A fluorine and chlorine substituted 2D lead bromide perovskite with high phase transition temperature

The Halogenation Effect Induces a Variety of Switchable Phase Transition and Second-Harmonic-Generation Materials

Halogen engineering offers a means of enhancing the physical properties of materials by fine-tuning the rotational energy barrier and dipole moment, which proved to be effective in achieving switchable phase transitions and optical responses in materials. In this work, by substituting the methyl group in ligand N-ethyl-1,5-diazabicyclo[3.3.0]octane (CH3CH2-3.3.0-Dabco) with halogen atoms X (Cl or Br) and then contining to react it with FeBr3 in a HBr aqueous solution, we successfully synthesized three kinds of organic-inorganic hybrid switchable phase-change materials, [CH3CH2-3.3.0-Dabco]FeBr4 (1), [ClCH2-3.3.0-Dabco]FeBr4 (2), and [BrCH2-3.3.0-Dabco]FeBr4 (3), which were fully characterized by single-crystal X-ray diffraction and variable-temperature powder X-ray diffraction. Compared to compound 1, compounds 2 and 3 show two pairs of reversible phase transitions, dielectric anomalies, and a second-harmonic-generation effect, which are successfully induced due to the halogen substitution. This study offers an effective molecular design strategy for the exploration and construction of iron halide organic-inorganic hybrid materials with temperature-adjustable physical properties.

Temperature-dependent excited states for detecting reversible phase transitions in 2D lead(II) iodide perovskites

Significant interest exists in water-tolerant 2D lead iodide perovskites owing to their stability and proven potential in photovoltaic and photonic applications. These materials have solid-state phase transitions that are accessible below 100 °C. Here, the study witnesses the multiple phase transitions of the last members of a series of organic-inorganic hybrid materials, [(CnH2n+1NH3)2PbI4], with even n as n = 14, 16, and 18, once again. By employing temperature-dependent steady-state photoluminescence (PL) and temperature-dependent time-resolved photoluminescence (TRPL) spectroscopy in the temperature range of -18 to +90 °C and at -196 °C, we explore the thermal responses of these materials. The investigation reveals reversible phase transitions occurring between room temperature (RT) and elevated temperatures, impacting the optical properties and emitting colors of the perovskite compounds. The longer the alkyl chain, the higher the phase transition temperature, attributed to increased conformational disorder and enhanced perovskite symmetry. The decay constants for all compounds are very close in value, which confirms the underlying excited-state dynamics, pointing to contributions primarily from inorganic components across different phases. We anticipate that our results on the detection of phase transitions in 2D perovskites will not only motivate the use of these techniques for detecting phase transitions but also would help to understand their excited states in more details to selectively use them for solar cell and next-generation display technologies.

Alkali Metal Organic-Inorganic Hybrid Compounds with Different Crystal Dimensions Show Phase-Transition, Dielectric, and SHG Properties Based on a Quasi-Spherical Amine (1S,4S)-2,5-Diazabicyclo[2.2.1]heptane

Reactions of a chiral and quasi-spherical molecule [1S,4S-2,5-2.2.1-H2dabch]I2 (1) with alkali metal halide MX (M = Na, K, Cs; X = Cl, Br) at room temperature produced a series of organic-inorganic hybrid (OIH) materials [1S,4S-2,5-2.2.1-H2dabch]NaBr3 (2), [1S,4S-2,5-2.2.1-H2dabch]CsCl3·H2O (3) and [1S,4S-2,5-2.2.1-H2dabch]KBr3·H2O (4). The single-crystal X-ray diffraction analysis revealed that the organic-inorganic framework structures comprised of the templating ligand and alkali metal halides (NaBr, CsCl, KBr) displayed dimensions spanning from one-dimensional (1D) to three-dimensional (3D). Moreover, the results of both differential scanning calorimetry (DSC) and dielectric measurements demonstrated that compounds 1-4 displayed reversible, high-temperature phase transitions and noticeable dielectric anomalies. In addition, the temperature-dependent second harmonic generation (SHG) results revealed crystals 1 and 3 can switch from the SHG-ON to the SHG-OFF state, which was proved by the variable-temperature X-ray diffraction. This research aims to streamline the exploration of multifunctional second harmonic generation (SHG) and dielectric materials that have been synthesized using chiral ligands and alkali metals. This will provide researchers with enhanced opportunities to delve further into this specific research domain.

Three-Dimensional Perovskite Phase Transition Materials with Switchable Second Harmonic Generation Properties Introduced by Homochiral (1S,4S)-2,5-Diazabicyclo[2.2.1]-heptane

Switchable second harmonic generation (SHG) materials have potential applications in information storage, signal processing, and so on because they can switch between SHG-on and SHG-off states. In this work, we designed and synthesized three organic-inorganic hybrid Rb halide three-dimensional (3D) perovskite materials [1S,4S 2,5-2.2.1-H₂dabch]RbX₃·0.5H₂O (X = Cl, 1; Br, 2; I, 3) based on the chiral 1S,4S-2,5-diazabicyclo[2.2.1]heptane (1S,4S-2,5-2.2.1-dabch). The selection of homochiral organic cations ensures that the compounds 1∼3 crystallize in the noncentrosymmetric and chiral space group P2₁2₁2₁, which further leads to reversible SHG responses of the three compounds. Through differential scanning calorimetry (DSC) and dielectric measurements, it revealed that the phase transition point of the compounds 1∼3 increased with RbCl, RbBr, and RbI. This is because the hydrogen interaction H···X between the inorganic framework [RbX₃]_n and the organic cation [1S,4S-2,5-2.2.1-H₂dabch]²⁺ is increased with the order of I > Br > Cl. This study can provide an effective molecular design strategy for the exploration and construction of temperature-tunable SHG switching materials.

3D Perovskite (1,5-3.2.2-H2dabcn)CsBr3 with Reverse Symmetry Breaking

Even though hybrid organic-inorganic perovskites (HOIPs) have been studied by many scholars in recent years, there are not many reports on three-dimensional (3D) alkali metal cesium halide perovskites. Here, we report an unprecedented 3D HOIP molecule (1,5-3.2.2-H₂dabcn)CsBr₃ (1), in which the 3D anionic framework is constructed by corner-sharing CsBr₆ octahedra and organic cations [1,5-3.2.2-H₂dabcn]²⁺ are located in the cavities formed by the octahedra. Organic cations interact with an inorganic metal frame via two N-H···Br hydrogen bonds. Compound 1 undergoes a reversible order-disorder phase transition and exhibits switchable dielectric and second-harmonic generation (SHG) properties. Interestingly, product 1 crystallizes in a non-centrosymmetric space group Pmn2₁ at the low-temperature phase (LTP) and transforms into a centrosymmetric space group P2/m at the high-temperature phase (HTP). The space group Pmn2₁ in the LTP has a higher symmetry than P2/m in the HTP. This inverted symmetry breaking is very unusual.