Phase transitions, prominent dielectric anomalies, and negative thermal expansion in three high thermally stable ammonium magnesium-formate frameworks

A Metal-Free Molecular Ferroelectric [4-Me-cyclohexylamine]ClO4 Introduced by Boat and Chair Conformations of Cyclohexylamine

Organic ferroelectrics have received a great deal of interest due to their exclusive properties. However, organic ferroelectrics have not been fully explored, which hinders their practical application. Here, we presented a novel metal-free organic molecular ferroelectric [4-MCHA][ClO4 ] (1) (4-MCHA=trans-4-methylcyclohexylamine), which exhibits an above-room-temperature of 328 K. Strikingly, the single crystal structure analysis of 1 shows that the driving force of phase transition is related to the interesting chair-boat conformation change of 4-MCHA cation, in addition to the order-disorder transition of ClO4 - anion. Using piezoelectric response force microscopy (PFM), the presence of domains and the implemented polarization switching were clearly observed, which explicitly determined the presence of room-temperature ferroelectricity of 1. As far as we know, the ferroelectric phase transition mechanism attributed to the conformational change in a trans isomeric cation is very rare. This research enriched the path of designing ferroelectric materials and smart materials.

Metal-Organic Framework Optical Thermometer Based on Cr3+ Ion Luminescence

Metal-organic frameworks with perovskite structures have recently attracted increasing attention due to their structural, optical, and phonon properties. Herein, we report the structural and luminescence studies of a series of six heterometallic perovskite-type metal-organic frameworks with the general formula [EA]₂NaCr_xAl_1-x(HCOO)₆, where x = 1, 0.78, 0.57, 0.30, 0.21, and 0. The diffuse reflectance spectral analysis provided valuable information, particularly on crystal field strength (Dq/B) and energy band gap (E_g). We showed that the Dq/B varies in the 2.33-2.76 range depending on the composition of the sample. Performed Raman, XRD, and lifetime decay analyses provided information on the relationship between those parameters and the chemical composition. We also performed the temperature-dependent luminescence studies within the 80-400 K range, which was the first attempt to use an organic-inorganic framework luminescence thermometer based solely on the luminescence of Cr³⁺ ions. The results showed a strong correlation between the surrounding temperature, composition, and spectroscopic properties, allowing one to design a temperature sensing model. The temperature-dependent luminescence of the Cr³⁺ ions makes the investigated materials promising candidates for noncontact thermometers.

Dielectric switching, SHG response and Pd(ii) adsorption of a multifunctional phase-transition complex

One-dimensional coordination polymer [(C2H4OH)C4H9NS]CdCl3 has the performance of dielectric switching and SHG response. And the compound can adsorb precious Pd(ii) and the adsorption could be monitored by dielectric anomaly and SHG signal.

Thermo- and light-triggered reversible interconversion of dysprosium-anthracene complexes and their responsive optical, magnetic and dielectric properties

Artificial smart materials with switchable multifunctionality are of immense interest owing to their wide application in sensors, displays and memory devices. Lanthanide complexes are promising multifunctional materials integrating optical and magnetic characteristics. However, synergistic manipulation of different physical properties in lanthanide systems is still challenging. Herein we designed and synthesized a mononuclear complex [DyIII(SCN)3(depma)2(4-hpy)2] (1), which incorporates 9-diethylphosphonomethylanthracene (depma) as a photo-active component and 4-hydroxypyridine (4-hpy) as a polar component. This compound shows several unusual features: (a) reversible thermo-responsive phase transition associated with the order-disorder transition of 4-hpy and SCN-, which leads to thermochromic behavior and dielectric anomaly; (b) reversible photo-induced dimerization of anthracene groups, which leads to synergistic switching of luminescence, magnetic and dielectric properties. To our knowledge, compound 1 is the first example of lanthanide complexes that show stimuli-triggered synergistic and reversible switching of luminescence, magnetic and dielectric properties.

Anisotropic Thermal Expansion in an Anionic Framework Showing Guest-Dependent Phases

Crystalline materials generally show small positive thermal expansion along all three crystallographic axes because of increasing anharmonic vibrational amplitudes between bonded atoms or ions pairs on heating. In very rare cases, structural peculiarities may give rise to negative, anomalously large or zero thermal expansion behaviors, which remain poorly understood. Host-guest composites may exhibit such anomalous behavior if guest motions controllable. Here we report an anionic framework of helical nanotubes comprising three parallel helical chains. The anisotropic interaction between the guest and the framework, results in anisotropic thermal expansion in this framework. A series of detailed structural determination at 50 K intervals enable process visualization at the molecular level and the observed guest-dependent phases of the framework strongly support our proposed mechanism.

On the impact of metal ion proportion on the physical properties of heterometallic metal–organic frameworks

In the studied heterometallic structures, the interactions between the built-in EtA⁺ cation and the environment are modified by the K/Na ratio.

Nitroprusside as a promising building block to assemble an organic–inorganic hybrid for thermo-responsive switching materials

A new nitroprusside-based hybrid (Me₂NH₂)[KFe(CN)₅(NO)] exhibits thermo-responsive switching behaviours on uniaxial expansivity and SHG signal owing to flexible host–guest hydrogen bonds and the synchronously deformable inorganic framework.

Tunable Ferromagnetic Strength in Niccolite Structural Heterometallic Formate Framework Based on Orthogonal Magnetic Orbital Interactions

A series of heterometallic formate framework templated by amines were solvothermally prepared. They feature the formula of [A^I][CrM^II(HCO₂)₆] (A^I = NH₄H₂O^I and M = Mn for 1, A^I = CH₃NH₃^I and M = Fe for 2, A^I = CH₃NH₂CH₃^I and M = Co for 3, A^I = CH₃NH₃^I and M = Ni for 4). The title compounds exhibit isostructural niccolite architectures with differences only in the host metal ions and guest amines. Tunable ferromagnetic (FO) strength was realized in the resulting framework under the guidance of orthogonal magnetic orbital analysis of Cr^III (t_2g³e_g) and M^II (t_2g³e_g² for Mn^II, t_2g⁴e_g² for Fe^II, t_2g⁵e_g² for Co^II, t_2g⁶e_g² for Ni^II) ions. The magnetic ordering temperatures derived from the experimental magnetic measurements for 1-4 are lower than 2, 10.3, 7.6, and 22.0 K, respectively. Notably, thanks to the weak FO coupling between Cr^III and Mn^II ions, compound 1 displays a large magnetocaloric effect bearing the maximum of magnetic entropy change (-Δ S_m^max) up to 43.9 J kg^-1 K^-1 with Δ H = 7 T and T = 3.5 K, larger than most reported transition metal-based complexes and commercial gadolinium gallium garnet (Gd₃Ga₅O₁₂) (-Δ S_m^max = 38.4 J kg^-1 K^-1 with Δ H = 7 T). From 1, 2/3, to 4, an enhancement of the magnetic ordering temperature is observable due to the increasing strength of FO interactions between Cr^III and M^II ions. Our work provides a successful instance to modulate the strength of FO exchange via analyzing the orthogonal magnetic orbitals of heterometallic ions.

Structural, phonon, magnetic and optical properties of novel perovskite-like frameworks of TriBuMe[M(dca)3] (TriBuMe = tributylmethylammonium; dca = dicyanamide; M = Mn2+, Fe2+, Co2+, Ni2+)

Tributylmethylammonium metal dicyanamides exhibit huge caloric effects due to order–disorder phase transitions, and the Mn analogue shows orange emission under 420 nm excitation.

Phase transition and switchable dielectric behaviours in an organic-inorganic hybrid compound: (3-nitroanilinium)2(18-crown-6)2(H2PO4)2(H3PO4)3(H2O)

An organic-inorganic hybrid compound with an extensive three-dimensional (3D) crystal structure, (3-nitroanilinium)₂(18-crown-6)₂(H₂PO₄)₂(H₃PO₄)₃(H₂O) (1), was synthesized under slow evaporation conditions. Differential scanning calorimetry measurements indicated that 1 underwent a reversible phase transition at ca 231 K with a hysteresis width of 10 K. Variable-temperature X-ray single-crystal diffraction revealed that the phase transition of 1 can be ascribed to coupling of pendulum-like motions of its nitro group with proton transfer in O-H···O hydrogen bonds of the 3D framework. The temperature dependence of its dielectric permittivity demonstrated a step-like change in the range of 150-280 K with remarkable dielectric anisotropy, making 1 a promising switchable dielectric material. Potential energy calculations further supported the possibility of dynamic motion of the cationic nitro group. Overall, our findings may inspire the development of other switchable dielectric phase transition materials by introducing inorganic anions into organic-inorganic hybrid systems.

Switchable Dielectric Phase Transition Triggered by Pendulum-Like Motion in an Ionic Co-crystal

Molecular-based ionic co-crystals, which have the merits of low-cost/easy fabrication processes and flexible structure and functionality, have already exhibited tremendous potential in molecular memory switches and other electric devices. However, dipole (ON/OFF switching) triggering is a huge challenge. Here, we introduce a pendulum-like dynamic strategy to induce the order-disorder transition of a co-crystal [C₅ H₇ N₃ Cl]₃ [Sb₂ Br₉ ] (compound 1). Here, the anion and cation act as a stator and a pendulum-like rotor (the source of the dielectric switch), respectively. The temperature-dependent dielectric and differential scanning calorimetry (DSC) analyses reveal that 1 undergoes a reversible phase transition, which stems from the order-disorder transition of the cations. The thermal ON/OFF switchable motions make 1 a promising candidate to promote the development of bulk crystals as artificial intelligent dielectric materials. In addition, the pendulum-like molecular dynamics and distinct arrangements of two coexisting ions with a notable offset effect promotes/hinders dipolar reorientation after dielectric transition and provides a rarely observed but fairly useful and feasible strategy for understanding and modulating the dipole motion in crystalline electrically polarizable materials.

Effect of solvent, temperature and pressure on the stability of chiral and perovskite metal formate frameworks of [NH2NH3][M(HCOO)3] (M = Mn, Fe, Zn)

We report the synthesis, crystal structure, and thermal, Raman, infrared and magnetic properties of [NH₂NH₃][M(HCOO)₃] (HyM) compounds (M = Mn, Zn, Fe). Our results show that synthesis from methanol solution leads to perovskite polymorphs while that from 1-methyl-2-pyrrolidinone or its mixture with methanol allows obtaining chiral polymorphs. Perovskite HyFe, chiral HyFe and chiral HyMn undergo phase transitions at 347, 336 and 296 K, respectively, with symmetry changes from Pnma to Pna2₁, P6₃ to P2₁2₁2₁ and P6₃ to P2₁. X-ray diffraction and Raman studies show that the phase transitions are governed by dynamics of the hydrazinium ions. Low-temperature magnetic studies show that these compounds exhibit magnetic ordering below 9-12.5 K. Since the low-temperature structures of chiral HyMn and perovskite HyFe are polar, these compounds are possible multiferroic materials. We also report high-pressure Raman scattering studies of chiral and perovskite HyZn, which show much larger stiffness of the latter phase. These studies also show that the ambient pressure polar phases are stable up to at least 1.4 and 4.1 GPa for the chiral and perovskite phase, respectively. Between 1.4 and 2.0 GPa (for chiral HyZn) and 4.1 and 5.2 GPa (for perovskite HyZn) pressure-induced transitions are observed associated with changes in the zinc-formate framework. Strong broadening of Raman bands and the decrease in their number for the high-pressure phase of chiral HyZn suggest that this phase is disordered and has higher symmetry than the ambient pressure one.

Heterometallic perovskite-type metal–organic framework with an ammonium cation: structure, phonons, and optical response of [NH4]Na0.5CrxAl0.5−x(HCOO)3 (x = 0, 0.025 and 0.5)

We report the synthesis, crystal structure, dielectric, vibrational and emission spectra of heterometallic perovskite-type AmNaCr and AmNaAl MOFs.

Heat-sensitive structural phase transitions of hybrid halide perovskite with double dielectric ON/OFF switches

A novel one-dimensional perovskite-type hybrid compound (trimethylamine N-oxide)CdCl₃ exhibited the sensitivity of dielectric switches at high and low temperatures.

Uniaxial negative thermal expansion induced by moiety twisting in an organic crystal

Anomalous thermal expansion of a new diyn-diol molecule was studied by means of variable-temperature single-crystal X-ray diffraction. Analysis of the unit cell axes as a function of temperature shows that the material experiences uniaxial negative thermal expansion. Packing analysis of the crystal structures reveals twisting of the cyclopentyl moiety relative to the diyne spine with increasing temperature.

Magnetic layered perovskites of [CH3C(NH2)2]2[M(HCOO)4] (M = Co2+ and Ni2+): synthesis, structures and properties

Two layered perovskites of the formula [CH₃C(NH₂)₂]₂[M(HCOO)₄] (M = Co and Ni) exhibit anisotropic thermal expansion behavior and weak ferromagnetism.

Ferroelastic Phase Transition and Switchable Dielectric Constant in Heterometallic Niccolite Formate Frameworks

Four heterometallic formate frameworks templated by various alkylamine cations with the general formula [cat][Ga^IIIMn^II(HCOO)₆] {cat is MA (CH₃NH₃⁺) for 1, DMA [(CH₃)₂NH₂⁺] for 2, EtA (CH₃CH₂NH₃⁺) for 3, and DEtA [(CH₃CH₂)₂NH₂⁺] for 4} have been prepared and characterized by X-ray diffraction, differential scanning calorimetry, and dielectric studies. All of the complexes have niccolite-like structures, which possess the same [GaMn(HCOO)₆]^- anionic framework with binodal (4¹²·6³)(4⁹·6⁶) topology; only the counterions in the cavity are different. Complex 4 undergoes a reversible ferroelastic phase transition around 220 K accompanied by a thermally switchable dielectric constant transition triggered by the freezing of the order-disorder DEtA cations.

Dielectric transitions and relaxations in Ca(ii)Co(iii)-based cyanometallate frameworks with a rare (6,6)-connected nia topology

A series of cyanometallate frameworks A[CaCo(CN)₆] (A = protonated guanidine, urea or thiourea guest) exhibit a rare (6,6)-connected nia topology with the Schläfli symbol of (4¹²·6³)(4⁹·6⁶) and thermally driven dielectric transitions and relaxations without evident structural phase transitions.

Multifunctional Material with Efficient Optoelectronic Integrated Molecular Switches Based on a Flexible Thin Film/Crystal

Switchable materials, due to their potential applications in the fields of sensors, photonic devices, digital processing, etc., have been developed drastically. However, they still face great challenges in effectively inducing multiple molecular switching. Herein organic-inorganic hybrid compounds, an emerging class of hydrosoluble optoelectronic-active materials, welcome a new member with smart unique optical/electrical (fluorescence/dielectric) dual switches (switching ON/OFF), that is, [C₅H₁₃NBr][Cd₃Br₇] (1) in the form of both a bulk crystal and an ultraflexible monodirectional thin film, which simultaneously exhibits fast dielectric/fluorescent dual switching triggered by an optical/thermal/electric signal with a high signal-to-noise ratio of 35 (the highest one in the known optical/dielectric dual molecular switches). Additionally, the exceptional stability/fatigue resistance as well as the fantastic extensibility/compactness of thin films (more than 10000 times folding over 90°), makes 1 an ideal candidate for single-molecule intelligent wearable devices and seamlessly integrated optoelectronic multiswitchable devices. This opens up a new route toward advanced light/electric high-performance switches/memories based on organic-inorganic hybrid compounds.

A room temperature reversible phase transition containing dielectric switching of a host-guest supramolecular metal-halide compound

Following our recent findings on dielectric materials, we synthesized a new host-guest supramolecular metal-halide compound, [(2-AMPD)(18-crown-6)]CuCl₄ (1, 2-AMPD = 2-aminomethylpiperidinium). Systematic characterization techniques such as variable-temperature crystal structure analyses, differential scanning calorimetry (DSC) measurements, temperature-dependent dielectric measurements and powder X-ray diffraction (PXRD) measurements demonstrate that 1 undergoes a reversible phase transition at room temperature, accompanied by switchable dielectric responses and remarkable anisotropy along three different crystallographic axes. The structural phase transition mechanism is triggered by the order-disorder transition of the 18-crown-6 molecules. We believe that these findings might further promote the application of a host-guest inclusion compound in the field of switchable dielectric materials.

Lead-free Single-molecule Switching Material with Electric, Optical, Thermal Triple Controllable Multifunction Based on Perovskite-like Crystal and Flexible Thin Film

With the flourishing development of star molecule (CH₃NH₃)PbI₃, organic-inorganic perovskites with multifunction and flexibility have become a worldwide focus. However, the controllable photoelectric switchable material (especially electric, optical, thermal multifunctional switches) still face great challenges, and most of them are ceramic and toxic lead-based series. Herein a lead-free perovskite-like crystal and flexible thin film, ImMC (ImMC = (HIm)₆∙[MnCl₄∙MnCl₆]) (1), with many advantages over inorganic ceramics and lead-based perovskites, performs ideal optical and dielectric duple switching properties simultaneously. The order-disordered HIm (Im = imidazole) cations of α-type occupy two lattice sites corresponding to "Switch-ON/0" and "Switch-OFF/1" states, respectively. Interestingly, the optical and dielectric "ON/OFF or 0/1" switches can be integrated into one single-molecule single/duple channel module with high signal-noise ratio, in which the "ON/OFF" response can be precisely controlled by temperature or/and light wavelength signal to realize automatically multiple switching. In brief, the lead-free multifunctional switch opens up a brand new route and shows the mark of its real genius as a highly desirable material for its advanced applications in highly integrated circuit and ultrahigh-encrypted storage in flexible photoelectric devices.

A Series of Bimetallic Ammonium AlNa Formates

A series of AlNa bimetallic ammonium metal formate frameworks (AlNa AMFFs) have been prepared by employing various ammoniums from NH₄⁺ to large linear polyammoniums. The series consists of six perovskites of (4¹² ⋅6³ ) topology for mono-ammoniums, two chiral (4⁹ ⋅6⁶ ) frameworks incorporating polyethylene ammoniums, two niccolites with (4¹² ⋅6³ )(4⁹ ⋅6⁶ ) topology containing diammoniums, and two layered compounds made of 2D (4,4) AlNa formate sheets intercalated by small diammoniums. The first ten compounds present the structural hierarchy of (4¹² ⋅6³ )_m (4⁹ ⋅6⁶ )_n framework topologies for (m, n)=(1, 0), (0, 1), and (1, 1), respectively, in parallel to the homometallic AMFFs for divalent metals. The symmetry lowering, asymmetric formate bridges, and different hydrogen-bonding strengths appeared in the bimetallic structures owing to the different charge and size of Al³⁺ and Na⁺ seemingly inhibits the occurrence of phase transitions for more than half the AlNa AMFFs within the series, and the bimetallic members undergoing phase transitions show different transition behaviors and dielectric properties compared with the homometallic analogs. Anisotropic/negative/zero thermal expansions of the materials could be rationally attributed to the librational motion, or flip movement between different sites, of the ammonium cations, and the coupled change of AlNa formate frameworks. The thermal and IR spectroscopic properties have also been investigated.

Phase Transition, Dielectric Properties, and Ionic Transport in the [(CH3)2NH2]PbI3 Organic-Inorganic Hybrid with 2H-Hexagonal Perovskite Structure

In this work, we focus on [(CH₃)₂NH₂]PbI₃, a member of the [AmineH]PbI₃ series of hybrid organic-inorganic compounds, reporting a very easy mechanosynthesis route for its preparation at room temperature. We report that this [(CH₃)₂NH₂]PbI₃ compound with 2H-perovskite structure experiences a first-order transition at ≈250 K from hexagonal symmetry P6₃/mmc (HT phase) to monoclinic symmetry P2₁/c (LT phase), which involves two cooperative processes: an off-center shift of the Pb²⁺ cations and an order-disorder process of the N atoms of the DMA cations. Very interestingly, this compound shows a dielectric anomaly associated with the structural phase transition. Additionally, this compound displays very large values of the dielectric constant at room temperature because of the appearance of a certain conductivity and the activation of extrinsic contributions, as demonstrated by impedance spectroscopy. The large optical band gap displayed by this material (E_g = 2.59 eV) rules out the possibility that the observed conductivity can be electronic and points to ionic conductivity, as confirmed by density functional theory calculations that indicate that the lowest activation energy of 0.68 eV corresponds to the iodine anions, and suggests the most favorable diffusion paths for these anions. The obtained results thus indicate that [(CH₃)₂NH₂]PbI₃ is an electronic insulator and an ionic conductor, where the electronic conductivity is disfavored because of the low dimensionality of the [(CH₃)₂NH₂]PbI₃ structure.

Uniaxial Negative Thermal Expansion and Mechanical Properties of a Zinc-Formate Framework

The thermal expansion behavior of a metal-formate framework, Zn(HCOO)₂·2(H₂O) (1), has been systematically studied via variable temperature single-crystal X-ray diffraction. Our results demonstrate that this formate exhibits significant negative thermal expansion (NTE, -26(2) MK^-1) along its c-axis. Detailed structural analyses reveal that the large NTE response is attributed to the 'hinge-strut' like framework motion. In addition, the fundamental mechanical properties of framework 1 have been explored via nanoindentation experiments. The measured elastic modulus and hardness properties on the (00-2)/(100)/(110) facets are 35.5/35.0/27.1 and 2.04/1.83/0.47 GPa, respectively. The stiffness and hardness anisotropy can be correlated well with the underlying framework structure, like its thermoelastic behavior.

Dielectric relaxation and anhydrous proton conduction in [C2H5NH3][Na0.5Fe0.5(HCOO)3] metal–organic frameworks

First experimental direct evidence of anhydrous conductivity in the perovskite-like metal-formate framework in its ferroelectric phase.

Role of crystallization water molecules on hydrogen-bonded structures and dielectric phase transitions in amino trimethylene phosphonic acid-based crystals

Amino trimethylene phosphonic acid-based organic salts show crystallization water molecule-triggered dielectric transitions and relaxations.

Cation-templated cyanometallate-based supramolecular rectangular cage compounds showing dielectric transitions

Relatively small displacements or thermal vibrations of the polar guests in Cd(ii)–Co(iii) cyanometallate frameworks A₂{H(CdCl₂)[Co(CN)₆]} (A = monovalent cation) induce dielectric transitions and relaxations.

Growth of centimeter-sized [(CH3)2NH2][Mn(HCOO)3] hybrid formate perovskite single crystals and Raman evidence of pressure-induced phase transitions

The fewer the number of the nucleation sites formed in the vessel, the larger the size of the obtained crystals.

Dielectric and nonlinear optical dual switching in an organic–inorganic hybrid relaxor [(CH3)3PCH2OH][Cd(SCN)3]

[(CH₃)₃PCH₂OH][Cd(SCN)₃] exhibits dielectric and nonlinear optical dual switching with reversible phase transition as an organic–inorganic hybrid relaxor.

Modulating molecular structures and dielectric transitions in organic–inorganic hybrid crystals

Three hybrids display tunable structural transitions, achieving the regulation of phase transition temperature and dielectric characteristics.

Distinct room-temperature dielectric transition in a perchlorate-based organic–inorganic hybrid perovskite

Distinct room-temperature dielectric transition is realized in a perchlorate-based organic–inorganic hybrid perovskite with the introduction of a polar cationic guest.

The effect of K+ cations on the phase transitions, and structural, dielectric and luminescence properties of [cat][K0.5Cr0.5(HCOO)3], where cat is protonated dimethylamine or ethylamine

We report the synthesis, crystal structure, and dielectric, vibrational and emission spectra of novel heterometallic DMAKCr and EtAKCr MOFs.

Thermal conductivity of a perovskite-type metal–organic framework crystal

The thermal conductivity of a perovskite-type MOF crystal has been investigated for the first time.