Reversible Thermochromism and Strong Ferromagnetism in Two‐Dimensional Hybrid Perovskites

Two‐dimensional (2D) hybrid perovskites have shown many attractive properties associated with their soft lattices and multiple quantum well structure. Herein, we report the synthesis and characterization of two new multifunctional 2D hybrid perovskites, (PED)CuCl₄ and (BED)₂CuCl₆, which show reversible thermochromic behavior, dramatic temperature‐dependent conductivity change, and strong ferromagnetism. Upon temperature change, the (PED)CuCl₄ and (BED)₂CuCl₆ crystals exhibit a reversible color change between yellow and red‐brown. The associated structural changes were monitored by in situ temperature‐dependent powder X‐ray diffraction (PXRD). The (BED)₂CuCl₆ exhibits superior thermal stability, with a thermochromic working temperature up to 443 K. The conductivity of (BED)₂CuCl₆ changes over six orders of magnitude upon temperature change. The 2D perovskites exhibit ferromagnetic properties with Curie temperatures around 13 K.     

An organic-inorganic hybrid thermochromic ferroelastic with multi-channel switches

Multifunctional switchable materials are attracting tremendous interest because of their great application potential in signal processing, information encryption, and smart devices. Here, we reported an organic-inorganic hybrid thermochromic ferroelastic crystal, [TMIm][CuCl₄] (TMIm = 1,1,3,3-tetramethylimidazolidinium), which undergoes two reversible phase transitions at 333 K and 419 K, respectively. Intriguingly, these three phases experience a remarkable ferroelastic-paraelastic-ferroelastic (2/m-mmm-2/m) transition, which remains relatively unexplored in ferroelastics. Moreover, the ferroelastic domains can be simultaneously switched under temperature and stress stimuli. Meanwhile, [TMIm][CuCl₄] exhibits thermochromic phenomenon, endowing it with extra spectral encryption possibilities during information processing. Combined with dielectric switching behavior, [TMIm][CuCl₄] are promising for practical applications in memory devices, next-generation sensors, and encryption technology.     

A Chiral Thermochromic Ferroelastic with Seven Physical Channel Switches

Switchable materials play an invaluable role in signal processing and encryption of smart devices. The development of multifunctional materials that exhibit switching characteristics in multiple physical channels has attracted widespread attention. Now, two chiral thermochromic ferroelastic crystals (S‐CTA)₂CuCl₄ and (R‐CTA)₂CuCl₄ (CTA=3‐chloro‐2‐hydroxypropyltrimethylammonium) have been prepared with switchable properties in dielectricity, conductivity, second harmonic generation (SHG), piezoelectricity, ferroelasticity, chiral, and thermochromic properties. Compared with traditional phase‐transition materials with switching features, thermochromism brings additional spectral encryption possibilities for future information processing. To the best of our knowledge, this is the first chiral thermochromic ferroelastic that exhibits switching properties in seven physical channels. This work is expected to promote further exploration of multifunctional molecular switchable materials.     

Temperature-Dependent Photoluminescence of Hexafluorobenzene-Intercalated Phenethylammonium Tin Iodide 2D Perovskite

Tin halide perovskites are potential alternatives of lead halide perovskites. However, the easy oxidation of Sn²⁺ to Sn⁴⁺ brings in a challenge. Recently, layered two-dimensional hybrid tin halide perovskites have been shown to partially resist the oxidation process because of the presence of hydrophobic organic molecules. Consequently, such layered hybrid perovskites are being explored for optoelectronic applications. The optical properties of layered tin halide perovskites depend on the interlayer separation and the dielectric mismatch between the organic and inorganic layers. Intercalation (insertion) of a molecular species between the layers modifies the interlayer interactions affecting the optical properties of layered hybrid perovskites. We investigated the effect of hexafluorobenzene (HFB) intercalation in phenethylammonium tin iodide [(PEA)₂SnI₄] using temperature-dependent (6 K to 300 K) photoluminescence (PL). HFB intercalation increases the bandgap. A strong PL quenching is observed in pristine (PEA)₂SnI₄ below 150 K, probably because of the presence of non-emissive states. HFB intercalation suppresses the influence of such non-emissive states resulting in an increase in PL intensity at the cryogenic temperatures. Our results highlight that a simple molecular intercalation (non-covalent interaction) into layered hybrid perovskites can significantly tailor the electronic and optical properties.     

Metal Coordination Sphere Deformation Induced Highly Stokes-Shifted,Ultra Broadband Emission in 2D Hybrid Lead-Bromide Perovskites and Investigation of Its Origin

Published studies of layered (2D) (100)-oriented hybrid lead-bromide perovskites evidence a correlation between increased inter-octahedral (Pb-Br-Pb) distortions and the appearance of broadband white light emission. However, the impact of distortions within their constituent [PbBr₆]⁴⁻ octahedra has yet to be assessed. Herein, we report two new (100)-oriented 2D Pb-Br perovskites, whose structures display unusually high intra-octahedral distortions, whilst retaining minimal inter-octahedral distortions. Using a combination of temperature-dependent, power-dependent and time-resolved photoluminescence spectroscopic measurements, we show that increased intra-octahedral distortion induces exciton localization processes and leads to formation of multiple photoinduced emissive colour centres. Ultimately, this leads to highly Stokes-shifted, ultrabroad white light emission at room temperature.     

SPECTROSCOPIC INVESTIGATION ON THE TRICHLORO-, TETRAHALO- AND MIXED-TETRAHALO-CUPRATES OF THE PIPERIDINIUM AND MORPHOLINIUM CATIONS

Abstract

Some halo- and mixed-halo-cuprates of the type CuCl₃ · L, CuCl₄ · L₂, CuCl₃Br · L₂, CuCl₂Br₂ · L₂ and CuBr₄ · L₂, were prepared and characterized by means of far-i.r., i.r. and n.i.r. spectroscopy, magnetic moments and conductivity measurements. A band at 19000 cm^?1, which appears only in the CuCl₃ · L complexes, is characteristic of Cu₂Cl^2- ₆ dimer or polymer species. For these complexes and for CuCl₄(Morph)₂ and CuCl₃Br(Morph)₂ (Morph = morpholinium cation) an approaching square-planar geometry, while for all the other complexes a distorted-tetrahedral geometry, may be suggested from their spectroscopic properties, compared to those of other tri- and tetra-halo-cuprates of known structure. An intermediate structure between these geometries may be proposed for the CuCl₃ · Pipd complex. The planar geometry is probably stabilized by extensive NH … Cl hydrogen bonding interactions greater with the morpholinium cation than with the piperidinium cation. No thermochromic behaviour in the solid state in the 290–370° K temperature range is observed. The room-temperature magnetic moments agree with the proposed configurations.     

Pressure-Induced Phase Transition,Jahn-Teller Suppression,Optical and Electronic Property Evolutions in Ruddlesden-Popper Perovskites Rb2CuCl4-xBrx

Transition-metal containing halides with Ruddlesden-Popper (RP) perovskite structures have received extensive attention owing to their emerging and anisotropic photoelectric functionalities. Among them, A₂CuX₄ (A=alkali metal or organic cations, X=Cl, Br, I) series are particular, because of the Jahn-Teller distortion of Cu²⁺ sensitive to external stimuli such as temperature and pressure. In this article, we report the structure evolution and physical property responses of RP perovskites Rb₂CuCl_4-xBr_x (x=1, 2) to external pressure. Dramatic structural phase transitions from orthorhombic to monoclinic occur around 3.0 GPa in both materials regardless of their distinct compositions. Structure analyses reveal the suppression and final vanishing of the Jahn-Teller distortion of Cu²⁺ cations under compression and crossing the phase transition, respectively. Rb₂CuCl_4-xBr_x perovskites exhibit abrupt bandgap narrowing (from reddish-brown to black) along with the structural phase transition, and an overall bandgap narrowing of 75% up to ∼27 GPa but still keeping semiconductive. During the compression processes, the resistances of Rb₂CuCl_4-xBr_x have been greatly reduced by 5-orders of magnitude. Moreover, all of the pressure-induced phenomena in Rb₂CuCl_4-xBr_x perovskites are reversible upon decompression and no obvious difference is observed for the pressure responses between [CuCl₄Br₂] and [CuCl₄(Cl,Br)₂] coordination environments. The impact of pressure on the structural and physical properties in two-dimensional Rb₂CuCl_4-xBr_x provides in-depth understanding on the structure design of functional halide perovskites at ambient conditions.     

A Molecular Thermochromic Ferroelectric

Molecular ferroelectrics have attracted considerable interests because of their easy and environmentally friendly processing, low acoustical impedance and mechanical flexibility. Herein, a molecular thermochromic ferroelectric, N,N′‐dimethyl‐1,4‐diazoniabicyclo[2.2.2]octonium tetrachlorocuprate(II) ([DMe‐DABCO]CuCl₄) is reported, which shows both excellent ferroelectricity and intriguing thermochromism. [DMe‐DABCO]CuCl₄ undergoes a ferroelectric phase transition from Pca2₁ to Pbcm at a significantly high Curie temperature of 413 K, accompanied by a color change from yellow to red that is due to the remarkable deformation of [CuCl₄]^2? tetrahedron, where the ferroelectric and paraelectric phases correspond to yellow and red, respectively. Combined with multiple bistable physical properties, [DMe‐DABCO]CuCl₄ would be a promising candidate for next‐generation smart devices, and should inspire further exploration of multifunctional molecular ferroelectrics.     

Structural phase transition,electrical and semiconducting properties in a lead-free 2D hybrid perovskite-like compound: [Cl-(CH2)2-NH3]2[CuCl4]

A new layered perovskite-type organic–inorganic hybrid compound: [Cl-(CH₂)₂-NH₃]₂[CuCl₄] ( 1 ), in which 2-chloroethylammonium cation occupies the space enclosed by the CuCl₆ octahedra, has been successfully synthesized. It is found that the compound exhibits the semiconducting properties with the optical band gap equal to 1.98 eV, confirmed by AC conductivity measurements, which varies between 10⁻⁵ and 10⁻⁴ Ω⁻¹ m⁻¹. Also, the low activation energy at low temperatures (0.26 eV) can indicate the electronic conduction of this material. Compound ( 1 ) displays phase transitions at T₁ = 281 K and T₂ = 380 K, confirmed by DSC, electrical and dielectric measurements. Single-crystal X-ray diffraction data at variable temperatures reveal that symmetry-breaking occurs from P2₁/c (at 296 K ˃ T₁) to P-1 (at 150 K ˂ T₁), originating directly from the [Cl-(CH₂)₂-NH₃]⁺ cation conformational changes and the distortions of CuCl₆⁴⁻ octahedra caused by the Jahn-Teller effect in the inorganic layers. Meanwhile, organic 2-chloroethylammonium moieties display some boat-like conformation below T₁, which transforms to a chair-like structure above T₁. This study paves the pathway to explore new lead-free hybrid perovskites with targeted properties for thermoelectric, supercapacitors, batteries, environmentally friendly processing and semiconductor applications.     

High-Temperature and Large-Polarization Ferroelectric with Second Harmonic Generation Response in a Novel Crown Ether Clathrate

Molecular ferroelectrics of high-temperature reversible phase transitions are very rare and have attracted increasing attention in recent years. In this paper is described the successful synthesis of a novel high-temperature host-guest inclusion ferroelectric: [(C₆H₅NF₃)(18-crown-6)][BF₄] ( 1 ) that shows a pair of reversible peaks at 348 K (heating) and 331 K (cooling) with a heat hysteresis about 17 K by differential scanning calorimetry measurements, thus indicating that 1 undergoes a reversible structural phase transition. Variable-temperature PXRD and temperature-dependent dielectric measurements further prove the phase-transition behavior of 1 . The second harmonic response demonstrates that 1 belongs to a non-centrosymmetric space group at room temperature and is a good nonlinear optical material. In its semiconducting properties, 1 shows a wide optical band gap of about 4.43 eV that comes chiefly from the C, H and O atoms of the crystals. In particular, the ferroelectric measurements of 1 exhibit a typical polarization-electric hysteresis loop with a large spontaneous polarization (P_s) of about 4.06 μC/cm². This finding offers an alternative pathway for designing new ferroelectric-dielectric and nonlinear optical materials and related physical properties in organic-inorganic and other hybrid crystals.     

[(18‐Crown‐6)K][Fe(1)Cl(1)4]0.5[Fe(2)Cl(2)4]0.5: A Multifunctional Molecular Switch of Dielectric,Conductivity and Magnetic Properties

Multifunctional materials that exhibit different physical properties in a single phase have potential for use in multifunctional devices. Herein, we reported an organic–inorganic hybrid compound [(18‐crown‐6)K][Fe(1)Cl(1)₄]_0.5[Fe(2)Cl(2)₄]_0.5 ( 1 ) by incorporating KCl and FeCl₃ into a 18‐crown‐6 molecule, which acts as a host of the six O atoms providing a lone pair of electrons to anchor the guest potassium cation, and [FeCl₄]^? as a counterion for charge balance to construct a complex salt. This salt exhibited a one‐step reversible structural transformation giving two separate high and low temperature phases at 373 K, which was confirmed by systematic characterizations including differential scanning calorimetry (DSC) measurements, variable‐temperature structural analyses, and dielectric, impedance, variable‐temperature magnetic susceptibility measurements. Interestingly, the structural transformation was coupled to both hysteretic dielectric phase transition, conductivity switch and magnetic‐phase transition at 373 K. This result gives an idea for designing a new type of phase‐transition materials harboring technologically important magnetic, conductivity and dielectric properties.     

Dimensional Reduction of Cs2AgBiBr6: A 2D Hybrid Double Perovskite with Strong Polarization Sensitivity

By dimensional reduction of the 3D motif of Cs₂AgBiBr₆, a lead‐free 2D hybrid double perovskite, (i‐PA)₂CsAgBiBr₇ ( 1, i‐PA=isopentylammonium), was successfully designed. It adopts a quantum‐confined bilayered structure with alternating organic and inorganic sheets. Strikingly, the unique 2D architecture endows it highly anisotropic nature of physical properties, including electric conductivity and optical absorption (the ratio α_b/α_c=1.9 at 405 nm). Such anisotropy attributes result in the strong polarization‐sensitive responses with large dichroic ratios up to 1.35, being comparable to some 2D inorganic materials. This is the first study on the hybrid double perovskites with strong polarization sensitivity. A crystal device of 1 also exhibits rapid response speed (ca. 200 μs) and excellent stabilities. The family of 2D hybrid double perovskites are promising optoelectronic candidates, and this work paves a new pathway for exploring new green polarization‐sensitive materials.     

H/F Substitution Induced Large Increase of Tc in a 3D Hybrid Rare-Earth Double Perovskite Multifunctional Compound

Increasing attention has been devoted to studying perovskite-type multifunctional stimuli-responsive materials with multiple channel physical characteristics. However, it remains challenging to simultaneously achieve multifunction and regulate structural phase transition temperature in hybrid perovskites. Here, we report two three-dimensional organic–inorganic hybrid rare-earth double perovskite compounds, (HQ)₂RbEu(NO₃)₆ ( 1 , HQ=quinuclidium) and (4FHQ)₂RbEu(NO₃)₆ ( 2 , 4FHQ=4-fluoro-quinuclidium), which exhibit ferroelasticity, dielectric switch, and excellent photoluminescence response. The phase transition temperature of 2 increases 169 K compared with 1 through H/F substitution. This result is attributed to the H/F substitution inducing the generation of the Rb-F coordination bond between cations and anions. Meanwhile, the photoluminescence emission intensity of 2 shows no quench with the increase of temperature, in particular, the emission spectrum achieves fine regulation at high temperatures. This work provides a new solution for the realization of multi-functions and regulations of the properties based on hybrid perovskite materials with high critical temperatures.     

Tailored Engineering of an Unusual (C4H9NH3)2(CH3NH3)2Pb3Br10 Two‐Dimensional Multilayered Perovskite Ferroelectric for a High‐Performance Photodetector

Two‐dimensional (2D) layered hybrid perovskites have shown great potential in optoelectronics, owing to their unique physical attributes. However, 2D hybrid perovskite ferroelectrics remain rare. The first hybrid ferroelectric with unusual 2D multilayered perovskite framework, (C₄H₉NH₃)₂(CH₃NH₃)₂Pb₃Br₁₀ ( 1 ), has been constructed by tailored alloying of the mixed organic cations into 3D prototype of CH₃NH₃PbBr₃. Ferroelectricity is created through molecular reorientation and synergic ordering of organic moieties, which are unprecedented for the known 2D multilayered hybrid perovskites. Single‐crystal photodetectors of 1 exhibit fascinating performances, including extremely low dark currents (ca. 10⁻¹² A), large on/off current ratios (ca. 2.5×10³), and very fast response rate (ca. 150 μs). These merits are superior to integrated detectors of other 2D perovskites, and compete with the most active CH₃NH₃PbI₃.     

Electrically Switchable Persistent Spin Texture in a Two-Dimensional Hybrid Perovskite Ferroelectric

As a momentum-independent spin configuration, persistent spin texture (PST) could avoid spin relaxation and play an advantageous role in spin lifetime. Nevertheless, manipulation of PST is a challenge due to the limited materials and ambiguous structure–property relationships. Herein, we present electrically switchable PST in a new 2D perovskite ferroelectric, (PA)₂CsPb₂Br₇ (where PA is n-pentylammonium), which has a high Curie temperature of 349 K, evident spontaneous polarization (3.2 μC cm⁻²) and a low coercive electric field of 5.3 kV cm⁻¹. The combination of symmetry-breaking in ferroelectrics and effective spin-orbit field facilitates intrinsic PST in the bulk and monolayer structure models. Strikingly, the directions of spin texture are reversible by switching the spontaneous electric polarization. This electric switching behavior relates to the tilting of PbBr₆ octahedra and the reorientation of organic PA⁺ cations. Our studies on ferroelectric PST of 2D hybrid perovskites afford a platform for electrical spin texture manipulation.     

Study the electrical conductivity of crosslinked polyester doped with different metal salts

Three crosslinked polymers were prepared via condensation polymerization between triethanolamine and glycerol [(25:75%), (50:50%) and (0:100%) (G:TEA)] with maleic anhydride which produced polymers I, II and III consequently. All the prepared polymers were doped with metal salts (CuCl₂, NiCl₂ and FeCl₂). D.C. conductivity was measured in the temperature range of (298–373 K), the result showed that the electrical conductivity increased several orders of magnitude with increasing temperature, the activation energy decreased with increasing conductivity. A.C. measurement is used to calculate the dielectric constant for the polymers in both pure and doped state.     

X-ray single crystal study of reversible thermochromism and the pecularities of atomic thermal motion in n,n-diphenylguanidinium(1+) hexabromotellurate (IV)

Peculiarities of the atomic structure of N,N-diphenylguanidium (1+) hexabromotellurate (IV), (C₁₃H₁₄N₃)₂[TeBr₆], which has reversible thermochromic properties, have been studied by X-ray single crystal method at different temperatures. It has been revealed that temperature-dependent distortions of the geometry of the thermochrom compound are not expressed anyhow substantially; therefore, the temperature behavior of thermal parameters of Br atoms was studied in detail. For the first time the interrelation between the thermochromic properties of the complex and peculiarities of atomic thermal vibrations has been disclosed: abnormally great increase in the deflection angle of the axis σ₃ of the ellipsoid of thermal vibrations of apical bromine atoms from Te-Br bond direction under the temperature reduction reflects the decrease of vibronic interaction between the ground and the first excited state of Te(IV) (the display of Jahn-Teller dynamic effect of the second order) and, correspondingly, results in asymmetry lessening of the band A in the absorption spectrum and causes the change in compound color determined by it. There has been revealed the presence of C-H…Br hydrogen bond via hydrogen atoms of phenyl rings of diphenylguanidium. Basing on the analysis of atomic thermal parameters, it has been established that the apical atom Br(4) having the shortest distance in the octahedron is linked with the central atom relatively weaker than other atoms.     

A Quasi-Two-Dimensional Trilayered CsPbBr3-based Dion-Jacobson Hybrid Perovskite toward High-Performance Photodetection

Quasi-two-dimensional (Q-2D) Dion-Jacobson (DJ) organic-inorganic hybrid perovskites based on CsPbBr₃ are promising candidates for photodetection. Previous studies have predicted that the photoresponse of such materials with high inorganic-layer numbers (n) will be more protruding in this portfolio. However, until now, only bilayered (n=2) CsPbBr₃-based DJ-type hybrid perovskites are obtained and the higher number of layers (n>2) remain completely unexplored, owing to the relatively high formation energies. Here, by incorporating diamine into the 3D CsPbBr₃ motif, a new Q-2D trilayered CsPbBr₃-based DJ-type hybrid perovskite that contains organic cation and inorganic Cs metal, namely (4-AMP)Cs₂Pb₃Br₁₀ ( 1 , 4-AMP²⁺=4-(aminomethyl)piperidinium, n=3), is obtained. Excitingly, 1 exhibits excellent photoresponse, superior to its single-layered and bilayered counterparts. The resulting photodetectors thus exhibit a large on/off ratio (>10³), high photodetectivity (6.5×10¹⁰ Jones) and fast response speed (193 μs). As far as we know, 1 is the first Q-2D CsPbBr₃-based DJ-type hybrid perovskites with high n numbers. Our results may widen the range of the potential material in application of photodetection and will be helpful to design hybrid perovskites for other advanced optoelectronic devices.     

Predicting and Screening Dielectric Transitions in a Series of Hybrid Organic–Inorganic Double Perovskites via an Extended Tolerance Factor Approach

Extended Goldschmidt tolerance factor t is applied to the hybrid double perovskites (MA)₂[B′B′′(CN)₆] (MA=methylammonium cation) to predict and screen dielectric transitions in 121 compounds through the correlations among t, the radius of the B component r_B and the transition temperature T_c, based on experimental results from model compounds. For (MA)₂[B′Co(CN)₆], it is concluded that: i) when t>0.873, the cubic phase would be stable below 298 K; ii) when 0.873>t>0.805, the cubic phase would be stable between 298 and 523 K; iii) the larger the r_B, the higher the T_c of the perovskite (T_c^1/2∝r_B); and iv) the T_c of the hybrid perovskites can be well tuned by doping the B components.     

(C6H13NH3)2(NH2CHNH2)Pb2I7: A Two‐dimensional Bilayer Inorganic–Organic Hybrid Perovskite Showing Photodetecting Behavior

Inorganic–organic hybrid perovskites, especially two‐dimensional (2D) layered halide perovskites, have attracted significant attention due to their unique structures and attractive optoelectronic properties, which open up a great opportunity for next‐generation photosensitive devices. Herein, we report a new 2D bilayered inorganic–organic hybrid perovskite, (C₆H₁₃NH₃)₂(NH₂CHNH₂)Pb₂I₇ ( HFA , where C₆H₁₃NH₃⁺ is hexylaminium and NH₂CHNH₂⁺ is formamidinium), which exhibits a remarkable photoresponse under broadband light illumination. Structural characterizations demonstrate that the 2D perovskite structure of HFA is constructed by alternant stacking of inorganic lead iodide bilayered sheets and organic hexylaminium layers. Optical absorbance measurements combined with density functional theory (DFT) calculations suggest that HFA is a direct band gap semiconductor with a narrow band gap (E_g) of ≈2.02 eV. Based on these findings, photodetectors based on HFA crystal wafer are fabricated, which exhibit fascinating optoelectronic properties including large on/off current ratios (over 10³), fast response speeds (τ_rise=310 μs and τ_decay=520 μs) and high responsivity (≈0.95 mA W^?1). This work will contribute to the design and development of new two‐dimensional bilayer inorganic–organic hybrid perovskites for high‐performance photosensitive devices.