Two‐Step Structure Phase Transition, Dielectric Anomalies, and Thermochromic Luminescence Behavior in a Direct Band Gap 2D Corrugated Layer Lead Chloride Hybrid of [(CH 3 ) 4 N ] 4 Pb 3 Cl 10

Tellurium(IV) Halide Achieving Effective Nonlinear-Optical Activity: The Role of Chiral Ligands and Lattice Distortion

Chiral organic-inorganic hybrid metal halides are a promising class of nonlinear-optical materials with unique optical properties and flexible crystal structures. However, the structures and properties of chiral hybrid tellurium halides, especially second harmonic generation (SHG), have not been reported. Here, by introducing chiral organic molecule (R/S)-methylbenzylammonium (R/S-MBA), we synthesized a pair of novel zero-dimensional (0D) chiral tellurium-based hybrid halides with noncentrosymmetric space group C2, (R/S-MBA)2TeCl6 (R/S-Cl). Single-crystal X-ray diffraction analysis and solid-state circular dichroism (CD) spectra confirm that R/S-Cl shows obvious enantiomer enrichment. Moreover, the resulting chiral products present an efficient SHG response. Interestingly, through manipulation of halogen atoms, two pairs of achiral tellurium halides, (R/S-MBA)2TeBr6 (R/S-Br) and (R/S-MBA)2TeI6 (R/S-I), were obtained, both of which crystallize in the centrosymmetric space group R3̅. It is noteworthy that R/S-I has a narrow band gap of 1.55 eV, which is smaller than that of most 0D metal halides and comparable to that of three-dimensional lead halide, showing its potential as a highly efficient light absorber.

Luminescence and Dielectric Switchable Properties of a 1D (1,1,1-Trimethylhydrazinium)PbI3 Hybrid Perovskitoid

The synthesis and investigation of the physicochemical properties of a novel one-dimensional (1D) hybrid organic-inorganic perovskitoid templated by the 1,1,1-trimethylhydrazinium (Me₃Hy⁺) cation are reported. (Me₃Hy)[PbI₃] crystallizes in the hexagonal P6₃/m symmetry and undergoes two phase transitions (PTs) during heating (cooling) at 322 (320) and 207 (202) K. X-ray diffraction data and temperature-dependent vibrational studies show that the second-order PT to the high-temperature hexagonal P6₃/mmc phase is associated with a weak change in entropy and is related to weak structural changes and different confinement of cations in the available space. The second PT to the low-temperature orthorhombic Pbca phase that corresponds to the high change in entropy and dielectric switching is associated with an ordering of the trimethylhydrazinium cations, re-arrangement and strengthening of hydrogen bonds, and slightly shifted lead-iodide octahedral chains. The high-pressure Raman data revealed two additional PTs, one between 2.8 and 3.2 GPa, related to the symmetry decrease, ordering of the cations, and inorganic chain distortion, and the other in the 6.4-6.8 GPa range related to the partial and reversible amorphization. Optical studies revealed that (Me₃Hy)[PbI₃] has a wide band gap (3.20 eV) and emits reddish-orange excitonic emission at low temperatures with an activation energy of 65 meV.

[SMe3]2[Bi2Ag2I10], a silver iodido bismuthate with an unusually small band gap

Iodido metalates of heavy main group elements have seen much research interest in the last years due to their possible application as absorbers in photovoltaics. However, for materials based on the non-toxic element bismuth one challenge lies in narrowing the optical band gap for sufficient solar absorption. Here, we present a new iodido silver bismuthate, [SMe₃]₂[Bi₂Ag₂I₁₀] (1), which is prepared from solution and characterized regarding its structure, thermal stability and optical absorption. While compounds with similar anion compositions are known, the band gap of 1.82 eV is the smallest in chain-like Bi/Ag/I-compounds that has been reported to date. To support our experimental findings we carried out computational investigations and were able to reproduce the surprisingly narrow band gap, highlighting the subtle influence of the connectivity of different building units in multinary bismuthates. We also prepared and characterized the simple iodido pentelates [SMe]₃[E₂I₉] (E = Bi, Sb; 2, 3) to provide a point of comparison.

Inorganic Chain Mediated Excitonic Properties in One-Dimensional Lead Halide Hybrid Perovskites

Semiconductors that emit intrinsic white light are considered next-generation lighting sources. Herein, the broadband emission of one-dimensional (1D) lead halide perovskites, TMAPbBr_3-xI_x (x = 0, 1, 1.5, 2, 3; TMA⁺ = tetramethylammonium), is systematically investigated. Lattice distortion causes the conversion of dark excitons to bright self-trapped excitons. Owing to its strongly localized exciton recombination and high absorption probability, TMAPbBr₃ is the most viable in this family. A delocalized hole increases the nonradiative recombination rate of excitons in TMAPbBr_3-xI_x alloys. In 1D TMAPbBr_3-xI_x perovskites, the vibration mode of the Pb-X bond stretching of the PbX₆ octahedra contributes more to the effect on exciton-phonon coupling than the mode of the X-Pb-X angle bending. Pb-X bond stretching and spontaneous polarization can tune exciton binding energy. This systematic study of excitonic behavior in 1D compounds relates the nature of ground states to the unknown excited states and provides the rational design of materials with stable and efficient broadband emission.

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.

Tunable thermotropic phase transition triggering large dielectric response and superionic conduction in lead halide perovskites

Lead halide perovskites show tunable structural phase transition, accompanied by large dielectric response and superionic conduction.

A Rotorlike Supramolecular Assembly, {[K(18-crown-6)]PbI3}∞, with a Reversible Breaking-Symmetry Phase Transition near Room Temperature

A rotorlike supramolecular crystal, {[K(18-crown-6)]PbI₃}_∞, is composed of a linear [PbI₃]_∞ chain acting as a stator and [K(18-crown-6)]⁺ cations fastened to the [PbI₃]_∞ chain and K-I bond like rotators and axes, respectively. A reversible breaking-symmetry phase transition occurs at ∼305 K. Variable-temperature ¹H NMR spectra and dielectrics were used for the dynamic analysis of [K(18-crown-6)]⁺ cations in the crystal.

Structural variations in (001)-oriented layered lead halide perovskites, templated by 1,2,4-triazolium

A new hybrid lead(ii) halide perovskite, (TzH)2PbCl4, ([TzH+] = 1,2,4-triazolium), adopts a (001)-oriented layered perovskite structure, which can be considered as derived from the n = 1 Ruddlesden-Popper (RP) type. Variable temperature single crystal X-ray diffraction reveals a structural phase transition in the region 125 K < T < 173 K between a high temperature, high symmetry polymorph, space group Cmcm, and a low temperature, low symmetry chiral polymorph, space group P212121, which has a tripled unit cell volume. UV-Vis spectra suggest a band gap of 3.30 eV for (TzH)2PbCl4. A second polymorph of the bromide analogue, (TzH)2PbBr4-II, is also reported, and structural relationships between all three variants are discussed.

Luminescent perovskites: recent advances in theory and experiments

This review summarizes previous research on luminescent perovskites, including oxides and halides, with different structural dimensionality. The relationship between the crystal structure, electronic structure and properties is discussed in detail.