Ba2Ga2F6(IO3)(PO4): the first fluoride-containing iodate-phosphate with a 1D [Ga2F6(IO3)(PO4)]4- helix chain

Herein, the first F-containing iodate-phosphate, namely Ba2Ga2F6(IO3)(PO4), was prepared via a hydrothermal reaction, in which HPF6 (70 wt% solution in water) was used as the source of both fluoride and phosphate anions for the first time. Ba2Ga2F6(IO3)(PO4) features an unprecedented 1D [Ga2F6(IO3)(PO4)]4- helix chain, composed of a 1D Ga(1)(IO3)O4F chain via the bridging of 0D Ga(2)(PO4)F5. The UV-Vis spectrum shows that Ba2Ga2F6(IO3)(PO4) has a wide bandgap with a short-UV absorption edge (4.35 eV; 253 nm). Birefringence measurement under a polarizing microscope shows that Ba2Ga2F6(IO3)(PO4) displays a moderate birefringence of 0.072@550 nm, which is consistent with the value (0.070@550 nm) obtained by DFT calculations, indicating that Ba2Ga2F6(IO3)(PO4) has potential applications as a short-UV birefringent material. This study highlights the crucial role played by the incorporation of specific functional groups into compounds, shedding light on their contribution to promising inorganic functional materials.

Record Second-Harmonic Generation and Birefringence in an Ultraviolet Antimonate by Bond Engineering

Oxides have attracted considerable attention owing to their potential for nonlinear optical (NLO) applications. Although significant progress has been achieved in optimizing the structural characteristics of primitives (corresponding to the simplest constituent groups, namely, cations/anions/neutral molecules) comprising the crystalline oxides, the role of the primitives' interaction in determining the resultant functional structure and optical properties has long been underappreciated and remains unclear. In this study, we employ a π-conjugated organic primitive confinement strategy to manipulate the interactions between primitives in antimonates and thereby significantly enhance the optical nonlinearity. Chemical bonds and relatively weak H-bonding interactions promote the formation of cis- and trans-Sb(III)-based dimer configurations in (C5H5NO)(Sb2OF4) (4-HPYSOF) and (C5H7N2)(Sb2F7) (4-APSF), respectively, resulting in very different second-harmonic generation (SHG) efficiencies and birefringences. In particular, 4-HPYSOF displays an exceptionally strong SHG response (12 × KH2PO4 at 1064 nm) and a large birefringence (0.513 at 546 nm) for a Sb(III)-based NLO oxide as well as a UV cutoff edge. Structural analyses and theoretical studies indicate that polarized ionic bond interactions facilitate the favorable arrangement of both the inorganic and organic primitives, thereby significantly enhancing the optical nonlinearity in 4-HPYSOF. Our findings shed new light on the intricate correlations between the interactions of primitives, inorganic primitive configuration, and SHG properties, and, more broadly, our approach provides a new perspective in the development of advanced NLO materials through the interatomic bond engineering of oxides.

Guanidinium Vanadate [C(NH2)3]3VO4·2H2O Revealing Enhanced Second-Harmonic Generation and Wide Band Gaps

A new guanidinium-templated vanadate, [C(NH2)3]3VO4·2H2O, has been synthesized in a phase-pure form. It crystallizes in a noncentrosymmetric polar space group, Cc, and the crystal structure is built upon a framework of guanidinium, vanadate tetrahedra, and water molecules linked by hydrogen bonds. Notably, optical measurements reveal that the material exhibits an approximately 9.6-fold enhancement in second-harmonic generation efficiency compared to its phosphate analogue. The enhancement can be attributed to the increased geometrical distortion of the VO4 tetrahedra. Furthermore, we found that the coordination number of the central vanadium atom significantly affects the optical band gaps. Among various coordination numbers, the 4-coordinate VO4 tetrahedra are found to be more favorable for widening the optical band gap of materials compared to the 5- and 6-coordinate vanadium polyhedra, as demonstrated by this work.

Covalent-Organic Framework-Based Materials in Theranostic Applications: Insights into Their Advantages and Challenges

Nanomedicine has been essential in bioimaging and cancer therapy in recent years. Nanoscale covalent-organic frameworks (COFs) have been growing as an adequate classification of biomedical nanomaterials with practical application prospects because of their increased porosity, functionality, and biocompatibility. The high sponginess of COFs enables the incorporation of distinct imaging and therapeutic mechanisms with a better loading efficiency. Nevertheless, preliminary biocompatibility limits their possibility for clinical translation. Thus, cutting-edge nanomaterials with high biocompatibility and improved therapeutic efficiency are highly expected to fast-track the clinical translation of nanomedicines. The inherent effects of nanoscale COFs, such as proper size, modular pore geometry and porosity, and specific postsynthetic transformation through simple organic changes, make them particularly appealing for prospective nanomedicines. The organic building blocks of COFs may also be postmodified for particular binding to biomarkers. The exceptional features of COFs cause them to be an encouraging nanocarrier for bioimaging and therapeutic applications. In this review, we have systematically discussed the advances of COFs in the field of theranostics by providing essential features of COFs along with their synthetic methods. Further, the applications of COFs in the field of theranostics (such as drug delivery systems, photothermal, and photodynamic therapy) are discussed in detail with the help of available literature to date. Furthermore, the advantages of COFs over other materials for therapeutics and drug delivery are discussed. Finally, the review concludes with potential future COF applications in the theranostic field.

Fluorination Strategy Towards Symmetry Breaking of Boron-centered Tetrahedron for Poly-fluorinated Optical Crystals

Borate crystals can be chemically and functionally modified by the fluorination strategy, which encourages the identification of emerging fluorooxoborates with a structure and set of characteristics not seen in any other oxide parents. However, the bulk of fluorooxoborates have been found accidentally, rational methods of synthesis are required, particularly for the infrequently occurring poly-fluorinated components. Herein, we reported the use of bifluoride salts as a potent source of fluorine to prepare fluorooxoborates that contain rarely tri-fluorinated [BF3 X] (X=O and CH3 ) tetrahedra and eleven compounds were found. We identified the optical properties of the organofluorinated group [CH3 BF3 ] and their potential for nonlinear optics for the first time. Among these, two non-centrosymmetric components hold potential for the production of 266 nm harmonic coherent light for nonlinear optics, and more crucially, have the benefit of growing large size single crystals. Our study establishes experimental conditions for the coexistence of the diverse functional groups, enabling the production of poly-fluorinated optical crystals.

NaMoO3(IO3)(H2O): water molecule introduction induces strong second harmonic generation response, widened band gap and large anisotropy

Exploring feasible tactics to induce the formation of non-centrosymmetric (NCS) structures, especially from centrosymmetric (CS) structures, is essential for the development of nonlinear optical crystals with more potential. An NCS alkali metal-containing molybdenum iodate hydrate, namely, NaMoO3(IO3)(H2O), was designed based on the CS matrix NaMoO3(IO3) via introducing a water molecule into the structure. The introduction of one crystalline water molecule results in the rearrangement of Λ-shaped cis-[MoO4(IO3)2] units, and the proper array of the cis-[MoO4(IO3)2] units in NaMoO3(IO3)(H2O) results in its strong SHG response of 4.6 × KH2PO4. In addition, NaMoO3(IO3)(H2O) exhibits a wider optical bandgap of 3.44 eV and a larger birefringence of 0.231 than its matrix. Furthermore, the framework of NaMoO3(IO3)(H2O) is highly similar to that of α-KMoO3(IO3), with water molecules assisting Na+ cations in occupying the position of K+. However, due to the extra hydrogen bond of water molecules, the [MoO3(IO3)]∞ layers in NaMoO3(IO3)(H2O) retain a parallel-stacking arrangement, different from the antiparallel arrangement of layers in α-KMoO3(IO3) with a centric structure. This study confirms the feasibility of applying a water molecule to adjust the orientation of basic building block units to assemble an NCS structure based on CS crystals.

From NaGa(IO3)3F to NaGa(IO3)2F2 and NaGa(IO3)4: The Effects of Chemical Substitution between F- Anions and IO3- Groups on the Structures and Properties of Gallium Iodates

Introducing F- anions or substituting F- anions with IO3- groups has been proven to be ideal strategies for designing novel noncentrosymmetric (NCS) and polar materials, yet systematic investigation into the effect of F- anions or the substitution of IO3- for F- anions on structures and properties remains rarely explored. Herein, two new gallium iodates, NaGa(IO3)2F2 (1) and NaGa(IO3)4 (2), were successfully designed and synthesized based on NaGa(IO3)3F by introducing more F- anions and replacing F- anions with IO3 groups, respectively. Structurally, in compound 1, the adjacent [GaF3(IO3)3]3- polyanions are connected in an antiparallel manner, resulting in a complete cancellation of local polarity. While in compound 2, all IO3 groups in 2D [Ga(IO3)4]∞- layers are aligned, leading to large macroscopic polarization. Additionally, chemical substitution also results in a qualitative improvement in the functional properties of compound 2. It possesses strong SHG response (12 × KDP @1064 nm) and broad optical transparency, coupled with large birefringence (0.21 @1064 nm), showcasing its promise as a promising nonlinear optical (NLO) crystal. The effects of chemical substitution between F- anions and IO3- groups on the structures and properties are discussed in detail.

Ultrashort Phase-Matching Wavelength and Strong Second-Harmonic Generation in Deep-UV-Transparent Oxyfluorides by Covalency Reduction

The development of urgently-needed ultraviolet (UV)/deep-UV nonlinear optical (NLO) materials has been hindered by contradictory requirements of the microstructure, in particular the need for a strong second-harmonic generation (SHG) response as well as a short phase-matching (PM) wavelength. We herein employ a "de-covalency" band gap engineering strategy to adjust the optical linearity and nonlinearity. This has been achieved by assembling two types of transition-metal (TM) polyhedra ([TaO2 F4 ] and [TaF7 ]), affording the first tantalum-based deep-UV-transparent NLO materials, A5 Ta3 OF18 (A = K (KTOF), Rb (RTOF)). Experimental and theoretical studies reveal that the highly ionic bonds and strong electropositivity of tantalum in the two oxyfluorides induce record short PM wavelengths (238 (KTOF) and 240 (RTOF) nm) for d0 -TM-centered oxides, in addition to strong SHG responses (2.8 × KH2 PO4 (KTOF) and 2.6 × KH2 PO4 (RTOF)), and sufficient birefringences (0.092 (KTOF) and 0.085 (RTOF) at 546 nm). These results not only broaden the available strategies for achieving deep-UV NLO materials by exploiting the currently neglected d0 -TMs, but also push the shortest PM wavelength into the short-wavelength UV region.

Synthesis, structures and properties of two new selenite optical materials: K2Zn3Se4O12 and K4Zn3V4Se2O19

Two new selenites, K2Zn3Se4O12 (compound 1) and K4Zn3V4Se2O19 (compound 2), have been successfully synthesized by solid-state reactions in vacuum tubes. Compound 1 consists of a three-dimensional (3D) framework with [SeO3] triangular pyramids and [ZnO4] tetrahedra in the monoclinic space group P21/c (No. 14). Compound 1's cut-off edge is below 344 nm, based on its UV-Vis-NIR diffuse reflectance studies, and theoretical calculations indicate a birefringence of around 0.043 at 1064 nm. The two-dimensional layer of compound 2, in contrast, is made up of [SeO3] triangular pyramids, [ZnO4] tetrahedra, and [V4O13] tetrahedra. It crystallizes in the monoclinic space group C2/c (No. 15). Its UV-Vis-NIR diffuse reflectance studies demonstrate that the compound's cut-off edge is lower than 330 nm.

Optically Anisotropic Mixed-Metal Fluoroiodate Ba2[GaF5(IO3F)] with a Wide Optical Transparent Window and a Moderate Birefringence

An optically anisotropic alkali-earth-metal gallium fluoroiodate, Ba2[GaF5(IO3F)] (1), was ingeniously obtained by integrating fluoride and fluoroiodate functional units under moderate hydrothermal conditions. It features a three-dimensional (3D) structure constructed by the highly polarizable fluoroiodate unit [IO3F] and the fluoride groups [GaOF5] and [BaO3Fx] (x = 6, 7). The compound is stable at temperatures up to 500 °C. With the synergistic interaction between [IO3F] and the fluoride groups, the mixed-metal fluoroiodate induces a short ultraviolet cutoff edge at about 230 nm, a medium measured birefringence of 0.068 @ 550 nm, and a wide optical transparent window (0.34-11.9 μm), indicating that 1 has potential applications as a birefringent material from near-UV to mid-infrared. Theoretical calculations prove that the optical characteristics of the compound are mainly attributed to [IO3F] and the fluoride functional groups. This work demonstrates that the presence of various specific functional groups in compounds will help to develop promising inorganic functional materials possessing good optical performance.

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.

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.

From Centrosymmetry to Noncentrosymmetry: Precise Structural Regulation and Characterization on ZnHPO3·2H2O Polymorphs

Polymorphs of ZnHPO₃·2H₂O with centrosymmetry (Cmcm) and noncentrosymmetry (C2) structures were prepared by modified solution evaporation and seed-crystal-induced secondary nucleation methods. In Cmcm-ZnHPO₃·2H₂O, the zinc atoms are only octahedrally coordinated, while in C2-ZnHPO₃·2H₂O, they feature both tetrahedral and octahedral coordination. As a result, Cmcm-ZnHPO₃·2H₂O features a 2D layered structure with lattice water molecules located in the interlayer space, while C2-ZnHPO₃·2H₂O features a 3D electroneutral framework of tfa topology connected by Zn(1)O₄, Zn(2)O₆, and HPO₃ units. The UV-visible diffuse reflectance spectra associated with Tauc's analyses give a direct bandgap of 4.24 and 4.33 eV for Cmcm-ZnHPO₃·2H₂O and C2-ZnHPO₃·2H₂O, respectively. Moreover, C2-ZnHPO₃·2H₂O exhibits a weak second harmonic generation (SHG) response and a moderate birefringence for phase matching, indicating its potential as a nonlinear optical material. Detailed dipole moment calculation and analysis confirmed that the SHG response mainly derived from the HPO₃ pseudo-tetrahedra.

Alignment of Λ-Shaped Basic Building Units to Construct One New KMoO3(IO3) Polar Polymorph

Nonlinear optical (NLO) crystals assume an irreplaceable role in the development of laser science and technologies, yet the reasonable design of a high-performance NLO crystal remains difficult owing to the unpredictability of inorganic structures. In this research, we report the fourth polymorph of KMoO₃(IO₃), i.e., δ-KMoO₃(IO₃), to understand the effect of different packing patterns of basic building units on structures and properties. Among four polymorphs of KMoO₃(IO₃), the different stacking patterns of Λ-shaped cis-MoO₄(IO₃)₂ units result in α- and β-KMoO₃(IO₃) featuring nonpolar layered structures, whereas γ- and δ-KMoO₃(IO₃) display polar frameworks. Theoretical calculations and structure analysis reveal that IO₃ units can be regarded as the main source of its polarization in δ-KMoO₃(IO₃). Further property measurements show that δ-KMoO₃(IO₃) exhibits a large second-harmonic generation response (6.6 × KDP), a wide band gap (3.34 eV), and a broad mid-infrared transparency region (∼10 μm), which demonstrates that adjusting the arrangement of the Λ-shaped basic building units is an effective approach for rationally designing NLO crystals.

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.

Intense d-p Hybridization in Nb3 O15 Tripolymer Induced the Largest Second Harmonic Generation Response and Birefringence in Germanates

We herein report two asymmetric germanate crystals, KNbGe₃ O₉ and K₃ Nb₃ Ge₂ O₁₃ , with different structures and optical properties derived from divergent polymerized forms of GeO₄ and NbO₆ groups. Remarkably, K₃ Nb₃ Ge₂ O₁₃ achieved a rare combination of the strongest second harmonic generation (SHG) response of 17.5×KDP @ 1064 nm and the largest birefringence of 0.196 @ 546 nm in germanates. It features unique [Nb₃ O₁₂ ]_∞ tubular chains constructed by circular Nb₃ O₁₅ tripolymers. Theoretical calculations reveal that the d-p interactions in the Nb₃ O₁₅ group are responsible for outstanding optical properties. This work emphasizes the significance of the polymerizable functional units in obtaining high-performance nonlinear optical (NLO) crystals.

Polar Bismuth Selenite Iodate Oxide BiSeIO6 with Three Types of Lone Pair Cations in One Structure

Novel bismuth selenite iodate oxide BiSeIO₆ was synthesized in a mild hydrothermal condition. BiSeIO₆ was crystallized in the polar space group Pna2₁ of an orthorhombic system. The crystal structure features a three-dimensional framework composed of three types of lone pair cations with distorted BiO₇ polyhedra, SeO₃ pyramids, and IO₃ pyramids in one structure. Interestingly, BiSeIO₆ exhibits a strong and phase-matchable second-harmonic generation (SHG) of ∼6 times that of KH₂PO₄ (KDP). Dipole moment analysis shows that all three local acentric groups of BiO₇, SeO₃, and IO₃ cooperatively contribute to the large macroscopic polarization and thereby strong SHG efficiency of BiSeIO₆. In addition, BiSeIO₆ has a broad transparency range from 0.35 to 11 μm, indicating its promising nonlinear optical applications from visible to mid-infrared bands.

Growth and Optical Properties of Large-Sized NaVO2(IO3)2(H2O) Crystals for Second-Harmonic Generation Applications

Large-sized crystals of the quaternary iodate NaVO₂(IO₃)₂(H₂O) (NVIO) with centimeter-scale dimensions (23 mm × 18 mm × 6 mm as a representative) have been successfully grown by the top-seeded hydrothermal method. Linear optical properties have been measured, including the optical transmission spectrum and refractive index. The NVIO crystal possesses an optical window with high transmittance (above 80%) over the range of 500-1410 nm and exhibits strong optical anisotropy with large birefringence Δn (n_z - n_x) of 0.1522 at 1064 nm and 0.1720 at 532 nm. Based on the measured refractive indices, the phase-matching conditions for second-harmonic generation (SHG) have been calculated, and SHG devices have further been fabricated along the calculated type I and type II phase-matching directions of (θ = 39.0°, φ = 3.8°) and (θ =53.8°, φ = 1.3°). Laser experiments of extra-cavity frequency doubling have been performed on these NVIO devices. It has been confirmed that the effective SHG conversion from 1064 to 532 nm could be achieved with an energy conversion efficiency of 8.1%. Our work demonstrates that large-sized NVIO crystals are promising in the frequency-doubling application.

Isoreticular Design of KTiOPO4-Like Deep-Ultraviolet Transparent Materials Exhibiting Strong Second-Harmonic Generation

Second-harmonic generation (SHG) is of great technological importance for applications in nonlinear optics, but it remains challenging to engineer large SHG responses in the short-wavelength ultraviolet (UV) region owing to competing microstructure requirements. Herein, we report the first examples of d⁰ transition-metal-based (TM-based) deep-UV-transparent nonlinear optical (NLO) crystals MOF₄H₂ (M = Zr (ZOF), Hf (HOF)), which exhibit unprecedented short UV absorption edges (below 190 nm). Evolving from the KTiOPO₄ (KTP) structure by an isoreticular node substitution strategy, the three-dimensional frameworks of ZOF and HOF consist of corner-sharing [MO₂F₆] moieties that are new functional units in deep-UV NLO material design, conferring wide UV transparency and strong phase-matchable SHG response (2.2 × KH₂PO₄ (ZOF) and 1.8 × KH₂PO₄ (HOF) at 1064 nm). Such d⁰-TM-based [MO₂F₆] polyhedra preclude deleterious d-d electronic transitions, resulting in significantly blue-shifted UV absorption edges of ZOF and HOF (<190 nm). The d⁰-TM-based [MO₂F₆] polyhedra introduced in this work offer a new perspective in the construction of deep-UV transparent NLO materials, demonstrating the feasibility of an isoreticular design strategy in developing functional NLO materials.

Ba2[WO3F(IO3)][WO3F2]: the first polar fluorinated tungsten iodate featuring a direct W-O-I bond

The introduction of the transition metal cations with d⁰ electron configurations and F in the iodate systems generates a new polar compound, Ba₂[WO₃F(IO₃)][WO₃F₂], which features the first example of a direct W-O-I bond in the structure. It exhibits excellent properties, including a large second harmonic generation response (∼3.5 × KH₂PO₄), a wide visible and mid-infrared transparency region (0.28-10.74 μm), and a moderate birefringence of 0.061@532 nm.

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.

Metal oxyhalides: an emerging family of nonlinear optical materials

Second-order nonlinear optical (NLO) materials have drawn enormous academic and technological attention attributable to their indispensable role in laser frequency conversion and other greatly facilitated applications. The exploration of new NLO materials with high performances thus has long been an intriguing research field for chemists and material scientists. However, an ideal NLO material should simultaneously satisfy quite a few fundamental yet rigorous criteria including a noncentrosymmetric structure, large NLO coefficients, desired transparent range, large birefringence, high laser damage threshold, and availability of a large-size single crystal. Therefore, the identification of promising compound systems, targeted design, and experience-based syntheses are crucial to discover novel NLO materials working in the spectral region of interest. As an important family of mixed-anion compounds, versatile metal oxyhalides containing metal-centered oxyhalide functional units ([MO m X n ] (X = F, Cl, Br, and I)) are becoming a marvelous branch for interesting NLO materials. Especially, when the central metals are d0/d10 transition metals or heavy post-transition metals, a number of novel NLO materials with superior functionalities are expected. Our thorough review on the recent achievements of metal oxyhalides for NLO materials are divided into the fast-growing NLO metal oxyhalides with single type halogen anions and the newly identified NLO metal oxyhalides with mixed halogen anions. Here we mainly focus on the design strategy, structural chemistry, NLO-related properties, and structure-property correlation of the metal oxyhalides with relatively large NLO responses. We hope this review can provide an insight on the rational design and future development of emerging metal oxyhalides for NLO and other applications.

Unprecedented boat-shaped [Mo2O5(IO3)4]2- polyanions induced a strong second harmonic generation response

The first organic-inorganic hybrid guanidine molybdenyl iodate [C(NH₂)₃]₂Mo₂O₅(IO₃)₄·2H₂O was successfully synthesized via an improved moderate hydrothermal method. It features an unprecedented boat-shaped zero-dimensional [Mo₂O₅(IO₃)₄]^2- polyanion cluster, which induces a wide band gap, moderate birefringence and strong second harmonic generation response, indicating that it is a potential nonlinear optical material.

K3V2O3F4(IO3)3: a high-performance SHG crystal containing both five and six-coordinated V5+ cations

The combination of d⁰ transition metal oxofluorides with iodate anions helps to synthesize polar crystals. Herein, a novel polar crystal, K₃V₂O₃F₄(IO₃)₃, which is the first metal vanadium iodate with two types of V⁵⁺-centered polyhedra (VO₄F₂ octahedron and VO₃F₂ trigonal bipyramid), has been prepared hydrothermally. It crystallizes in the polar space group of Cmc2₁ and its structure displays an unprecedented 0D [V₂O₃F₄(IO₃)₃]^3- anion, which is composed of Λ-shaped cis-[VO₂F₂(IO₃)₂]^3- and [VO₂F₂(IO₃)]^2- anions interconnected via the corner-sharing of one oxo anion. The synergy gained from the VO₄F₂, VO₃F₂ and IO₃ groups resulted in K₃V₂O₃F₄(IO₃)₃ exhibiting both a strong second-harmonic generation (SHG) response (1.3 × KTiOPO₄) under 2050 nm laser irradiation and a large birefringence (0.158 @ 2050 nm). This study provides a facile route for designing SHG materials by assembling various vanadium oxide-fluoride motifs and iodate anions into one compound.

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.

Wide Bandgaps and Strong SHG Responses of Hetero-Oxyfluorides by Dual-Fluorination-Directed Bandgap Engineering

A wide bandgap is an essential requirement for a nonlinear optical (NLO) material. However, it is very challenging to simultaneously engineer a wide bandgap and a strong second-harmonic generation (SHG) response, particularly in NLO materials containing second-order Jahn–Teller (SOJT) distorted units. Herein, we employ a bandgap engineering strategy that involves the dual fluorination of two different types of SOJT distorted units to realize remarkably wide bandgaps in the first examples of 5d⁰-transition metal (TM) fluoroiodates. Crystalline A₂WO₂F₃(IO₂F₂) (A = Rb (RWOFI) and Cs (CWOFI)) exhibit the largest bandgaps yet observed in d⁰-TM iodates (4.42 (RWOFI) and 4.29 eV (CWOFI)), strong phase-matching SHG responses of 3.8 (RWOFI) and 3.5 (CWOFI) × KH₂PO₄, and wide optical transparency windows. Computational studies have shown that the excellent optical responses result from synergism involving the two fluorinated SOJT distorted units ([WO₃F₃]³⁻ and [IO₂F₂]⁻). This work provides not only an efficient strategy for bandgap modulation of NLO materials, but also affords insight into the relationship between the electronic structure of the various fluorinated SOJT distorted units and the optical properties of crystalline materials.

Wide bandgaps, strong SHG responses, and sufficient birefringence are observed in the first examples of 5d⁰-transition metal fluoroiodates, A₂WO₂F₃(IO₂F₂) (A = Rb, and Cs), which were constructed by dual-fluorination-directed bandgap engineering.

Two molybdenyl carbonates with different dimensional structures exhibiting huge differences in band gaps

Two molybdenyl carbonates with different dimensional structures exhibit huge differences in band gaps, 0D Cs3MoO4(HCO3) exhibiting a much larger band gap than 1D Cs2MoO3(CO3).

KLi2CO3F: a Beryllium-free KBBF-type Deep-UV Carbonate with Enhanced Interlayer Interaction and Large Birefringence

KBe2BO3F2 (KBBF) with perfect honeycomb hexagonal layered structure is the sole practical deep-ultraviolet (DUV) nonlinear optical material which could generate DUV coherent light through direct frequency doubling. However, the layered...

A new I3O93− group constructed from IO3− and IO55− anion units in Cs3[Ga2O(I3O9)(IO3)4(HIO3)]

Cs3[Ga2O(I3O9)(IO3)4(HIO3)] with a novel I3O93− fundamental building block (FBB) constituted by two IO3 and one IO5 polyhedra exhibited a wide band gap.

A Lanthanum Ammonium Sulfate Double Salt with a Strong SHG Response and Wide Deep-UV Transparency

The targeted synthesis of deep-ultraviolet (deep-UV) nonlinear optical (NLO) materials, especially those with non-π-conjugated sulfates, has experienced considerable difficulties due to the need to reconcile the oft-competing requirements for deep-UV transparency and strong second-harmonic generation (SHG). We report herein the designed synthesis of the first rare-earth metal-based deep-UV sulfate La(NH₄ )(SO₄ )₂ by a double-salt strategy involving introduction of complementary cations, together with optical studies that reveal a short-wavelength deep-UV absorption edge (below 190 nm) and the strongest SHG response among deep-UV NLO sulfates (2.4×KDP). Theoretical calculations and crystal structure analysis suggest that the excellent balance between SHG response and deep-UV transparency can be attributed to a synergistic interaction of the hetero-cations La³⁺ and [NH₄ ]⁺ , which optimize alignment of the [SO₄ ] tetrahedra and highly polarizable [LaO₈ ] polyhedra.

Synergistic Effect of π-Conjugated [C(NH2)3] Cation and Sb(III) Lone Pair Stereoactivity on Structural Transformation and Second Harmonic Generation

The search for nonlinear optical (NLO) crystals with excellent comprehensive properties is a formidable challenge. In this work, two guanidine antimony fluorides, C(NH₂)₃Sb₂F₇ and C(NH₂)₃SbF₄, were obtained by conjunction of [C(NH₂)₃] groups with π-conjugated configuration and stereochemically active Sb³⁺ cations. Due to the different coordination modes of Sb-F bonds and H-F hydrogen bonds, the crystal structure of C(NH₂)₃Sb₂F₇ is centrosymmetric (CS), while C(NH₂)₃SbF₄ is noncentrosymmetric (NCS). Optical measurements show that the UV cutoff wavelengths of the title compounds were both less than 240 nm. Thermal studies indicate that these crystals are stable up to 250 °C. In addition, the second harmonic generation (SHG) response of C(NH₂)₃SbF₄ is 2 times that of KH₂PO₄ (KDP) with the phase-matchable capacity. Theoretical calculations reveal that the large SHG effects of C(NH₂)₃SbF₄ were attributed to the synergy between the planar [C(NH₂)₃] units and the distorted [SbF₄] groups. These results demonstrate that the guanidine antimony fluorides will have potential value as UV NLO materials.

Study of Integer Spin S = 1 in the Polar Magnet β-Ni(IO3)2

Polar magnetic materials exhibiting appreciable asymmetric exchange interactions can potentially host new topological states of matter such as vortex-like spin textures; however, realizations have been mostly limited to half-integer spins due to rare numbers of integer spin systems with broken spatial inversion lattice symmetries. Here, we studied the structure and magnetic properties of the S = 1 integer spin polar magnet β-Ni(IO₃)₂ (Ni²⁺, d⁸, ³F). We synthesized single crystals and bulk polycrystalline samples of β-Ni(IO₃)₂ by combining low-temperature chemistry techniques and thermal analysis and characterized its crystal structure and physical properties. Single crystal X-ray and powder X-ray diffraction measurements demonstrated that β-Ni(IO₃)₂ crystallizes in the noncentrosymmetric polar monoclinic structure with space group P2₁. The combination of the macroscopic electric polarization driven by the coalignment of the (IO₃)⁻ trigonal pyramids along the b axis and the S = 1 state of the Ni²⁺ cation was chosen to investigate integer spin and lattice dynamics in magnetism. The effective magnetic moment of Ni²⁺ was extracted from magnetization measurements to be 3.2(1) µ_B, confirming the S = 1 integer spin state of Ni²⁺ with some orbital contribution. β-Ni(IO₃)₂ undergoes a magnetic ordering at T = 3 K at a low magnetic field, μ₀H = 0.1 T; the phase transition, nevertheless, is suppressed at a higher field, μ₀H = 3 T. An anomaly resembling a phase transition is observed at T ≈ 2.7 K in the C_p/T vs. T plot, which is the approximate temperature of the magnetic phase transition of the material, indicating that the transition is magnetically driven. This work offers a useful route for exploring integer spin noncentrosymmetric materials, broadening the phase space of polar magnet candidates, which can harbor new topological spin physics.

An Effective Strategy for Designing Nonlinear Optical Crystals by Combining the Structure-Directing Property of Oxyfluorides with Chemical Substitution

Rationally designing a high-performance nonlinear optical (NLO) crystal remains a great ongoing challenge. It involves not only the design of noncentrosymmetric structures but also property optimization. In this communication, a new strategy for effectively designing the NLO crystal has been put forward, that is, using the structure-directing property of oxyfluoride anions to obtain a noncentrosymmetric and polar structure, and then by substitution of IO₃ ^- for isovalent F^- anions to further enhance the SHG response. With this strategy, a new iodate fluoride, Ba₂ [MoO₃ F(IO₃ )](MoO₃ F₂ ) has been successfully designed from the Ba₂ (MoO₃ F₂ )F₂ with the cis-directing [MoO₄ F₂ ]^4- groups. It exhibits a large SHG response (≈8×KDP), a wide transparent region (0.30-10.92 μm), a high laser-induced damage threshold (LDT) (88.53 MW cm^-2 ), and a large birefringence (≈0.264@532 nm). These indicate Ba₂ [MoO₃ F(IO₃ )](MoO₃ F₂ ) would be a promising NLO crystal.

Heterobimetallic Cluster-Based Coordination Polymers: Assembly, Structures and Third-Order Nonlinear Optical Properties

Reactions of (NH₄ )₂ WS₄ with CuCN, CuCN/1,2-bis(4-pyridyl)propane (bppa) or [Cu(MeCN)₄ ]PF₆ /bppa under different reaction conditions afforded a set of two- or three-dimensional W/Cu/S cluster-based coordination polymers including {[Et₄ N]₂ [WS₄ Cu₄ (μ-CN)₂ (μ-I)₂ ]}_n (1), [WS₄ Cu₄ (μ-CN)₂ (bppa)₂ ]_n (2) and {[WS₄ Cu₄ (bppa)₄ ](PF₆ )₂ }_n (3), respectively. Compound 2 can be readily formed from reaction of 1 with bppa under solvothermal conditions. Compounds 1 and 2 feature two-dimensional networks with a "sql" topology, while 3 possesses a two-fold interpenetrated three-dimensional net with a rare "reo" topology. Compounds 1-3 in DMF exhibited different third-order nonlinear optical responses, and they all showed a reverse saturable absorption while 2 held a strong self-focusing effect.

NaGaI3O9F: a new alkali metal gallium iodate combined with IO3- and IO3F2- units

Birefringent crystals are very important optical materials, which are widely used in the fields of optics and communication. In this work, we reported a new iodate, NaGaI₃O₉F, synthesized by mild hydrothermal method. NaGaI₃O₉F crystallized in the monoclinic space group P2₁/c (No. 14) and it featured a novel _∞[Ga₂(IO₃F)₂(IO₃)₄]^2- layer stacked with Na⁺ cations located in the void maintaining charge balance. Notably, IO₃^- and IO₃F^2- anionic units appeared in the same time in the A-Ga-I-O/F (A = alkali metal) system. According to the experimental characterization and theoretical calculations, NaGaI₃O₉F showed a wide bandgap (4.27 eV) and large birefringence (Δn_exp∼ 0.203, Δn_cal = 0.197 at 1064 nm), indicating its potential use as a birefringent material.

[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

An organic-inorganic hybrid polyiodate, namely, [o-C₅ H₄ NHOH]₂ [I₇ O₁₈ (OH)]⋅3 H₂ O (I), featuring a novel branched polyiodate chain has been obtained by evaporation method. [o-C₅ H₄ NHOH]₂ [I₇ O₁₈ (OH)]⋅3 H₂ O (I) crystalizes in the polar space group Ia and features an [I₇ O₁₈ (OH)] ∞ 2 - branched polyiodate chain in which [I₃ O₉ ]^3- trimers are grafted on the same side of the one-dimensional (1D) chain based on [I₄ O₁₁ (OH)]^3- tetramers. The asymmetric organic amine groups are beneficial to the polymerization of iodate groups and inducing the formation of the non-centrosymmetric (NCS) structure. Compound I exhibits a rather large Second-Harmonic- Generation (SHG) signal of 8.5×KH₂ PO₄ (KDP) upon 1064 nm laser radiation, a moderate band gap of 3.90 eV and a high laser-induced-damage-threshold (LIDT) of 182.34 MW cm^-2 , hence it is a promising new SHG material. The relationship between the structures of the organic amine groups and the overall structures has been also analyzed.