Pb4(OH)4(BrO3)3(NO3): An Example of SHG Crystal in Metal Bromates Containing π-Conjugated Planar Triangle

A Two-Dimensional Hybrid Perovskite With Heat Switching Birefringence

Birefringent crystals that can switch light polarization have important applications in optoelectronics. In the last decades, birefringence is mostly optimized by chemical strategies. Recently, switching birefringence by physical means has attracted much attention. Here, this work reports the observation of heat switching birefringence in a 2D layered hybrid halide perovskite (C2 N3 H4 )2 PbCl4 ((C2 N3 H4 )+ =1,2,4-triazolium). This heat switching birefringence leads to a significant change in the interference color for the crystal plate under the illumination of orthogonal polarized light. Structure analyses reveal a heat dependent structure transition in (C2 N3 H4 )2 PbCl4 , whose birefringence is switched by the change in the distortion degree of PbCl6 octahedron. This discovery may be beneficial to the further development of stimuli-responsive polarization optical devices.

Halogen Bonding Assembles Anion⋅⋅⋅Anion Architectures in Non-centrosymmetric Iodate and Bromate Crystals

Single crystal X-ray diffraction of iodate and bromate salts shows that the I and Br atoms in IO3 - and BrO3 - anions form short and linear O-I/Br⋅⋅⋅O contacts with the O atoms of nearby anions. Non-centrosymmetric systems are formed wherein anions are orderly aligned into supramolecular 1D and 2D networks. Theoretical evidences, namely the outcome of QTAIM and NCIplot studies, prove the attractive nature of these contacts and the ability of iodate and bromate anions to act as robust halogen bond (HaB) donors. The HaB is proposed as a general and effective assisting tool to control the architecture of acentric iodate salts.

On closed-shell interactions between heavy main-group elements

A series of di- and polymetal complexes involving closed-shell, heavy main-group atoms and ions shows a selection of special physical properties. These involve short metal-metal contacts, low entropies of formation and, most interestingly, strong Raman bands at low wavenumbers. These results together with the constitution of the coordination compounds, where the majority of electrons are assembled on the highly polarizable metal atoms and ions, experimental results have been interpreted in terms of direct, partial covalent metal-metal bonding. Previous theoretical studies have challenged this view and instead attributed the obvious attractive forces involved to secondary-type of interactions, such as dispersion. This study utilizes a multitude of theoretical tools, such as natural bond order (NBO) and natural energy decomposition analysis (NEDA), non-covalent interaction (NCI) analysis, electron localization functions (ELFs), and atoms-in-molecules (AIM) to characterize the interactions in models comprising closed-shell dimers, as well as experimentally studied ring and cage systems constituting the main reason for the hypotheses on metal-metal interactions. The results show that all experimental results can be attributed to the covalent interactions between the electron-rich, metal centers and the bridging anions in ring and cage coordination compounds of high symmetry, where the experimentally observed effects can be traced to the combination of covalent interactions between the metal centers and the anions along the edges of the ring and cage systems in combination with the cooperative effects generated by the high symmetry of these ring and cage systems.

A Hybrid Halide Perovskite Birefringent Crystal

Birefringent crystals that can modulate the polarization of light play a significant role in modern optical devices including polarizing microscopes, optical isolators, and achromatic quarter-wave plates. To date, commercial birefringent crystals are exclusively limited to purely inorganic compounds. Here we report a new organic-inorganic hybrid halide, MLAPbBr₄ (MLA=melamine), which features a (110)-oriented layered perovskite structure. Although the 6s² lone-pair electrons of Pb²⁺ cations are stereochemically inert, MLAPbBr₄ exhibits a birefringence of 0.322@550 nm, which exceeds those of all commercial birefringent crystals. The first-principles calculations reveal that this birefringence should be ascribed to the highly dislocated π-conjugation of MLA cations and high distortion of PbBr₆ octahedra. This work highlights the persistently neglected great potential of hybrid halide perovskites as birefringent crystals.

Linear unit BN2: a novel birefringence-enhanced fundamental module with sp orbital hybridization

Inorganic planar π-conjugated groups are advantageous to generate large birefringence in optical functional materials, and many excellent materials contain CO₃, BO₃ or B₃O₆, such as CaCO₃, α/β-BaB₂O₄ (α/β-BBO), and KBe₂BO₃F₂ (KBBF). In view of their microscopic structures, the common characteristics are the planar structures, which are regarded as birefringence-enhanced fundamental modules (FMs). Nowadays, exploring novel birefringence-enhanced FMs is becoming a burning issue. Herein, we investigated the birefringence-enhanced FMs in B–N systems and found that the BN₂ linear unit could produce great birefringence. Through the investigation based on the Inorganic Crystal Structure Database, some compounds with the BN₂ linear group were screened out with the formulas A₃BN₂ (A = Li, Na), A₃BN₃ (A = Mg, Ca), and Ba₃(BN₂)₂. Particularly, Ca₃(BN₂)N exhibits a great birefringence of about 0.411 at 1064 nm, which is 3.5, 2.5 and 2.0 times those of the most commercially used birefringent crystals α-BaB₂O₄ (Δn = 0.116 at 1064 nm), CaCO₃ (Δn = 0.164 at 1064 nm) and YVO₄ (Δn = 0.208 at 1064 nm), respectively. To find the origins of the optical properties of compounds with the BN₂ linear group, the first-principles, REDA and polarizability anisotropy analysis methods were used. Owing to the structural arrangement and the polarization anisotropy of the BN₂ linear group, it can influence the birefringence significantly. This work will provide a general way for exploring birefringence-enhanced FMs in B–N compounds.

The linear unit BN₂ is discovered as a novel birefringence-enhanced fundamental module. Particularly, Ca₃(BN₂)N exhibits a large birefringence (0.411 at 1064 nm), which is about 2.0–3.5 times of the commercial used birefringent crystals, such as α-BaB₂O₄, CaCO₃ and YVO₄.

KPb3(o-C5H4NCOO)2Cl5: A Brand-New Stable Lead Chloride with Good Comprehensive Nonlinear Optical Performances

The discovery of nonlinear optical (NLO) crystals with excellent performances has still been a great challenge. Herein, a brand-new stable lead chloride, KPb3(o-C5H4NCOO)2Cl5, has been successfully synthesized and well characterized....

(R)- and (S)-[C8H10NO3]2[NbOF5]: Noncentrosymmetric niobium oxyfluorides with large optical anisotropy

Huge crystals of noncentrosymmetric (NCS) organic-inorganic hybrid niobium oxyfluorides, (R)-[C8H10NO3]2[NbOF5] [(R)-Nb] and (S)-[C8H10NO3]2[NbOF5] [(S)-Nb], have been easily grown via a slow evaporation method in high yields through the systematic driving...

Distinctive modulation of optical anisotropy by halogens in α/β-Cd-P-X (X = Cl, Br, and I)

Birefringent materials are widely applied as photoelectric functional field devices to modulate the polarization of lasers. The introduction of a halogen into the structure of crystals could balance the relationship between the band gap E_g and nonlinear optical (NLO) coefficient owing to their outstanding electronegativity and control the optical anisotropy. In this work, the optical properties of phosphohalides α/β-Cd₂P₃X (X = Cl, Br, I) were studied. It was found that the birefringences of α/β-Cd₂P₃Cl (0.23/0.24 @ 1064 nm) are unexpectedly 8 times larger than those of α/β-Cd₂P₃I (0.04/0.03 @ 1064 nm). To find the optical property origins and explore the contributions of microscopic groups to the optical anisotropy and NLO responses in Cd-P-X (X = Cl, Br, I), the first-principles, real-space atom-cutting, and polarizability anisotropy analysis methods were used. This reveals that the electron distribution is susceptible to halogen electronegativity. Halogen atoms can modulate the polarization anisotropy of the active polyhedron and influence the birefringence significantly, owing to the synergistic effect of the anion size and strong covalent interactions between halogens and metal cations. This work clarifies the optical anisotropy origin mechanism and provides a general strategy for finding promising birefringent crystals in phosphohalide systems.

KCs2 [Pb2 Br5 (HCOO)2 ]: A Polar 3D Lead-Bromide Framework Exhibiting Strong Second-Harmonic Generation Response

The discovery of new nonlinear optical (NLO) crystals with excellent properties is in urgently demand because of their ability to generate coherent light. Herein, we report an unique NLO lead bromide formate, KCs₂ [Pb₂ Br₅ (HCOO)₂ ], which has been synthesized by a mix-solvothermal method. KCs₂ [Pb₂ Br₅ (HCOO)₂ ] exhibits strong phase-matching second-harmonic generation (SHG) response (6.5×KDP), large birefringence (0.16@ 1064 nm), and a wide transparent window in most visible light and mid-IR region. Interestingly, KCs₂ [Pb₂ Br₅ (HCOO)₂ ] features a polar 3D lead-bromide framework in which adjacent Pb-Br layers containing coplanar Pb₆ Br₆ rings are not only parallel to each other, but also orient in the same direction. These oriented arrangements are responsible for the strong SHG response and large birefringence that are elucidated by both local dipole moment and theoretical calculations. This research provides a new strategy to explore subsequent NLO crystals.

An Antimony(III) Fluoride Oxalate with Large Birefringence

Birefringent materials play a key role in modulating the polarization of light and thus in optical communication as well as in laser techniques and science. Designing new, excellent birefringent materials remains a challenge. In this work, we designed and synthesized the first antimony(III) fluoride oxalate birefringent material, KSb₂ C₂ O₄ F₅ , by a combination of delocalized π-conjugated [C₂ O₄ ]^2- groups, stereochemical active Sb³⁺ cations, and the most electronegative element, fluorine. The [C₂ O₄ ]^2- groups are not in an optimal arrangement in the crystal structure of KSb₂ C₂ O₄ F₅ ; nonetheless, KSb₂ C₂ O₄ F₅ exhibits a large birefringence (Δn=0.170 at 546 nm) that is even better than that of the well-known commercial birefringent material α-BaB₂ O₄ , even though the latter features an optimal arrangement of π-conjugated [B₃ O₆ ]^3- groups. Based on first-principles calculations, this prominent birefringence should be attributed to the alliance of planar π-conjugated [C₂ O₄ ]^2- anions, highly distorted SbO₂ F₂ and SbOF₃ polyhedra with a stereochemically active lone pair. The combination of lone-pair electrons and π-conjugated systems boosts the birefringence to a large extent and will help the development of high-performance birefringent materials.

Hydrogen-Bond-Assisted Alignment of [MCu(SeO3)4Cl(H2O)]4- (M = Fe, Ga) Anionic Layers to Form Two Polar Oxychlorides: Pb2MCu(SeO3)4Cl(H2O)

Two novel selenite oxychlorides Pb₂MCu(SeO₃)₄Cl(H₂O) (M = Fe, Ga) were hydrothermally synthesized and structurally characterized. They are isostructural and crystallize in the two-dimensional [MCu(SeO₃)₄Cl(H₂O)]^4- anionic layer structure mediated with hydrogen bonds and aligned between neighboring layers which assist in building the three-dimensional framework with a polar space group. Optical properties measurements revealed that the optical band gaps are 2.61 and 3.22 eV for Pb₂FeCu(SeO₃)₄Cl(H₂O) (1) and Pb₂GaCu(SeO₃)₄Cl(H₂O) (2) and the SHG responses are about 0.12 and 0.18 times that of KDP, respectively. Furthermore, 1 exhibits an interesting metamagnetic phenomenon under varied applied fields from around 1 to 4 T at 2 K, and 2 behaves with potential ferromagnetic ordering at low temperature.

Barium fluoroiodate crystals with a large band gap and birefringence

BaI₂O₅F₂ and BaIO₂F₃ have large birefringence of 0.174 and 0.133 at 1064 nm, respectively, which is owing to the rare [IO₃F]²⁻ units with high anisotropic polarizability in BaI₂O₅F₂ and the orderly arranged [IO₂F₂]⁻ units in BaIO₂F₃.

Pb2Cd(SeO3)2X2 (X = Cl and Br): two halogenated selenites with phase matchable second harmonic generation

The second harmonic generation (SHG) efficiencies of Pb₂Cd(SeO₃)₂X₂ (X = Cl and Br) are higher than that of commercial KDP (KH₂PO₄) and their laser damage thresholds are 30 times more than that of AGS (AgGaS₂).

Two low-dimensional metal halides: ionothermal synthesis, photoluminescence, and nonlinear optical properties

An organic–inorganic hybrid metal halide with a chain-like structure has been prepared under ionothermal conditions, which shows a large second harmonic generation (SHG) efficiency.

PbCdF(SeO3)(NO3): A Nonlinear Optical Material Produced by Synergistic Effect of Four Functional Units

A new nonlinear optical material, the first fluoride selenite nitrate PbCdF(SeO₃)(NO₃), was successfully synthesized by traditional solid state reactions. This compound crystallizes in the polar space group of Pca2₁, and its structure features a novel 2D layered structure consisting of 1D lead nitrate chains and 2D cadmium selenite layers. PbCdF(SeO₃)(NO₃) exhibits a phase-matchable SHG efficiency of about 2.6 times that of KDP and a wide band gap of 4.42 eV. Its laser damage threshold was measured to be 135.6 MW/cm², which is comparative to that of the reported Li₇(TeO₃)₃F with a short ultraviolet cutoff edge. Theoretical calculation confirmed that the synergistic effects of all the four functional units make PbCdF(SeO₃)(NO₃) a remarkable SHG material.

Hydrogen bond-assisted crystallization: structure, growth and characterization of a new mixed-anion transition metal fluoride Na3NH4(TiF6)(SO4)·H2O

A new type of mixed-anion TM fluoride was grown with a size up to 5.0 × 2.5 × 1.0 mm³ by a facile hydrothermal method.