[o-C5 H4 NHOH]2 [I7 O18 (OH)]⋅3 H2 O: An Organic-Inorganic Hybrid SHG Material Featuring an [I7 O18 (OH)] ∞ 2 - Branched Polyiodate Chain

A Noncentrosymmetric Metal-Free Borophosphate: Achieving a Large Birefringence and Excellent Stability by Covalent-Linkage

Organic-inorganic hybrid linear and nonlinear optical (NLO) materials have received increasingly wide spread attention in recent years. Herein, the first hybrid noncentrosymmetric (NCS) borophosphate, (C5H6N)2B2O(HPO4)2 (4PBP), is rationally designed and synthesized by a covalent-linkage strategy. 4-pyridyl-boronic acid (4 PB) is considered as a bifunctional unit, which may effectively improve the optical properties and stability of the resultant material. On the one hand, 4 PB units are covalently linked with PO3(OH) groups via strong B-O-P connections, which significantly enhances the thermal stability of 4PBP (decomposition at 321, vs lower 200 °C of most of hybrid materials). On the other hand, the planar π-conjugated C5H6N units and their uniform layered arrangements represent large structural anisotropy and hyperpolarizability, achieving the largest birefringence (0.156 @ 546 nm) in the reported borophosphates and a second-harmonic generation response (0.7 × KDP). 4PBP also exhibits a wide transparency range (0.27-1.50 µm). This work not only provides a promising birefringent material, but also offers a practical covalent-attachment strategy for the rational design of new high-performance optical materials.

Advancing nonlinear optics: discovery and characterization of new non-centrosymmetric phenazine-based halides

Organic-inorganic metal halides (OIMHs) have drawn considerable attention due to their remarkable optoelectronic properties and substantial promise for nonlinear optical applications. In this research, phenazine has been selected as the organic cation because of its π-conjugated feature. Three compounds, (C12H9N2)PbCl3, (C12H9N2)SbCl4, and (C12H9N2)2InBr4·Br, were synthesized. Initial space group assignments were centrosymmetric for (C12H9N2)PbCl3 and (C12H9N2)SbCl4. However, under 1550 nm laser excitation, (C12H9N2)PbCl3 and (C12H9N2)SbCl4 exhibited second harmonic generation intensities ∼1.7 times greater than that of the benchmark KH2PO4. Structural reevaluation ultimately confirmed non-centrosymmetric P1 and P21 space groups for (C12H9N2)PbCl3 and (C12H9N2)SbCl4, respectively. Upon excitation at 335 nm and 470 nm, (C12H9N2)PbCl3, (C12H9N2)SbCl4, and (C12H9N2)2InBr4·Br emit fluorescence at room temperature. (C12H9N2)2InBr4·Br exhibits reversible phase transitions, showing potential for phase change energy storage. Our research underscores the critical role of comprehensive experimental validation in determining the precise crystallographic space groups and reveals the extensive potential of OIMHs as versatile candidates for advanced optoelectronic applications.

[(C5H6N2)2H](Sb4F13): a polyfluoroantimonite with a strong second harmonic generation effect

It is of great difficulty to create a new antimonite with second-harmonic-generation (SHG) intensity larger than 6 times that of KDP. In this study, a polyfluoroantimonite strategy has been proposed to explore fluoroantimonites with large nonlinear optical (NLO) coefficients. Under the cooperation of chemical (highly asymmetric π-conjugated organic amine) and physical (viscous reaction medium ethylene glycol) methods, two novel polyfluoroantimonites, namely, (3PC)2(Sb4F14) and (3AP)2(Sb4F13), have been achieved. Interestingly, these two structures contain two new polyfluoroantimonite groups respectively, an isolated (Sb4F14)2- four-member polyhedral ring and an infinite [Sb4F13]∞- helical chain. More importantly, the polar (3AP)2(Sb4F13) displays a strong SHG intensity of 8.1 × KDP, a large birefringence of 0.258@546 nm and a high laser-induced damage threshold (LIDT) value of 149.7 MW cm-2. Theoretical calculations indicated that its strong SHG effect stems from the synergistic effect of the helical [Sb4F13]∞- polyfluoroantimonite chain and π-conjugated 3AP+ cation, with a contribution ratio of 48.93% and 50.77% respectively. This work provides a new approach for the design and synthesis of high-performance fluoroantimonites.

Broadband Green Luminescence and Phase Transition in Low-Dimensional Organic-Inorganic Hybrid Iodate

Organic-inorganic hybrid iodide systems, which can form highly ordered chromophores and uniformly oriented transition dipole moments, serve as optimal host-guest systems for the fabrication of micrometer-scale optical devices. In particular, those with low-dimensional structures can exhibit strong quantum-limited and highly localized charges, enabling the generation of high exciton energies and stable excitation emission. In this study, we report a novel instance of an organic-inorganic hybrid iodate, (C13H11N2)(IO3), which was synthesized by incorporating the optically active organic compound, 9-aminoacridine. Upon crystallization in the monoclinic space group P21/c, this compound exhibits a direct optical band gap of 2.66 eV. The incorporation of discrete organic units within the low-dimensional structures induces pronounced local charges, culminating in broadband green luminescence with a peak at 540 nm under UV excitation. This corresponds to the CIE coordinates (0.37, 0.56). A potential phase transition was inferred through a comprehensive analysis of the variable temperature structure and emission spectra. Furthermore, first-principles calculations revealed the pivotal role of organic cations in facilitating broadband luminescence.

Secondary-Bond-Driven Construction of a Polar Material Exhibiting Strong Broad-Spectrum Second-Harmonic Generation and Large Birefringence

Considerable effort has been invested in the development of non-centrosymmetric (NCS) inorganic solids for ferroelectricity-, piezoelectricity- and, particularly, optical nonlinearity-related applications. While great progress has been made, a persistent problem is the difficulty in constructing NCS materials, which probably stems from non-directionality and unsaturation of the ionic bonds between metal counter-cations and covalent anionic modules. We report herein a secondary-bond-driven approach that circumvents the cancellation of dipole moments between adjacent anionic modules that has plagued second-harmonic generation (SHG) material design, and which thereby affords a polar structure with strong SHG properties. The resultant first NCS counter-cation-free iodate, VO2 (H2 O)(IO3 ) (VIO), a new class of iodate, crystallizes in a polar lattice with∞ 1 [ ${{}_{{\rm { \infty }}}{}^{{\rm { 1}}}{\rm { [}}}$ VO2 (H2 O)(IO3 )] zigzag chains connected by weak hydrogen bonds and intermolecular forces. VIO exhibits very large SHG responses (18 × KH2 PO4 @ 1200 nm, 1.5 × KTiOPO4 @ 2100 nm) and sufficient birefringence (0.184 @ 546 nm). Calculations and crystal structure analysis attribute the large SHG responses to consistent polarization orientations of the∞ 1 [ ${{}_{{\rm { \infty }}}{}^{{\rm { 1}}}{\rm { [}}}$ VO2 (H2 O)(IO3 )] chains controlled by secondary bonds. This study highlights the advantages of manipulating the secondary bonds in inorganic solids to control NCS structure and optical nonlinearity, affording a new perspective in the development of high-performance NLO materials.

Giant Optical Anisotropy in a Covalent Molybdenum Tellurite via Oxyanion Polymerization

Large birefringence is a crucial but hard-to-achieve optical parameter that is a necessity for birefringent crystals in practical applications involving modulation of the polarization of light in modern opto-electronic areas. Herein, an oxyanion polymerization strategy that involves the combination of two different types of second-order Jahn-Teller distorted units is employed to realize giant anisotropy in a covalent molybdenum tellurite. Mo(H2O)Te2O7 (MTO) exhibits a record birefringence value for an inorganic UV-transparent oxide crystalline material of 0.528 @ 546 nm, which is also significantly larger than those of all commercial birefringent crystals. MTO has a UV absorption edge of 366 nm and displays a strong powder second-harmonic generation response of 5.4 times that of KH2PO4. The dominant roles of the condensed polytellurite oxyanions [Te8O20]8- in combination with the [MoO6]6- polyhedra in achieving the giant birefringence in MTO are clarified by structural analysis and first-principles calculations. The results suggest that polymerization of polarizability-anisotropic oxyanions may unlock the promise of birefringent crystals with exceptional birefringence.

[C(NH2)3]6Mo7O24: A Guanidinium Molybdate as a UV Nonlinear Optical Crystal with Large Birefringence

The key to searching novel nonlinear optical (NLO) crystals was effectively combining the NLO-active units to obtain a noncentrosymmetric structure. Nevertheless, the present predicament lies in the growing challenge of discovering novel crystals within conventional inorganic frameworks that surpass the properties of the current NLO materials. In view of this, researchers expanded their research focus to the organic-inorganic hybridization system; it is foreseeable to concentrate the advantages from several kinds of NLO-active units to acquire novel NLO crystals with superior properties. We herein report an organic-inorganic hybrid molybdate crystal, namely, [C(NH2)3]6Mo7O24 (GMO). It was successfully obtained via combining inorganic NLO-active MoO6 octahedra and organic π-conjugated [C(NH2)3]+ groups. GMO demonstrates a moderate second-harmonic-generation response, specifically measuring about 1.3 times the value of KDP. Additionally, it exhibits a significant birefringence value of 0.203 at the wavelength of 550 nm and possesses a wide band gap of 3.31 eV. Theoretical calculations suggest that the optical properties of the GMO are primarily influenced by the synergy effect of [C(NH2)3]+ groups between MoO6 octahedra.

Designing Optical Anisotropy: Silver-Aminoalkylpyridine Nitrate Complexes with Tunable Structures

To introduce a design strategy for improving optical properties, two silver-amino alkylpyridine nitrate complexes, AgC6H8N3O3 and Ag2C14H20N6O6, were successfully synthesized using a recrystallization method. By employing polarizable π-conjugated [NO3-] ions, two types of pyridine ligands, and silver cations with a high affinity for pyridine, we obtained a one-dimensional chain structure with 4-aminomethylpyridine (AgC6H8N3O3) and a zero-dimensional molecular compound by introducing a relatively flexible aliphatic chain with 4-(2-aminoethyl)pyridine (Ag2C14H20N6O6). The compounds crystallize in the triclinic crystal system with the centrosymmetric P-1 space group, exhibiting a change in orientation between the π-conjugated system and the silver ion. Despite similar optical band gaps (3.69 eV for AgC6H8N3O3 and 3.73 eV for Ag2C14H20N6O6), AgC6H8N3O3 shows higher absorption in the 350-600 nm range. Electronic structure calculations support the ultraviolet absorption findings, suggesting that charge transfer with π-conjugated systems influences birefringence. Ag2C14H20N6O6 exhibits experimental birefringence (0.261@546.1 nm) surpassing that of AgC6H8N3O3 (0.212@546.1 nm), placing it among the highest recorded values within metal-pyridine incorporating nitrate complexes. The nonconventional orientation of π-conjugated [NO3-] ions contributes to this phenomenon, enhancing the action of free π-conjugated orbitals. This design strategy for micromodulating the alignment of the π-conjugated system promises to be an effective approach for enhancing optical properties, such as birefringence.

From C4H7N2Ge0.4Sn0.6Br3 to C6H11N2Ge0.4Sn0.6Br3: Effective Modulation of the Second Harmonic Generation Effect and Optical Band Gap by Planar π-Conjugated Organic Cation Size

In the search for nonlinear optical (NLO) materials with excellent overall performance, we have devoted ourselves to organic-inorganic hybrids consisting of anionic groups containing stereochemically active lone-pair (SCALP) electron cations and organic planar π-conjugated group cations. Accordingly, in this paper, two novel organic-inorganic hybrid metal halides, C4H7N2Ge0.4Sn0.6Br3 (I) and C6H11N2Ge0.4Sn0.6Br3 (II), have been synthesized. The powder second-harmonic technique shows that both C4H7N2Ge0.4Sn0.6Br3 and C6H11N2Ge0.4Sn0.6Br3 have moderately strong second-order nonlinear optical effects, which are about 2.03 (I) and 1.16 (II) times that of KH2PO4 (KDP), respectively. They also have different optical band gaps of 2.75 (I) and 2.88 eV (II) due to the different sizes of the organic cations, and their photoluminescent and thermal properties were also investigated. This work provides new structural insights for the design and modulation of organic-inorganic hybrid halide materials with multiple excellent optical properties.

(C5H6.16N2Cl0.84)(IO2Cl2): a birefringent crystal featuring unprecedented (IO2Cl2)- anions and π-conjugated organic cations

Birefringent crystals can manipulate the polarization state of lasers and have vital application in polarizers, optical isolators, phase compensators, etc. The design and synthesis of crystals with large birefringence remains a challenging task. To design crystals with large birefringence, we combine an unprecedented chloroiodate(v) group (IO2Cl2)- featuring large polarizability anisotropy and a strong stereochemically active lone pair (SCALP) with the π-conjugated 2-amino-5-chloropyridine group. The superior synergy effect of (IO2Cl2)- and 2-amino-5-chloropyridine groups produces a new birefringent crystal, namely (C5H6.16N2Cl0.84)(IO2Cl2). It exhibits remarkably large birefringence of 0.67 at 546 nm, far exceeding those of most visible birefringent materials reported. This work discovers the first chloroiodate(v) group and provides a new synthetic route for birefringent materials.

(NH4 )2 (I5 O12 )(IO3 ) and K1.03 (NH4 )0.97 (I5 O12 )(IO3 ): Mixed-Valent Polyiodates with Unprecedented I5 O12 - Unit Exhibiting Strong Second-Harmonic Generation Responses and Giant Birefringence

Second-harmonic generation (SHG) response and birefringence are crucial properties for linear and nonlinear optical (NLO) materials, while it is difficult to further optimize these two key properties by using a single traditional functional building block (FBB) in one compound. Herein, a novel IO4 5- unit is identified, which possesses a square-planar configuration and two stereochemically active lone-pairs (SCALPs). By combining IO4 5- and IO3 - units, the first examples of mixed-valent polyiodates featuring an unprecedented bowl-shaped I5 O12 - polymerized unit, namely (NH4 )2 (I5 O12 )(IO3 ) and K1.03 (NH4 )0.97 (I5 O12 )(IO3 ), are successfully synthesized. Excitingly, both crystals exhibit strong SHG responses (16 × KDP and 19.5 × KDP @1064 nm) as well as giant birefringence (∆nexp  = 0.431 and 0.405 @546 nm). Detailed structure-property analyses reveal that the parallel aligned planar IO4 5- units induce the properly aligned high-density SCALPs, leading to strong SHG response and giant birefringence for both materials. This work not only provides two new potential NLO and birefringent crystals, but also discovers a novel promising FBB (IO4 5- ) for developing high-performance linear and nonlinear optical materials.

From H12C4N2CdI4 to H11C4N2CdI3: a highly polarizable CdNI3 tetrahedron induced a sharp enhancement of second harmonic generation response and birefringence

In this study, we identify a novel class of second-order nonlinear optical (NLO) crystals, non-π-conjugated piperazine (H10C4N2, PIP) metal halides, represented by two centimeter-sized, noncentrosymmetric organic-inorganic metal halides (OIMHs), namely H12C4N2CdI4 (P212121) and H11C4N2CdI3 (Cc). H12C4N2CdI4 is the first to be prepared, and its structure contains a CdI4 tetrahedron, which led to a poor NLO performance, including a weak and non-phase-matchable second harmonic generation (SHG) response of 0.5 × KH2PO4 (KDP), a small birefringence of 0.047 @1064 nm and a narrow bandgap of 3.86 eV. Moreover, H12C4N2CdI4 is regarded as the model compound, and we further obtain H11C4N2CdI3via the replacement of CdI4 with a highly polarizable CdNI3 tetrahedron, which results in a sharp enhancement of SHG response and birefringence. H11C4N2CdI3 exhibits a promising NLO performance including 6 × KDP, 4.10 eV, Δn = 0.074 @1064 nm and phase matchability, indicating that it is the first OIMH to simultaneously exhibit strong SHG response (>5 × KDP) and a wide bandgap (>4.0 eV). Our work presents a novel direction for designing high-performance NLO crystals based on organic-inorganic halides and provides important insights into the role of the hybridized tetrahedron in enhancing the SHG response and birefringence.

Rational Design of Noncentrosymmetric Organic-Inorganic Hybrids with a π-Conjugated Pyridium-Type Cation for High Nonlinear-Optical Performance

Organic-inorganic hybrid materials have attracted increasing attention due to their unique superiority by combining the features of organic parts with inorganic parts. Herein, two organic-inorganic hybrid nonlinear-optical crystals, [C5H6O2N3]2[IO3]2 (I) and [C5H6O2N3][HSO4]·H2O (II), were successfully synthesized in aqueous solution by selecting 2-amino-3-nitropyridine as the cation and different anions of [IO3]- and [HSO4]-. The two compounds crystallized in the noncentrosymmetric space groups of P21 and P212121, respectively. I displays second-harmonic-generation (SHG) effects of 2.4 × KDP (KH2PO4) and a large birefringence (Δncal ∼ 0.22). Moreover, II exhibits a stronger SHG response of 5.2 × KDP, an enhanced band gap (2.81 eV), as well as a large birefringence (Δncal ∼ 0.25). This work points out a new feasible path for the rational design of high-performance organic-inorganic hybrid materials.

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.

α- and β-(C4H5N2O)(IO3)·HIO3: Two SHG Materials Based on Organic-Inorganic Hybrid Iodates

Organic-inorganic hybrid nonlinear optical (NLO) materials are highly anticipated because of the integration of both merits of the organic and inorganic moieties. Herein, the 2-pyrimidinone cation (C4H5N2O)+ has been incorporated into the iodate system to form two polymorphic organic-inorganic hybrid iodates, namely, α- and β-(C4H5N2O)(IO3)·HIO3. They crystallize in different polar space groups (Ia and Pca21), and their structures feature one-dimensional (1D) chain structures composed of (C4H5N2O)+ cations, IO3- anions, and HIO3 molecules interconnected via hydrogen bonds. α- and β-(C4H5N2O) (IO3)·HIO3 exhibit strong and moderate second-harmonic-generation (SHG) responses of 6.4 and 0.9 × KH2PO4 (KDP), respectively, the same band gaps of 3.65 eV, and high powder laser-induced damage threshold (LIDT) values [51 and 57 × AgGaS2 (AGS)]. The results of theoretical calculations revealed that the large SHG effect of α-(C4H5N2O)(IO3)·HIO3 originated from the IO3 and HIO3 groups. This work indicates that (C4H5N2O)+ is a potential group for designing new NLO materials with brilliant optical performances.

LiCs3AlB7O14: achieving enhanced optical anisotropy via [AlO4] tetrahedron introduction to rearrange the anionic framework

Borate has become a hot topic because of its rich structural chemistry and excellent properties for functional materials fields. The rearrangement of π-conjugated B-O units is key to enhancing the optical anisotropy, but it remains a challenge. Herein, by introducing [AlO₄] tetrahedra, a new congruent melting aluminoborate LiCs₃AlB₇O₁₄ with [B₇O₁₄] clusters was discovered. This work confirms that the introduction of [AlO₄] tetrahedra can lead to the rearrangement of anionic framework of the borate system and thereby enhance the birefringence of LiCs₃AlB₇O₁₄. The birefringence is about 4.1 times higher than that of its congener Li₄Cs₃B₇O₁₄ with the same [B₇O₁₄] clusters. Similarly, the effects of [AlO₄] tetrahedra on the rearrangement of the B-O anionic framework are also demonstrated in other known borates.

CsHfF4(IO3): A Hafnium Iodate Exhibiting a Strong Second-Harmonic-Generation Effect and a Wide Band Gap Achieved by Mixed-Ligand Acentric Coordination

A new polar hafnium iodate, CsHfF₄(IO₃), was successfully designed and synthesized by integrating fluorinated hafnium-oxygen polyhedra (HfF₆O₂) and IO₃^- anionic functional groups. Owing to the weak electronic effect of Hf⁴⁺ and the bond-network-induced out-of-center distortion of the HfF₆O₂ dodecahedra, CsHfF₄(IO₃) achieves a good balance between a strong second-harmonic-generation effect (3.5 × KH₂PO₄) and a rather large band gap (4.47 eV), which is the largest among the d⁰ transition-metal iodates. In addition, CsHfF₄(IO₃) possesses a wide transparent region (0.27-9.9 μm), a large birefringence for phase-matching (0.161), and a high laser-induced damage threshold (55.41 MW cm^-2, 26 × AgGaS₂) and is nonhygroscopic. This work indicates that the integration of mixed-ligand acentric coordination polyhedra and functional groups containing lone electron pairs is an effective strategy for developing novel inorganic nonlinear-optical materials with balanced overall properties.

Cs[B3O3F2(OH)2]: discovery of a hydroxyfluorooxoborate guided by selective organic-inorganic transformation

Selective transformation between organic and inorganic systems is crucial but still remains a challenge. Herein, we demonstrated that selective organic-inorganic transformation is a simple but effective strategy to find new hydroxyfluorooxoborates. By replacing the [Ph₄P]/[Ph₃MeP] organic cations with Cs atoms, a new hydroxyfluorooxoborate Cs[B₃O₃F₂(OH)₂] with three-membered [B₃O₃F₂(OH)₂] clusters was synthesized. Theoretical analysis confirmed the effects of different components in the lattice of reported structure on the optical properties.

Noncentrosymmetric γ-Cs2I4O11 Obtained from IO4 Polyhedral Rearrangements in the Centrosymmetric β-Phase

We report the synthesis and optical properties of noncentrosymmetric (NCS) γ-Cs₂I₄O₁₁ that was obtained through IO₄ polyhedral rearrangements from centrosymmetric (CS) β-Cs₂I₄O₁₁. Trifluoroacetic acid (TFA) acts as a structure-directing agent and plays a key role in the synthesis. It is suggested that the function of TFA is to promote rearrangement reactions found in the organic synthesis of stereoisomers. γ-Cs₂I₄O₁₁ crystallizes in the NCS monoclinic space group P2₁ (No. 4) and exhibits a strong second-harmonic-generation (SHG) response of 5.0 × KDP (KH₂PO₄) under 1064 nm laser radiation. Additional SHG experiments indicate that the material is type I phase matchable. First-principles calculations show that SHG intensity mainly comes from its d₃₄, d₂₁, and d₂₃ SHG tensor components. The synthetic strategy of discovering γ-Cs₂I₄O₁₁ provides a new way for designing novel NCS SHG materials.

C(NH2)3(I3O8)(HI3O8)(H2I2O6)(HIO3)4·3H2O: An Unprecedented Asymmetric Guanidinium Polyiodate with a Strong Second-Harmonic-Generation Response and a Wide Band Gap

Herein, we report an unprecedented asymmetric guanidinium polyiodate, namely, C(NH₂)₃(I₃O₈)(HI₃O₈)(H₂I₂O₆)(HIO₃)₄·3H₂O (1). The title compound was obtained via the hybridization of polyiodate anions and planar π-conjugated C(NH₂)₃⁺; meanwhile, its strong second-harmonic-generation (SHG) response (2.1 × KDP, where KDP = KH₂PO₄) and wide band gap (3.89 eV) were mainly dominated by the synergy effect of the aforementioned structural units.

A High-Performance Nonlinear Optical Crystal with a Building Block Containing Expanded π-Delocalization

Common nonlinear optical (NLO) crystals consist of traditional functional building blocks with inherent optical limitation. Herein, inspired by traditional (B₃ O₆ )^3- inorganic building block, we theoretically identified a new type of organic functional building blocks and then successfully synthesized the first cyamelurate NLO crystal, Ba(H₂ C₆ N₇ O₃ )₂  ⋅ 8 H₂ O. To our surprise, the constituent (H₂ C₆ N₇ O₃ )^- building block is not in structurally optimal arrangement, but Ba(H₂ C₆ N₇ O₃ )₂  ⋅ 8 H₂ O exhibits excellent optical properties including wide band gap of 4.10 eV, very large birefringence of 0.24@550 nm, and exceptionally strong second-harmonic generation (SHG) response of about 12×KH₂ PO₄ . Both the SHG response and birefringence are much larger than those of commercial NLO crystal β-BaB₂ O₄ with optimally aligned (B₃ O₆ )^3- building block. Theoretical calculations suggest that the expanded π-conjugation delocalization within (H₂ C₆ N₇ O₃ )^- vs (B₃ O₆ )^3- should be responsible to the enhanced performance. This work implies that there is still much room to develop new NLO crystals with excellent functional building blocks that may be longly neglected.

Dual-Ligand-Oriented Design of Noncentrosymmetric Complexes with Nonlinear-Optical Activity

When the N- and O-donor ligands are combined as coligands, two noncentrosymmetric (NCS) complexes of [Ni(p-bdc)(tipa)(H₂O)₂]₂·H₂O (1) and Ni(npdc)(tipa)H₂O (2) [tipa = tris[4-(1H-imidazol-1-yl)phenyl]amine, p-H₂bdc = 1,4-benzenedicarboxylic acid, and H₂npdc = 2,6-naphthalenedicarboxylic acid] were achieved under solvothermal conditions. For both structures, N-donor ligands are responsible for the generation of a layered structure, while the O-donor ligands are hung on the layers and are responsible for enhancing the polarity, giving rise to the NCS structures. Because of the different connection modes between the metal centers and different carboxylate ligands (p-bdc^2- in 1 and npdc^2- in 2), 1 and 2 show some structural differences. The p-bdc^2- ligands in 1 are suspended on the upper and lower sides of the [Ni(tipa)]_n layers, while all of the npdc^2- ligands in 2 hang on one side of the [Ni(tipa)]_n layers and point in the same direction, which makes the two NCS complexes show phase-matchable behavior with different second-harmonic-generation (SHG) responses of about 0.9 and 1.5 times that of KH₂PO₄ (KDP), respectively. Theoretical studies reveal that charge transfers between Ni²⁺ and carboxylate ligands make the dominant contribution to the optical properties. It is expected that a dual-ligand strategy may guide the design of novel superior-performing NCS complexes.

NaB3O4F(OH): A Hydroxyfluorooxoborate with One-Dimensional Chain Featuring Large Birefringence and Short Ultraviolet Cutoff Edge

A new sodium hydroxyfluorooxoborate, NaB₃O₄F(OH) (NBOFH), was discovered and synthesized. NBOFH features the unprecedented [B₃O₄F(OH)] infinite chain constructed by the novel fundamental building block (FBB) of [B₃O₅F(OH)]. NBOFH has a large birefringence of 0.097 at 1064 nm and short ultraviolet (UV) cutoff edge below 200 nm. First-principles calculations and response electron distribution anisotropy (REDA) were performed to explain the structure-property relationships. This work provides a novel strategy for the synthesis of deep-ultraviolet birefringent crystals and enriches the structural diversity of the emerging hydroxyfluorooxoborates.

Rearrangement of [B2O5] Dimers within [B7O14] Clusters Enables Enhanced Optical Anisotropy in Li3Cs6Al2B14O28F

Borates with tunable structure and property currently provide a new rich source for solid-state chemistry and materials science. Realization of property improvement via simple structural regulation is a rising hot spot of borate-based research. Herein, a new aluminoborate fluoride, Li₃Cs₆Al₂B₁₄O₂₈F, with [B₇O₁₄] clusters was discovered, and it was found to melt congruently. The optimally aligned [B₂O₅] dimers within [B₇O₁₄] clusters make Li₃Cs₆Al₂B₁₄O₂₈F an enhanced birefringence, which is about 4.3× higher than its congener compound Li₄Cs₃B₇O₁₄ with same [B₇O₁₄] clusters. Structural analysis and additional theoretical calculations have revealed the origin of enhanced optical anisotropy.

Cd2(IO3)(PO4) and Cd1.62Mg0.38(IO3)(PO4): metal iodate-phosphates with large SHG responses and wide band gaps

The first NLO-active metal iodate-phosphates, namely, Cd₂(IO₃)(PO₄) and Cd_1.62Mg_0.38(IO₃)(PO₄) (1 and 2), with three types of NLO groups, have been reported. 1 and 2 are isostructural and the structure of 1 features a 3D network formed by the Cd₄(IO₃)_8/4(PO₄)_6/3 groups. 1 and 2 with strong SHG signals of 4 × and 3.5 × KH₂PO₄ are promising SHG materials in the visible region.

CaCe(IO3)3(IO3F)F: a promising nonlinear optical material containing both IO3− and IO3F2− anions

A promising nonlinear optical material, CaCe(IO3)3(IO3F)F, containing both IO3− and IO3F2− anions, has been reported.

A novel 2-(aminomethyl)pyridineH2PO4 crystal with second-order nonlinear optical performance

Single-crystal structure diagram and SHG response of (2-Ampy)H2PO4 crystal.

KCs2Pb2(HCOO)2Cl5: A Lead Formate with Strong Second-Harmonic-Generation Response Obtained by an Anionic Substitution

Nonlinear optical (NLO) materials are playing an increasingly vital role in laser science and technology. Herein, we report a new lead formate, KCs₂Pb₂(HCOO)₂Cl₅, successfully synthesized by using mild mix-solvothermal methods. By the anionic substitution of a [CH₃COO] group with a [HCOO] group, the dipole moment of the distorted [PbCl₄O₂] polyhedron is increased. In contrast, polarization from the anionic group and the volume of the unit cell are reduced. Hence, KCs₂Pb₂(HCOO)₂Cl₅ possesses a strong second-harmonic-generation (SHG) response of 4.2 times that of KH₂PO₄ (KDP), a transparent window covering 0.43-1.87 μm, a large band gap of 3.52 eV, and moderate birefringence (0.11@1064 nm). Systematic theoretical calculations illustrate the origin of the linear and nonlinear optical properties of KCs₂Pb₂(HCOO)₂Cl₅. This research provides a viable strategy for the design and synthesis of new NLO crystals.

Two bismuth iodate sulfates with enhanced optical anisotropy

Two bismuth iodate sulfates crystallizing in the monoclinic space group P2₁/c, namely, Bi(IO₃)(SO₄) and CdBi(IO₃)(SO₄)₂, were synthesized via solvothermal reactions. Bi(IO₃)(SO₄) features 2D [Bi(SO₄)]⁺ layers, which are further linked by the IO₃^- groups to form a 3D network. CdBi(IO₃)(SO₄)₂ exhibits 1D [IO₃]^- chains built from IO₄^3- groups via corner-sharing and is the first example of a polyiodate sulfate as far as we know. These [IO₃]^- chains are interconnected by Bi³⁺ cations into [Bi(IO₃)]²⁺ layers parallel to the bc plane, whereas the neighbouring Cd²⁺ cations are interconnected by bridging SO₄^2- anions into [Cd(SO₄)₂]^2- layers, also parallel to the bc plane. These cationic and anionic 2D layers are held together through Bi-O-S bridges into a complicated 3D framework. Bi(IO₃)(SO₄) and CdBi(IO₃)(SO₄)₂ show wide band gaps of 3.91 and 4.03 eV and large birefringence values of 0.087 and 0.100 at 1064 nm, respectively. Our work indicates that the introduction of iodate group and lone pair cations, such as Bi³⁺, into metal sulfates can greatly enhance their birefringent properties.