Exploring Pyroelectricity, Thermal and Photochemical Switching in a Hybrid Organic-Inorganic Crystal by In Situ X-Ray Diffraction

The switching behavior of the novel hybrid material (FA)Na[Fe(CN)5(NO)].H2O (1) in response to temperature (T), light irradiation and electric field (E) is studied using in situ X-ray diffraction (XRD). Crystals of 1 display piezoelectricity, pyroelectricity, second and third harmonic generation. XRD shows that the FA+ are disordered at room-temperature, but stepwise cooling from 273-100 K induces gradual ordering, while cooling under an applied field (E=+40 kVcm-1) induces a sudden phase change at 140 K. Structural-dynamics calculations suggest the field pushes the system into a region of the structural potential-energy surface that is otherwise inaccessible, demonstrating that application of T and E offers an effective route to manipulating the crystal chemistry of these materials. Photocrystallography also reveals photoinduced linkage isomerism, which coexists with but is not correlated to other switching behaviors. These experiments highlight a new approach to in situ studies of hybrid materials, providing insight into the structure-property relationships that underpin their functionality.

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.

Molecular Design of a Metal-Nitrosyl Ferroelectric with Reversible Photoisomerization

The development of photo-responsive ferroelectrics whose polarization may be remotely controlled by optical means is of fundamental importance for basic research and technological applications. Herein, we report the design and synthesis of a new metal-nitrosyl ferroelectric crystal (DMA)(PIP)[Fe(CN)₅(NO)] (1) (DMA = dimethylammonium, PIP = piperidinium) with potential phototunable polarization via a dual-organic-cation molecular design strategy. Compared to the parent non-ferroelectric (MA)₂[Fe(CN)₅(NO)] (MA = methylammonium) material with a phase transition at 207 K, the introduction of larger dual organic cations both lowers the crystal symmetry affording robust ferroelectricity and increases the energy barrier of molecular motions, endowing 1 with a large polarization of up to 7.6 μC cm^-2 and a high Curie temperature (T_c) of 316 K. Infrared spectroscopy shows that the reversible photoisomerization of the nitrosyl ligand is accomplished by light irradiation. Specifically, the ground state with the N-bound nitrosyl ligand conformation can be reversibly switched to both the metastable state I (MSI) with isonitrosyl conformation and the metastable state II (MSII) with side-on nitrosyl conformation. Quantum chemistry calculations suggest that the photoisomerization significantly changes the dipole moment of the [Fe(CN)₅(NO)]^2- anion, thus leading to three ferroelectric states with different values of macroscopic polarization. Such optical accessibility and controllability of different ferroelectric states via photoinduced nitrosyl linkage isomerization open up a new and attractive route to optically controllable macroscopic polarization.

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.

Near-room-temperature dielectric switch and thermal expansion anomaly in a new hybrid crystal: (Me2NH2)[CsFe(CN)5(NO)]

A new hybrid crystal, (Me2NH2)[CsFe(CN)5(NO)], featuring a double-layered nitroprusside-based inorganic framework with cubic-like cages encapsulating organic cations, undergoes a near-room-temperature phase transition accompanying with dielectric switch and thermal expansion anomaly....

Structural Phase Transition and Switchable Dielectric Behaviour of a One-Dimensional Chains Niobium Oxyfluoride

tructural phase transition is always accompanied by measurable properties change of material, which provides us more opportunities for material modification. Herein, a new niobium oxyfluoride, [HIm][NbOF4] (HIm = imidazole cation,...

A new crown-ether clathrate [15-crown-5][Y(NO3)2(H2O)5][NO3] with switchable dielectric constant behaviour

A crown-ether clathrate employing a rare-earth ion as the central metal ion was developed, and shown to display switchable dielectric behaviours around a temperature of about 230 K.

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 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.

Stepwise Dielectric Switching Occurs in Two Photo-Responsive Complexes Possessing Two-Dimensional Structures

Two organic-inorganic hybrid complexes, (CH₃NH₃)Na[Fe(CN)₅NO]·H₂O (1) and (CH₃NH₃)₂[Fe(CN)₅NO] (2), which exhibit stepwise dielectric switching as well as photo-induced structural transformation, are synthesized and examined. In these two compounds, the photo-responsive complex anions, [Fe(CN)₅NO]^2-, connected by Na⁺ through N-Na coordination bonds or CH₃NH₃⁺ through N···H-N hydrogen bonds, form two-dimensional structures. One organic cation, CH₃NH₃⁺, that resides in the intralaminar cavity and plays a role as a template, undergoes a temperature-controlled order-disorder structural phase transition. As the frozen-thawed state change of the polar organic cations modifies the polarizability of materials, stepwise dielectric switching is observed at the phase transition temperature. Furthermore, the photo-induced linkage isomerism of [Fe(CN)₅NO]^2- building block survives in the new compounds at the low-temperature range, which is verified by variable-temperature IR spectra after photo-irradiation. The coexistence of switchable dielectric properties and photo-induced structural variation suggests multiple optical-electric roles of the present materials.