Unique Perovskitizer N─Pb Bond Switching Induced Polar Photovoltaic Effect in Trilayered Hybrid Perovskite

Polar photovoltaic effect (PPE) has attracted great attention in regulating desired optoelectronic properties, which can be driven by order-disorder and displacive phase transitions. Bond-switching is also a feasible method to induce PPE, but such investigation is very rare. Lead-halide hybrid perovskite (LHHP) is an outstanding photodetection material; lead atoms possess rich coordination modes to provide possibilities to construct switchable bonds. Here, a unique perovskitizer N─Pb bond-switching is disclosed to induce polar photovoltage in the emerging LHHP, PA2MHy2Pb3Br10 (1, PA = n-propylamine, MHy = methylhydrazine). Interestingly, the perovskitizer MHy+ provides 2s2 lone pair while the Pb atom affords empty d orbitals, which coordinate with each other to generate a flexible N─Pb bond. Further, the introduction of N─Pb bonds results in a high distortion of the PbBr6 octahedron to form local polarity and further orientation to induce spontaneous polarization. More importantly, such a flexible N─Pb bond switching mechanism drives a notable PPE and controllable polarized photo-response, a polarization ratio up to 9.7 at the polar phase in striking contrast with the non-polar phase (1.03). The work provides the first demonstration of bond-switching to induce polar phase transition and polar photovoltage in the photoconductive hybrid perovskites for photoelectric applications.

Designing dynamic coordination bonds in polar hybrid crystals for a high-temperature ferroelastic transition

Ferroelastic materials have gained widespread attention as promising candidates for mechanical switches, shape memory, and information processing. Their phase-transition mechanisms usually originate from conventional order-disorder and/or displacive types, while those involving dynamic coordination bonds are still scarce. Herein, based on a strategic molecular design of organic cations, we report three new polar hybrid crystals with a generic formula of AA'RbBiCl6 (A = A' = Me3SO+ for 1; A = Me3SO+ and A' = Me4N+ for 2; A = A' = Me3NNH2+ for 3). Their A-site cations link to the [RbBiCl6]n2n- inorganic framework with lon topology through Rb-O/N coordination bonds, while their significantly different interactions between A'-site cations and inorganic frameworks provide distinct phase-transition behaviour. In detail, the strongly coordinative A'-site Me3SO+ cations prevent 1 from a structural phase transition, while coordinatively free A'-site Me4N+ cations trigger a conventional order-disorder ferroelastic transition at 247 K in 2, accompanied by a latent heat of 0.63 J g-1 and a usual "high → low" second-harmonic-generation (SHG) switch. Interestingly, the A'-site Me3NNH2+ cations in 3 reveal unusual dynamic coordination bonds, driving a high-temperature ferroelastic transition at 369 K with a large latent heat of 18.34 J g-1 and an unusual "low → high" SHG-switching behaviour. This work provides an effective molecular assembly strategy to establish dynamic coordination bonds in a new type of host-guest model and opens an avenue for designing advanced ferroelastic multifunctional materials.

Colossal Barocaloric Effect of Binary Fatty Acid Methyl Esters under Low Pressures near Room Temperature

Refrigeration technology based on the caloric effect is one of the more environmentally friendly alternatives to gas compression refrigeration. The barocaloric effect utilizes pressure to induce phase transition and results in a large entropy change. In this work, a colossal barocaloric effect in the liquid-solid transition (L-S-T) of binary fatty acid methyl esters (BFAMEs) was discovered. At 295 K, an isothermal entropy change as high as 591 J kg-1 K-1 and a reversible entropy change of 356 J kg-1 K-1 at a hydrostatic pressure of 80 MPa were obtained by mixing methyl palmitate and methyl stearate with a specific ratio to synthesize a BFAME. The value of the isothermal entropy change of the BFAME is comparable to that of a commercial gas compression refrigerant, R134a. This work will provide a new L-S-T candidate material to replace commercial refrigerants for the potential application of caloric effect refrigeration technology.

Pressure-freezing of dodecane: exploring the crystal structures, formation kinetics and phase diagrams for colossal barocaloric effects in n-alkanes

Barocaloric (BC) materials provide cheaper and more energy efficient alternatives to traditional refrigerants. Some liquid alkanes were recently shown to exhibit a colossal BC effect, matching the entropy changes in commercial vapour-liquid refrigerants. Dodecane was predicted to have the largest entropy change among the studied alkanes. Using synchrotron powder and single-crystal X-ray diffraction, Raman spectroscopy, and lattice energy calculations, we investigated the BC effects of n-dodecane at high pressures and room temperature. Remarkably, a colossal entropy change |ΔS| of 778 J kg-1 K-1 at 0.15(3) GPa and 295 K was observed. Spectroscopic studies revealed that this entropy change correlates closely with the conformational transition from mixed gauche to all-trans forms during pressure-induced crystallization. Additionally, the usage of a diamond anvil cell allowed the determination of the crystal structures of in situ crystallized n-un- and dodecane, as well as evaluation of the pressure-dependent crystal growth kinetics. Furthermore, our research suggests that the entropy change (per kilogram) upon compression should be similar for all n-alkanes within the range of 9-18 carbon atoms in the molecule, based on their lattice energies. Even-numbered alkanes are predicted to exhibit superior BC properties compared to their odd-numbered counterparts due to the more symmetric crystal structures and lower propensity to form plastic phases with lower transition entropy.

Halogen-regulating induced reversible high-temperature dielectric and thermal transitions in novel layered organic-inorganic hybrid semiconducting crystals

Organic-inorganic hybrid metal halides for high-temperature phase transition have become increasingly popular owing to their wide operating temperature range in practical applications, e.g., energy storage, permittivity switches and opto-electronic devices. This paper describes the subtle assembly of two new hybrid perovskite crystals, [Cl-C6H4-(CH2)2NH3]2CdX4 (X = Br 1; Cl 2), undergoing high-T reversible phase transformations around 335 K/356 K. Differential scanning calorimetry (DSC), differential thermal analysis (DTA) and VT PXRD tests uncover their reversible first-order phase transition behaviors. Furthermore, the compounds exhibit switchable dielectricity near T, making them potential dielectric switching materials. Hirshfeld surface analysis well discloses a distinct difference in hydrogen-bonding interaction between 1 and 2. UV spectra and computational analysis demonstrate that the compounds are a type of direct-band-gap semiconductor. This research will contribute an effective approach to the structure and development of multifunctional molecular hybrid crystals.

An anomalous ferroelastic phase transition arising from an unusual cis-/anti-conformational reversal of polar organic cations

Hybrid ferroelastics have attracted increasing attention for their potential application as mechanical switches. The sporadically documented anomalous ferroelastic phase transitions, i.e., ferroelasticity that appears at a high-temperature phase rather than a low-temperature phase, are of particular interest but are not well understood at the molecular level. By judiciously choosing a polar and flexible organic cation (Me₂NH(CH₂)₂Br⁺) with cis-/anti- conformations as an A-site component, we obtained two new polar hybrid ferroelastics, A₂[MBr₆] (M = Te for 1 and Sn for 2). These materials undergo distinct thermal-induced ferroelastic phase transitions. The larger [TeBr₆]^2- anions anchor the adjacent organic cations well and essentially endow 1 with a conventional ferroelastic transition (P2₁ → Pm2₁n) arising from a common order-disorder transition of organic cations without conformational changes. Moreover, the smaller [SnBr₆]^2- anions can interact with the adjacent organic cations in energetically similar sets of intermolecular interactions, enabling 2 to undergo an anomalous ferroelastic phase transition (P2₁2₁2₁ → P2₁) arising from an unusual cis-/anti-conformational reversal of organic cations. These two instances demonstrate the importance of the delicate balance of intermolecular interactions for inducing anomalous ferroelastic phase transitions. The findings here provide important insights for seeking new multifunctional ferroelastic materials.

Ferroelectric Phase Transition Driven by Switchable Covalent Bonds

The mechanism on ferroelectric phase transitions is mainly attributed to the displacive and/or order-disorder transition of internal components since the discovery of the ferroelectricity in 1920, rather than the breaking and recombination of chemical bonds. Here, we demonstrate how to utilize the chemical bond rearrangement in a diarylethene-based crystal to realize the light-driven mm2F1-type ferroelectric phase transition. Such a photoinduced phase transition is entirely driven by switchable covalent bonds with breaking and reformation, enabling the reversible light-controllable ferroelectric polarization switching, dielectric and nonlinear optical bistability. Moreover, light as quantized energy can achieve contactless, nondestructive, and remote-control operations. This work proposes a new mechanism of ferroelectric phase transition, and highlights the significance of photochromic molecules in designing new ferroelectrics for photocontrol data storage and sensing.

Pressure-Induced Ferroelastic Transition Drives a Large Shape Change in a Ni(II) Complex Single Crystal

Crystals with significant length reduction at an accessible low pressure are highly desirable for piezo-responsive devices. Here, we show a molecular crystal [Ni(en)₃](ox) (en = ethylenediamine and ox = oxalate anion) that exhibits an abrupt shape change with a contraction rate of ∼4.7% along its c axis near the phase transition pressure of ∼0.2 GPa. High-pressure single-crystal X-ray diffraction and Raman spectroscopy measurements reveal that this material undergoes a first-order ferroelastic transition from high-symmetry trigonal P3̅1c to low-symmetry monoclinic P2₁/n at ∼0.2 GPa. The oxalate anions serve as unique components, and their disorder-order transformation and rotation of 90° through cooperative intermolecular hydrogen bonding triggered unconventional anisotropic microsize contraction under compression, which can be appreciated visually. Such a prominent directional deformation at a low pressure driven by molecular motors of oxalate anions provides insights for the design of novel molecular crystal-based piezo-responsive switches and actuators in deep-sea environments.

A colossal barocaloric effect induced by the creation of a high-pressure phase

As a promising environment-friendly alternative to current vapor-compression refrigeration, solid-state refrigeration based on the barocaloric effect has been attracting worldwide attention. Generally, both phases in which a barocaloric effect occurs are present at ambient pressure. Here, instead, we demonstrate that KPF₆ exhibits a colossal barocaloric effect due to the creation of a high-pressure rhombohedral phase. The phase diagram is constructed based on pressure-dependent calorimetric, Raman scattering, and neutron diffraction measurements. The present study is expected to provide an alternative routine to colossal barocaloric effects through the creation of a high-pressure phase.

Insights into the Molecular Dynamics of Quasi-Spherical (Chloromethyl)triethylammonium Confined in a Weakly Bound Ionic Cocrystal

Here, we report a weakly bound ionic cocrystal, (Et₃NCH₂Cl)₂[ZnCl₄], which undergoes a reversible structural phase transition owing to the switched molecular dynamics of the quasi-spherical (Et₃NCH₂Cl)⁺ cation from static to dynamic. Interestingly, a unique rolling and moving mechanism is uncovered for such a cation in the high-temperature phase, where its two methylene groups exhibit different kinetic energy barriers. This study provides a meaningful insight into the solid-state molecular dynamics of large-size quasi-spherical molecules that contain both a rigid core and flexible shell.

H/F Substitution induced switchable coordination bonds in a cyano-bridged hybrid double perovskite ferroelastic

A three-dimensional cyano-bridged double perovskite ferroelastic [(CH₃)₃NCH₂F]₂[KFe(CN)₆] was constructed by introducing unprecedented switchable C-F-K coordination bonds. H/F substitution not only preserves the basic structure of the parent [(CH₃)₄N]₂[KFe(CN)₆] but also affords an m3̄mF2/m-type ferroelastic phase transition.

Two quasi-spherical molecules [1,4-diazabicyclo(3.2.2)nonane]X (X = ClO4, ReO4) exhibit switchable phase transition, dielectric and second-harmonic-generation properties

Two quasi-spherical molecules [3.2.2-Hdabc]X (1,4-diazabicyclo[3.2.2]nonane = 3.2.2-dabcn, X = ClO4, ReO4) with a high phase transition temperature exhibited switchable phase transition as well as dielectric and SHG properties.

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...

An Exceptional Thermally Induced Four-State Nonlinear Optical Switch Arising from Stepwise Molecular Dynamic Changes in a New Hybrid Salt

Switching materials in channels of nonlinear optics (NLOs) are of particular interest in NLO material science. Numerous crystalline NLO switches based on structural phase transition have emerged, but most of them reveal a single-step switch between two different second-harmonic-generation (SHG) states, and only very rare cases involve three or more SHG states. Herein, we report a new organic-inorganic hybrid salt, (Me₃ NNH₂ )₂ [CdI₄ ], which is an unprecedented case of a reversible three-step NLO switch between SHG-silent, -medium, -low, and -high states, with high contrasts of 25.5/4.3/9.2 in a temperature range of 213-303 K. By using the combined techniques of variable-temperature X-ray single-crystal structural analyses, dielectric constants, solid-state ¹³ C nuclear magnetic resonance spectroscopy, and Hirshfeld surface analyses, we disclose that this four-state switchable SHG behavior is highly associated with the stepwise-changed molecular dynamics of the polar organic cations. This finding demonstrates well the complexity of molecular dynamics in simple hybrid salts and their potential in designing new advanced multistep switching materials.

A high-temperature halide perovskite molecular ferroelastic with evident dielectric switching

Under the quasi-spherical strategy, a hybrid halide perovskite (TMTB)CdCl₃ is designed and synthesized and shows evident high-temperature ferroelastic phase transition and dielectric switching.

A pillar-layered binuclear 3D cobalt(ii) coordination polymer as an electrocatalyst for overall water splitting and as a chemosensor for Cr(vi) anion detection

A 3D pillar-layered cobalt(ii) CP with a 3D (4,6)-connected {44·610·8}{44·62} fsc net was designed and it showed great potential as an electrocatalyst in the overall water splitting and as a chemosensor for Cr(vi) anion detection.

An unprecedented hexagonal double perovskite organic-inorganic hybrid ferroelastic material: (piperidinium)2[KBiCl6]

An unprecedented A2MIMIIIX6-type double perovskite adopting a fully hexagonal BaNiO3-type structure, (piperidinium)2[KBiCl6], undergoes a 2/mF1[combining macron] ferroelastic phase transition at 285 K with a spontaneous strain of 0.0615, arising from the order-disorder transition of organic cations together with the synchronous displacement of inorganic chains.