An Organic-Inorganic Hybrid Pyrrolidinium Ferroelectric Based on Solvent Selective Effect

Halide Anions Tuning of Lead-Free Perovskite-Type Ferroelectric Semiconductors with Inverse High-Temperature Symmetry Breaking

There is a noticeable gap in the literature regarding research on halogen-substitution-regulated ferroelectric semiconductors featuring multiple phase transitions. Here, a new category of 1D perovskite ferroelectrics (DFP)2SbX5 (DFP+ = 3,3-difluoropyrrolidium, X- = I-, Br-, abbreviated as I-1 and Br-2) with twophase transitions (PTs) is reported. The first low-temperature PT is a mmmFmm2 ferroelectric PT, while the high-temperature PT is a counterintuitive inverse temperature symmetry-breaking PT. By the substitution of iodine with bromine, the Curie temperature (Tc) significantly increases from 348 K of I-1 to 374 K of Br-2. Their ferroelectricity and pyroelectricity are improved (Ps value from 1.3 to 4.0 µC cm-2, pe value from 0.2 to 0.48 µC cm-2 K-1 for I-1 and Br-2), while their optical bandgaps increased from 2.1 to 2.7 eV. A critical slowing down phenomenon is observed in the dielectric measurement of I-1 while Br-2 exhibits the ferroelastic domain. Structural and computational analyses elucidate that the order-disorder movement of cations and the distortion of the chain perovskite [SbX5]2- anions skeleton lead to PT. The semiconductor properties are determined by [SbX5]2- anions. The findings contribute to the development of ferroelectric semiconductors and materials with multiple PTs and provide materials for potential applications in the optoelectronic field.

Tunability of Photovoltaic Functions via Halogen Substitution [(Ade)2 CdX4](X = Cl, Br): A Class of Three-Dimensional Organic-Inorganic Hybrid Materials

Two new three-dimensional organic-inorganic hybrid crystalline materials, [(Ade)2 CdCl4] (1) and [(Ade)2 CdBr4] (2), were obtained by the slow evaporation of adenine (Ade) and cadmium chloride in aqueous solution at room temperature with hydrochloric acid and hydrobromic acid used as halogen sources. The structural, thermal, optical, and electrical properties were characterized by single-crystal X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, variable-temperature-variable-frequency dielectric constant analysis, and electrochemical tests. With increasing the substitution of Cl by Br, the composition of the material changed and the space group shifted from P-1 to P21/m, with a significant blue-shift in the fluorescence emission. Changing the temperature induced the deformation of the three-dimensional framework structure formed by hydrogen bonding interactions, leading to dielectric anomalies. Cyclic voltammetry tests showed the good reversibility of the electrolysis process. The structural diversity of the complexes was realized by modulating the halogen composition, and a new method for designing novel organic-inorganic hybrids with controllable photoelectric functionality was proposed.

A room temperature ferroelectric material with photoluminescence: (1,3-dicyclohexylimidazole)2MnCl4

Molecular ferroelectric materials have been widely used in capacitors and sensors due to their low cost, light weight, flexibility and good biocompatibility. Organic-inorganic hybrid complexes, on the other hand, have received a great deal of attention in the luminescence field due to their low cost and simple preparation. The combination of ferroelectricity and photoluminescence in organic-inorganic hybrid materials not only leads to tunable optical properties, but also enriches potential applications of multifunctional ferroelectrics in optoelectronic devices. Here, we report a new luminescent ferroelectric material (1,3-dicyclohexylimidazole)₂MnCl₄ (DHIMC). Thermogravimetric analysis (TGA) was used to measure the mass change of the material at a measurement rate of 20 K min^-1 from room temperature to 900 K, and we found that this material has good thermostability, which is up to 383 K. Meanwhile, UV-vis measurements showed that it is also a fluorescent material emitting a strong green fluorescence at the wavelength of 525 nm. The ferroelectricity of the crystal was determined by two different methods: the Sawyer-Tower method and the double-wave method (DWM). Particularly, the single crystal experiences a phase transition from the ferroelectric phase to the paraelectric phase during the heating/cooling process at 318 K/313 K and the space group changes from P1̄ (centrosymmetric) to P1 (non-centrosymmetric). This work will enrich multifunctional luminescent ferroelectric materials and their application in display and sensing.

Lead-Free Hybrid Organic-Inorganic Ferroelectric: (3,3-Difluoropyrrolidinium)2ZnCl4·H2O

Hybrid organic-inorganic ferroelectrics (HOIFs) have a wide range of applications in the optoelectronic field in terms of rich optoelectronic properties. Particularly, lead-free HOIFs have attracted extensive attention due to their environmental friendliness, low heavy metal toxicity, and low synthesis cost. However, there are few reports about Zn-based HOIFs due to their uncontrollable ferroelectric synthesis and other reasons. Here, we designed and synthesized a zinc-based zero-dimensional (3,3-difluoropyrrolidine)₂ZnCl₄·H₂O (DFZC) single crystal, which undergoes a phase transition from ferroelectric to paraelectric phase (space group from Pna2₁ to Pnma) at 295.5 K/288.9 K during the heating/cooling process. The systematic study shows that the ferroelectric phase transition is a displacive type. The ferroelectric hysteresis loop of DFZC was obtained by the double-wave method and the Sawyer-Tower method, which has a spontaneous polarization (P_s) of ∼0.4 μC/cm². This work reveals the strategy to design new zinc-based lead-free HOIFs for potential applications in optoelectronic fields.

Solvent Selective Effect Occurs in Iodinated Adamantanone Ferroelectrics

Organic ferroelectrics, as a type of crystalline compound, are generally solution processing. However, for most crystalline compounds, the changing of solvent would not influence the crystalline phase, let alone their physical performance. Here, the solvent selective effect occurs in the iodinated adamantanone ferroelectrics. By changing the solvent with different polarities, the ferroelectric crystals can be induced in two different phases, which is unprecedented to the knowledge. More strikingly, this solvent-induced transformation could realize the physical performance optimization in the orthorhombic phase (orth-I-OA, obtained from ethanol) with a stronger second harmonic generation (SHG) response, greater piezoelectric coefficient d₃₃ of 5 pC N^-1 , and larger spontaneous polarization (P_s ) of 3.43 µC cm^-2 than those of monoclinic one (mono-I-OA, obtained from ethyl acetate). Such an intriguing phenomenon might be closely related to solvent polarity. Based on the quantitative and qualitative analyses, the similar interaction energies of these two phases suggest that their transformation could be easily realized via changing the solvent. This work provides new insights into the chemical design and performance optimization of organic ferroelectrics.

H/F Substitution on the Spacer Cations Leads to 1D-to-2D Increment of the Pyrrolidinium-Containing Lead Iodide Hybrid Perovskites

Hybrid organic-inorganic perovskites (HOIPs) have emerged as multifunctional materials with remarkable optical and electronic properties. In particular, 2D-layered lead iodide-based HOIPs possess great practical application potential in the photoelectric field. In this work, we report H/F substitution-induced 1D-to-2D increment of lead iodide HOIPs. The enantiomeric HOIPs, S- and R-FPPbI₃ (FP = 3-fluoropyrrolidinium), were achieved by monofluoride substitution on the spacer cations of the parent HOIP, PyPbI₃ (Py = pyrrolidinium), showing mirror image structural relationship and reversible solid-state phase transition. A 2D-layered HOIP, (DFP)₂PbI₄ (DFP = 3,3-difluoropyrrolidinium), was achieved with a low band gap of 2.09 eV through difluoride substitution, thanks to the expansion of the Pb-I network from 1D to 2D. This work highlights the exploration of 1D chiral and 2D-layered HOIP materials with reversible phase transitions through H/F substitution strategies.