Switching Dielectric Constant Near Room Temperature in a Molecular Crystal

Sequence of Structural and Magnetic Phase Transitions in (NO)Mn6(NO3)13

New nitrosonium manganese(II) nitrate, (NO)Mn6(NO3)13, has been synthesized and structurally characterized. In the temperature range of 45-298 K, the crystal is hexagonal (centrosymmetric sp. gr. P63/m). Mn2+ ions are assembled into tubes along axis c with both NO3- filling and coating. The nitrosonium cation is located in the framework cavity and is disordered by a 3-fold axis. At the temperature TS1 = 190 K, a structural phase transition related to the libration of the intertube NO3 group and a small variation of Mn polyhedron is observed. Moreover, the anomalies in physical properties of (NO)Mn6(NO3)13 allow suggesting that ordering of NO+ units occurs at low temperatures. The antiferromagnetic ordering in this compound is preceded by the formation of a short-range correlation regime at about 25 K and takes place in two steps at TN1 = 12.0 K and TN2 = 8.4 K.

Halogen Substitution Regulates High Temperature Dielectric Switch in Lead-Free Chiral Hybrid Perovskites

Hybrid metal halides (HMHs) based phase transition materials have received widespread attention due to their excellent performance and potential applications in energy harvesting, optoelectronics, ferroics, and actuators. Nevertheless, effectively regulating the properties of phase transitions is still a thorny problem. In this work, two chiral lead-free HMHs (R-3FP)2 SbCl5 (1; 3FP=3-fluoropyrrolidinium) and (R-3FP)2 SbBr5 (2) were synthesized. By replacing the halide ions in the inorganic skeleton, the phase transition temperature of 2 changes with an increase of about 20 K, compared with 1. Meanwhile, both compounds display reversible dielectric switching properties. Through crystal structure analysis and Hirshfeld surface analysis, their phase transitions are ascribed to the disorder of the cations and deformation of the inorganic chains.

Hydrogen Bond-Driven Order-Disorder Phase Transition in the Near-Room-Temperature Nonlinear Optical Switch [Ag(NH3)2]2SO4

Herein, we report a near-room-temperature nonlinear optical (NLO) switch material, [Ag(NH₃)₂]₂SO₄, exhibiting switching performance with strong room-temperature second harmonic generation (SHG) intensity that outperforms the UV-vis spectral region industry standard KH₂PO₄ (1.4 times stronger). [Ag(NH₃)₂]₂SO₄ undergoes a reversible phase transition (T _c = 356 K) from the noncentrosymmetric room-temperature phase (P4̅2₁ c, RTP) to a centrosymmetric high-temperature phase (I4/mmm, HTP) where both the SO₄ ^2- anions and [Ag(NH₃)₂]⁺ cations are highly disordered. The weakening of hydrogen bond interactions in the HTP is also evidenced by the lower energy shift of the stretching vibration of the N-H···O bonds revealed by the in situ FT-IR spectra. Such weakening leads to an unusual negative thermal expansion along the c axis (-3%). In addition, both the atomic displacement parameters of the single-crystal diffraction data and the molecular dynamics-simulated mean squared displacements suggest the motions of the O and N atoms. Such a structural disorder not only hinders the phonon propagation and dramatically drops the thermal conductivity to 0.22 W m^-1 K^-1 at 361 K but also significantly weakens the optical anisotropy and SHG as verified by the DFT theoretical studies.

Fluoride-bridged dinuclear dysprosium complex showing single-molecule magnetic behavior: supramolecular approach to isolate magnetic molecules

Using Na-encapsulated benzo[18]crown-6 (Na)(B18C6) as a counter cation, we successfully magnetically isolated a fluoride-bridging Dy dinuclear complex {[(PW11O39)Dy(H2O)2]2F} (Dy2POM) with lacunary Keggin ligands. (Na)(B18C6) formed two types of tetramers through C-H⋯O, π⋯π and C-H⋯π interactions, and each tetramer aligned in one dimension along the c-axis to form two types of channels. One channel was partially penetrated by a supramolecular cation from the ±a-axis direction, dividing the channel in the form of a "bamboo node". Dy2POM was spatially divided by this "bamboo node," which magnetically isolated one portion from the other. The temperature dependence of the magnetic susceptibility indicated a weak ferromagnetic interaction between the Dy ions bridged by fluoride. Dy2POM exhibited the magnetic relaxation characteristics of a single-molecule magnet, including the dependence of AC magnetic susceptibility on temperature and frequency. Magnetic relaxation can be described by the combination of thermally active Orbach and temperature-independent quantum tunneling processes. The application of a static magnetic field effectively suppressed the relaxation due to quantum tunneling.

A ferroelastic molecular rotor crystal showing inverse temperature symmetry breaking

A molecular rotor crystal shows a ferroelastic phase transition with unique inverse temperature symmetry breaking which is a result of concerted molecular movement triggered by anisotropic steric repulsion among adjacent molecules.

Toward the Prediction of Multi-Spin State Charges of a Heme Model by Random Forest Regression

The random forest regression (RFR) model was introduced to predict the multiple spin state charges of a heme model, which is important for the molecular dynamic simulation of the spin crossover phenomenon. In this work, a multiple spin state structure data set with 39,368 structures of the simplified heme-oxygen binding model was built from the non-adiabatic dynamic simulation trajectories. The ESP charges of each atom were calculated and used as the real-valued response. The conformational adapted charge model (CAC) of three spin states was constructed by an RFR model using symmetry functions. The results show that our RFR model can effectively predict the on the fly atomic charges with the varying conformations as well as the atomic charge of different spin states in the same conformation, thus achieving the balance of accuracy and efficiency. The average mean absolute error of the predicted charges of each spin state is <0.02 e. The comparison studies on descriptors showed a maximum 0.06 e improvement in prediction of the charge of Fe 2+ by using 11 manually selected structural parameters. We hope that this model can not only provide variable parameters for developing the force field of the multi-spin state but also facilitate automation, thus enabling large-scale simulations of atomistic systems.

Covalently Bonded Pillared Layered Bromoplumbate with High Thermal Stability: High Capacitance Gain after Photoinduced Electron Transfer

Improving the stability and photoelectric properties are current aims in the field of inorganic-organic hybrid lead halides. In this work, a new covalently bonded pillared layered bromoplumbate, [Pb₃Br₆(CV)]_n, was prepared using a photoactive zwitterion viologen, N,N'-4,4'-bipyridiniodipropionate (CV), as a ligand. It has a high thermal stability in air and shows a remarkable increase of capacitance after photoinduced electron transfer (PIET). The observed dielectric switch ratio of ∼689% at 3 kHz exceeded all reported values at room temperature for switchable dielectric materials. The capacitance gain was derived from the redistribution of electrons after PIET from the lead-halide layer to organic π-aggregates, so that the activation energy of hopping polarizability was significantly reduced.

A Small Lattice Change Induces Significant Dynamic Changes of CH3NH3+ Caged in Hybrid Perovskite Crystals: Toward Understanding the Interplay between Host Lattices and Guest Molecules

Two perovskite-type compounds, (MA)₂[B'Co(CN)₆] (MA = methylammonium, B' = K(I) and Na(I)), have very similar structures, but exhibit marked differences in the phase and dielectric transitions. Solid state ²H NMR studies reveal the detailed dynamic changes of the caged methylammonium (MA) cations before and after the phase transitions, which are correlated with the different dielectric states of the compounds. Using solid state ⁵⁹Co NMR, the dynamic changes of the host lattices before and after the transitions, which accompany the changes in the dynamics of the caged MA cations, are unveiled, demonstrating the intriguing interplay between the MA cations and the host lattices. On the basis of these observations, the molecular origins of the dielectric transitions are discussed in detail.

Molecular Dynamics, Phase Transition and Frequency-Tuned Dielectric Switch of an Ionic Co-Crystal

Dielectric switches that can be converted between high and low dielectric states by thermal stimuli have attracted much interest owing to their many potential applications. Currently one main drawback for practical application lies in the non-tunability of their switch temperatures (T_S ). We report here an ionic co-crystal (Me₃ NH)₄ [Ni(NCS)₆ ] that contains a multiply rotatable Me₃ NH⁺ ion and a solely rotatable one due to a more spacious supramolecular cage for the former one. This compound undergoes an isostructural order-disorder phase transition and it can function as a frequency-tuned dielectric switch with highly adjustable T_S , which is further revealed by the variable-temperature structure analyses and molecular dynamics simulations. In addition, the distinct arrangements and molecular dynamics of two coexisting Me₃ NH⁺ ions confined in different lattice spaces as well as the notable offset effect on the promoting/hindering of dipolar reorientation after dielectric transition provide a rarely observed but fairly good model for understanding and modulating the dipole motion in crystalline environment.

Thermal-Induced Dielectric Switching with 40K Wide Hysteresis Loop Near Room Temperature

A thermal-induced dielectric switching has been realized in two ion-pair crystal [C₂H₆N₅]⁺·[H₂PO₄]^- (1, C₂H₆N₅ = 3,5-diamino-1,2,4-triazolinium) through single-crystal-to-single-crystal phase transition (SCSC-PT). Upon cooling from room temperature, the 1D cation stripes that are composed of [C₂H₅N₅]⁺ cations have undergone a 90° sharp rotation around the c axis, accompanied by the transition of crystal stacking from loose unparallel (dynamic state) to compression parallel (static state) and reorientation of dipoles on the [C₂H₅N₅]⁺ cation, which thus resulted in high dielectric state to low dielectric state transformation. While on the warming run, the reverse process was rather sluggish, resulting in a reversible dielectric switching with ultralarge (about 40K wide) hysteresis loop near room temperature. It is thought that the large-sized polar cation stripes have a predominant influence on the switching properties of 1.

N-Methylpyrrolidinium hydrogen tartrate (NMPHT): an above-room-temperature order–disorder molecular switchable dielectric material

A new above-room-temperature molecular switchable dielectric material, which undergoes a first-order phase transition induced by the order-disorder transformation of cation, has been reported.

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.

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.

Prominent dielectric transitions in layered organic–inorganic hybrids: (isoamyl-ammonium)2CdX4 (X = Cl and Br)

Two new organic–inorganic layered perovskite-type hybrid compounds demonstrate different dynamic motions of cations, contributing to a significant difference in dielectric transitions.

Switchings of dielectric constant, second harmonic generation and polarization in a polar hybrid cyanometallate crystal

A polar-to-polar phase transition in a cyanometallate crystal (TA)₂{(H₃O)[Co(CN)₆]} (TA = thiazolium) triggers multiple switchings of the dielectric constant, second harmonic generation and polarization.

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 rB and the transition temperature Tc , 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 rB , the higher the Tc of the perovskite (Tc (1/2) ∝rB ); and iv) the Tc of the hybrid perovskites can be well tuned by doping the B components.

The cation-dependent structural phase transition and dielectric response in a family of cyano-bridged perovskite-like coordination polymers

The cation-dependent phase transition and the relevant dielectric response were demonstrated in a family of cyano-bridged perovskite-like coordination polymers, [(CH₃)_nNH_4−n]₂[KFe(CN)₆] (n = 1–4).

A perovskite-type cage compound as a temperature-triggered dielectric switchable material

The perovskite-type cage compound [C₃H₆NH₂]₂[KCo^III(CN)₆] shows a temperature-triggered dielectric switch. The switchable properties are derived from an order–disorder phase transition of a system, by changing the motions of the polar azetidine cation guests.

Temperature-triggered order–disorder phase transition in molecular-ionic material N-butyldiethanolammonium picrate monohydrate

We report an organic–ionic material that undergoes a first-order structural phase transition, induced by order–disorder of oxygen atoms in picrate anion. This strategy offers a potential pathway to explore new switchable dielectric materials.

Tuning dielectric transitions by B′-site mixing in hybrid double perovskite crystals (CH3NH3)2[K1−xRbxCo(CN)6] (x = 0.23–0.62)

Mixing of the B′-site metals in (CH₃NH₃)₂[K_1−xRb_xCo(CN)₆] tunes the phase transition temperatures and therefore the switchable dielectric constant properties.