-free organic-inorganic hybrids: bismuth(III) iodide and antimony(III) iodide: (S(CH3)3)3[Bi2I9] and (S(CH3)3)3[Sb2I9]

We have synthesized and characterized two novel lead-free organic-inorganic hybrid crystals: (S(CH3)3)3[Bi2I9] (TBI) and (S(CH3)3)3[Sb2I9] (TSI). Thermal DSC, TG, and DTA analyses indicate structural phase transitions (PTs) in both compounds; TBI undergoes two structural phase transitions at 314.2/314.8 K (cooling/heating) and at 181.5 K of first (I ↔ II) and second order (II ↔ III), respectively. The crystal structures of TBI are refined for phases I (325 K), II (200 K) and III (100 K). TBI exhibits ferroelastic properties since both PTs are accompanied by a change in the symmetry of crystals: P63/mmc → C2/c (I → II) and C2/c → P1̄ (II → III). The presence of a ferroelastic domain structure has been confirmed by optical observations. In turn, TSI also reveals two PTs: I ↔ II (at 303.9/304.1 K) and II ↔ III (212.9/221.4 K). To compare and obtain insight into the mechanism of the PTs of TBI, we have carried out temperature dependent single crystal X-ray diffraction studies. Additionally, to confirm the change in the dynamical states of molecules in PTs, dielectric measurements have been carried out between 100 K and 400 K in the frequency range of 200 Hz to 2 MHz. Moreover, the measurements of the 1H NMR spin-lattice relaxation time, T1, and a second moment, M2, of the 1H NMR line have been undertaken in the temperature range between 100 and 300 K.

A chiral two-dimensional perovskite-like lead-free bismuth(III) iodide hybrid with high phase transition temperature

Bismuth(III) iodide perovskites have attracted great attention as lead-free hybrid semiconductors, but they mainly show zero- and one-dimensional structures. Herein, we report the first two-dimensional chiral perovskite-like bismuth(III) iodide hybrid [(S)-3-aminopyrrolidinium I]2Bi2/3I4 (1) with a high phase transition temperature of 408.8 K, higher than most of the reported chiral lead-free hybrid semiconductors.

Structural, Electric and Dynamic Properties of (Pyrrolidinium)3[Bi2I9] and (Pyrrolidinium)3[Sb2I9]: New Lead-Free, Organic-Inorganic Hybrids with Narrow Band Gaps

Hybrid organic-inorganic iodides based on Bi(III) and Sb(III) provide integrated functionalities through the combination of high dielectric constants, semiconducting properties and ferroic phases. Here, we report a pyrrolidinium-based bismuth (1) and antimony (2) iodides of (NC₄H₁₀)₃[M₂I₉] (M: Bi(III), Sb(III)) formula which are ferroelastic at room temperature. The narrow band gaps (~2.12 eV for 1 and 2.19 eV for 2) and DOS calculations indicate the semiconducting characteristics of both materials. The crystal structure consists of discrete, face-sharing bioctahedra [M₂I₉]^3- and disordered pyrrolidinium amines providing charge balance and acting as spacers between inorganic moieties. At room temperature, 1 and 2 accommodate orthorhombic Cmcm symmetry. 1 displays a complex temperature-induced polymorphism. It is stable up to 525 K and undergoes a sequence of low-temperature phase transitions (PTs) at 221/222 K (I ↔ II) and 189/190 K (II ↔ III) and at 131 K (IV→III), associated with the ordering of pyrrolidinium cations and resulting in Cmcm symmetry breaking. 2 undergoes only one PT at T = 215 K. The dielectric studies disclose a relaxation process in the kilohertz frequency region, assigned to the dynamics of organic cations, described well by the Cole-Cole relation. A combination of single-crystal X-ray diffraction, synchrotron powder diffraction, spin-lattice relaxation time of ¹H NMR, dielectric and calorimetric studies is used to determine the structural phase diagram, cation dynamics and electric properties of (NC₄H₁₀)₃[M₂I₉]_.

A photoluminescent chiral lead-free hybrid ferroelastic semiconductor with switchable second-harmonic generation

Chiral organic-inorganic hybrid semiconductors (COIHSs) dominated by lead halides have recently gained tremendous interest. Here, we report a lead-free photoluminescent COIHS [R-3-hydroxylpiperidinium]₂SbCl₅ with a bandgap of 3.14 eV. It shows a ferroelastic phase transition at 341 K accompanied by a switchable second-harmonic generation response and presents clear ferroelastic domains, which are rarely found in lead-free COIHSs.

Metal ion induced dual switchable dielectric and luminescent properties in hybrid halides

A new multi-functional organic-inorganic hybrid compound was successfully obtained by regulating metal halides. Apart from excellent luminescence properties, in particular, the introduction of a Mn halide successfully achieved a dual-switchable dielectric property, which could lead to very interesting exploration in sensors.

Ferroelasticity in Organic-Inorganic Hybrid Perovskites

Molecular ferroelastics have received particular attention for potential applications in mechanical switches, shape memory, energy conversion, information processing, and solar cells, by taking advantages of their low-cost, light-weight, easy preparation, and mechanical flexibility. The unique structures of organic-inorganic hybrid perovskites have been considered to be a design platform for symmetry-breaking-associated order-disorder in lattice, thereby possessing great potential for ferroelastic phase transition. Herein, we review the research progress of organic-inorganic hybrid perovskite ferroelastics in recent years, focusing on the crystal structures, dimensions, phase transitions and ferroelastic properties. In view of the few reports on molecular-based hybrid ferroelastics, we look forward to the structural design strategies of molecular ferroelastic materials, as well as the opportunities and challenges faced by molecular-based hybrid ferroelastic materials in the future. This review will have positive guiding significance for the synthesis and future exploration of organic-inorganic hybrid molecular ferroelastics.

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.

Symmetry breaking of A3M2X9-type perovskite derivatives induced by polar quaternary ammonium cations: achieving efficient nonlinear optical properties

Low-dimensional organic-inorganic metal halides, especially lead-free perovskites, are attracting increasing attention because of their environmentally friendly processing, flexible structures, chemical stability, and promising nonlinear optical properties. Herein, we report a new stable polar 0D lead-free hybrid bismuth chloride to enable the second-harmonic generation (SHG) active material (BTA)₃Bi₂Cl₉ (BTA = benzyltriethylammonium, C₆H₅CH₂N(C₂H₅)₃⁺) that was obtained by the antisolvent vapor diffusion method and crystallized in the polar Cc space group. Its structure features organic cations surrounded by face-sharing [Bi₂Cl₉]^3- dimers. (BTA)₃Bi₂Cl₉ exhibits a wide direct bandgap (3.21 eV) and a strong phase-matchable SHG conversion efficiency (1.39 × KDP). Theoretical calculation reveals that the SHG response is owing to the synergistic effect of distorted inorganic [Bi₂Cl₉]^3- anions and polar organic BTA⁺ cations. This work not only enriches the family of organic-inorganic A₃M₂X₉ (A = monovalent cations; M = trivalent metal ions; and X = halide ions) NLO crystals but also provides the possibilities for further designing novel lead-free semiconducting piezoelectric, pyroelectric and ferroelectric materials.

(C3N2H5)3Sb2I9 and (C3N2H5)3Bi2I9: ferroelastic lead-free hybrid perovskite-like materials as potential semiconducting absorbers

We have synthesised and characterised novel organic-inorganic hybrid crystals: (C₃N₂H₅)₃Sb₂I₉ and (C₃N₂H₅)₃Bi₂I₉ (PSI and PBI). The thermal DSC and TG analyses indicate four structural phase transitions (PTs) at 366.2/366.8, 274.6/275.4, 233.3/233.3 and 142.8/143.1 K (on cooling/heating) for PSI and two reversible PTs at 365.2/370.8 and 252.6/257.9 K for PBI. Both analogues crystallize at room temperature in the orthorhombic Cmcm structure, which transforms, in the case of PBI, to monoclinic P2₁/n at low temperature. According to the X-ray diffraction results, the anionic component is discrete and built of face-sharing bioctahedra, [M₂I₉]^3-, in both compounds, whereas cations exhibit distinct dynamical disorder over high temperature phases. Dielectric spectroscopy and ¹H NMR spectroscopy have been used to characterise the dynamical state of the C₃N₂H₅⁺ cations. The ferroelastic domain structure has been characterised by observations under a polarized optical microscope. Both compounds are semiconductors with narrow bandgaps of 1.97 eV (PBI) and 2.10 eV (PSI).

Ferroic phase transition molecular crystals

Ferroic phase transition molecular crystals (FPTMCs), i.e., ferroelectrics and ferroelastics, are an important family of functional molecular materials, having merits of easy synthesis, structural tunability and flexibility, and biocompatibility. Both...

Crown Ether Host-Guest Molecular Ferroelectrics

In recent years, molecular ferroelectrics have received great attention due to their low weight, mechanical flexibility, easy preparation and excellent ferroelectric properties. Among them, crown-ether-based molecular ferroelectrics, which are typically composed of the host crown ethers, the guest cations anchored in the crown ethers, and the counterions, are of great interest because of the host-guest structure. Such a structure allows the components to occur order-disorder transition easily, which is beneficial for inducing ferroelectric phase transition. Herein, we summarized the research progress of crown ether host-guest molecular ferroelectrics, focusing on their crystal structure, phase transition, ferroelectric-related properties. In view of the small spontaneous polarization and uniaxial nature, we outlook the chemical design strategies for obtaining high-performance crown-ether-based molecular ferroelectrics. This minireview will be of guiding significance for the future exploration of crown ether host-guest molecular ferroelectrics.

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.

A lead-free bismuth iodide organic-inorganic ferroelectric semiconductor

Organic-inorganic metal halide ferroelectric semiconductors are mainly lead halide ones, suffering from the presence of toxic lead. Herein, we report a lead-free bismuth iodide ferroelectric semiconductor [1,4-butanediammonium]BiI5, showing a high Curie temperature of 365 K and a small band gap of 1.95 eV, smaller than those of most lead halide counterparts.

Synthesis and first-principles study of structural, electronic and optical properties of tetragonal hybrid halobismuthathes [Py2(XK)]2[Bi2Br10−xIx]

Tuning the optical properties of solid solutions by variation of the halogen composition.

Efficient Lone-Pair-Driven Luminescence: Structure-Property Relationships in Emissive 5s2 Metal Halides

Low-dimensional metal halides have been the focus of intense investigations in recent years following the success of hybrid lead halide perovskites as optoelectronic materials. In particular, the light emission of low-dimensional halides based on the 5s2 cations Sn2+ and Sb3+ has found utility in a variety of applications complementary to those of the three-dimensional halide perovskites because of its unusual properties such as broadband character and highly temperature-dependent lifetime. These properties derive from the exceptional chemistry of the 5s2 lone pair, but the terminology and explanations given for such emission vary widely, hampering efforts to build a cohesive understanding of these materials that would lead to the development of efficient optoelectronic devices. In this Perspective, we provide a structural overview of these materials with a focus on the dynamics driven by the stereoactivity of the 5s2 lone pair to identify the structural features that enable strong emission. We unite the different theoretical models that have been able to explain the success of these bright 5s2 emission centers into a cohesive framework, which is then applied to the array of compounds recently developed by our group and other researchers, demonstrating its utility and generating a holistic picture of the field from the point of view of a materials chemist. We highlight those state-of-the-art materials and applications that demonstrate the unique capabilities of these versatile emissive centers and identify promising future directions in the field of low-dimensional 5s2 metal halides.