Pb2BO3Cl: A Tailor-Made Polar Lead Borate Chloride with Very Strong Second Harmonic Generation

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.

Layered Perovskite-like Nitrate Cs2Pb(NO3)2Br2 as a Multifunctional Optical Material

A novel alkali metal lead halide nitrate, Cs₂Pb(NO₃)₂Br₂, has been successfully synthesized via a hydrothermal method. Interestingly, the title compound features a distinctive Ruddlesden-Popper perovskite-like layered structure, which induces the outstanding multifunctional optical properties, including a large birefringence (0.147@546 nm) and broad light-orange emission. Detailed structural analysis and theoretical calculations revealed that the large birefringence originates from the p-π interaction between the Pb²⁺ cations and NO₃ groups and that the excellent luminescence properties derive from the distortion of PbO₄Br₄ polyhedra. This work not only enriches the variant structure types of layered perovskites but also guides the further exploration of all-inorganic multifunctional optical materials.

Na2SrB16O26: a new borate with independent interpenetrating B-O networks and deep-ultraviolet cutoff edge

A new borate, Na₂SrB₁₆O₂₆, was synthesized by the high-temperature solution method. It exhibits complicated interpenetrating 3D B-O frameworks composed of the functional building block (FBB) [B₈O₁₆]. The UV-vis-NIR diffuse reflectance spectroscopy shows that it has a deep-ultraviolet (DUV) cutoff edge (<200 nm). The relationship between the structures and optical properties was uncovered by theoretical calculations. By the first-principles calculation, the birefringence is estimated to be 0.07 at 1064 nm. The response electron distribution anisotropy (REDA) analysis indicates that the [BO₃] units contribute mainly to the generation of the moderate birefringence.

Two Indium Iodate-Nitrates with Large Birefringence Induced by Hybrid Anionic Functional Groups and Their Favorable Arrangements

Two new indium iodate-nitrates, In(IO₃)₂(NO₃) (1) and [In(IO₃)(OH)(H₂O)](NO₃) (2), were rationally designed through the integration of hybrid anionic functional units. They exhibit large birefringences (1, 0.269; 2, 0.188, at 532 nm) and wide band gaps (1, 4.08 eV; 2, 4.39 eV), which is attributed to the synergistic effect of two types of birefringence-active units, namely, lone-pair IO₃ and π-conjugated NO₃ anionic groups. Through the substitution of OH and H₂O of 2 with IO₃, the hydrogen bonds of 2 are eliminated and the birefringence of 1 is greatly enhanced, highlighting the intriguing role of isovalent substitution in the discovery of fascinating optical materials.

Introducing a New d0 Sc3+ Asymmetric Ion for Functional Materials: Large Birefringence Enhancement by ScO6 in Ba3 Sc2 (BO3 )4

Transition metal ions with d⁰ electronic states (Ti⁴⁺ , Zr⁴⁺ , Nb⁵⁺ and Ta⁵⁺ ) are widely investigated as functional materials. This work first illustrates that Sc³⁺ ion, long-time ignored, displays a second-order Jahn-Teller (SOJT) effect similar to asymmetric oxide-coordinated transition metal ions, thus providing a new ground to seek for asymmetric functional materials with enhanced performances. In Ba₃ Sc₂ (BO₃ )₄ , BO₃ groups are parallelly arranged, satisfying the ideal arrangement to produce large birefringence. Importantly, distorted octahedral ScO₆ with Sc³⁺ ion in its d⁰ electronic state enlarges birefringence unexpectedly up to 0.149 @ 550 nm, which is larger than previously reported borates containing solely BO₃ , even to B₃ O₆ units. Subsequently, the SOJT influence of distorted ScO₆ octahedra on birefringence is verified by a comparison between experimental data and theoretical calculations. In addition, Ba₃ Sc₂ (BO₃ )₄ also displays a high transmittance in the range of 230 nm-3.5 μm with a UV cut-off wavelength at 198 nm and a large laser induced damage threshold (2.7 GW/cm² ), comparable to α-BaB₂ O₄ . Above characteristics imply that the title compound may be a promising birefringent material.

MM'B3O4F3 (M = K; M' = Na, K, Cs): Alkali-Metal Fluorooxoborates with ∞1[B3O4F3] Chains and Deep-Ultraviolet Cutoff Edges

Three mixed-alkali-metal fluorooxoborates, KNaB₃O₄F₃ (I), K₂B₃O₄F₃ (II), and KCsB₃O₄F₃ (III), were acquired in a closed system. I-III are isomorphic and adopt orthorhombic structures [Pbcn (No. 60)] with wavy parallelly arranged pseudolayers composed of _∞¹[B₃O₄F₃] chains, which exhibit slight differences in the arrangement modes of the fundamental building blocks. First-principles calculations illustrate that they all have moderate birefringence and large band gaps on the order of 7.0 eV, suggesting deep-ultraviolet (DUV) cutoff edges. In order to investigate the main source of the optical properties, the electronic structure and anisotropy of the response electron distribution were analyzed. Experimental characterizations for I confirm the structure and DUV transparence ability.

From Pb(H2C3N3O3)(OH) to Pb(H2C3N3O3)F: Homovalent Anion Substitution-Induced Band Gap Enlargement and Birefringence Enhancement

Birefringent materials capable of modulating the polarization of light have attracted intensive studies because of their wide utilization in optical communication and the laser industry. Herein, two new lead(II)-based cyanurates, namely, Pb(H₂C₃N₃O₃)X (X = OH, F), were synthesized by hydrothermal methods, and the first halogen-containing metal cyanurate Pb(H₂C₃N₃O₃)F was successfully obtained by the rational substitution of a homovalent anion. Pb(H₂C₃N₃O₃)X (X = OH, F) belong to space group P1̅, and their structures display a neutral [Pb(H₂C₃N₃O₃)X] (X = OH, F) layer. The Pb²⁺ ions in Pb(H₂C₃N₃O₃)(OH) are interconnected by hydroxyl groups and oxygen atoms of cyanurate anions into a 1D [PbO(OH)]^- chain, whereas the Pb²⁺ ions in Pb(H₂C₃N₃O₃)F are interconnected by F^- anions and oxygen atoms of cyanurate anions into a 2D [PbOF]^- layer. The π-π interactions between adjacent hydroisocyanurate rings and the hydrogen bonds between neighboring neutral layers provide additional stability to the structures. Luminescent studies show that Pb(H₂C₃N₃O₃)(OH) and Pb(H₂C₃N₃O₃)F emit yellow-green and blue light, respectively. Theoretical calculations unveiled their birefringences of 0.079 and 0.203@1064 nm and their band gaps of 3.96 and 4.96 eV, respectively, for OH^- and F^- containing materials. Obviously, the substitution of OH^- by F^- with the largest electronegativity can simultaneously improve both the birefringence and band gap.

HgTeO2F(OH): A Nonlinear Optical Oxyfluoride Constructed of Active [TeO2F(OH)]2- Pyramids and V-Shaped [HgO2]2- Groups

Oxyhalides possessing the merits of oxides and halides have widely received attention for their comprehensive physical performances, especially as potential nonlinear optical (NLO) crystals. Here, based on conventional strategy for obtaining acentric compounds, a Te⁴⁺ lone-pair cation was introduced into oxyhalides, and one oxyfluoride, HgTeO₂F(OH), was obtained via a hydrothermal reaction. Crystallized in the polar space group Pca2₁, the layered structure of HgTeO₂F(OH) is composed of V-shaped [HgO₂]^2- groups and [TeO₂F(OH)]^2- pyramids, in which the [TeO₂F(OH)]^2- pyramid first served as the NLO functional motif. Its powder sample exhibits a phase-matchable SHG response of 1.1 × KH₂PO₄ at 1064 nm, and its birefringence (0.09@1064 nm) is sufficient for phase-matchable behavior, which manifests its comprehensive capacity as a promising NLO candidate. Theoretical calculations about electronic structure and optical properties are also carried out, revealing that the Te⁴⁺ lone-pair cation makes the predominant contribution to the SHG effect and synergizes with the [HgO₂]^2- groups.

α-LiMB9O15 (M = Sr, Pb): Flexible [B3O7] Units Leading to the Low Temperatures Phase of β-LiMB9O15 (M = Sr, Pb)

Polymorphism is a very important phenomenon in borates and is helpful to study the structure and properties. The low-temperature phase α-LiMB9O15 (M = Sr, Pb) crystallized in chiral space groups...

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...

NH4(B6PO10(OH)4)·H2O: Exhibiting the Largest Birefringence in Borophosphates

Borophosphates are ideal materials class for the design of deep-UV optical funcitonal crystals yet their practical applications are limited by the small birefringence. Herein, a new mixed-coordinated pyrophosphate, NH4(B6PO10(OH)4)·H2O has...

Ba2HgTe5: a Hg-based telluride with giant birefringence induced by linear [HgTe2] units

Ba2HgTe5, the first Hg-based telluride birefringent material, was successfully synthesized. The analysis of the response electron distribution anisotropy illustrates that the large birefringence of Ba2HgTe5 originates from the linear [HgTe2] unit.

Enhancement of band gap and birefringence induced via π-conjugated chromophore with “tail effect”

The enhancement of band gap and birefringence of template compounds can be attributed to this “tail effect” caused by the extended electronic distribution from [CO3]2−/[BO3]3−/[C3N3O3]3− to [HCO3]−/[HBO3]2−/[HC3N3O3]2−.

Li6.58Na7.43Sr4(B9O18)(B12O24)Cl: Unprecedented combination of the largest two highly polymerized isolated B-O Clusters with novel isolated B9O18 FBB

A new borate, Li6.58Na7.43Sr4(B9O18)(B12O24)Cl (LNSBOC), is successfully obtained by spontaneous crystallization method in an open system. LNSBOC crystallizes in a hexagonal crystal system with the centrosymmetric space group of P63/m,...

K2(BeS)O4F2: a novel fluorosulfate with unprecedented 1D [(BeO3F)–(SO3F)]∞ chains exhibiting large birefringence

By introducing the strongly electronegative F− ions into [SO4] tetrahedral units, polar NLO-active units [(BeS)O4F2] were obtained, thus forming 1D zigzag [(BeO3F)–(SO3F)]∞ chains.

A2B6O9F2 (A = NH4, K): new members of A2B6O9F2 family with deep-UV cutoff edges and moderate birefringence

Two new fluorooxoborates A2B6O9F2 (A = NH4, K) with different two-dimensional [B6O9F2]∞ layered structure constructed by the fundamental building block [B6O11F2] were obtained.

An excellent lead oxyiodide with a strong second-harmonic generation response and a large birefringence induced by the oriented arrangement of highly distorted [PbO4I2] polyhedra

K2I[PbI(OOCCH2COO)] featuring the strongest second-harmonic generation response among malonates, a large birefringence, a wide transparent window and good stability is induced by the oriented arrangement of highly distorted bifunctional [PbO4I2] polyhedra.

Sr3B14O24: a new borate with a [B14O30] fundamental building block and an unwonted 2D double layer

A new alkaline-earth metal borate, Sr₃B₁₄O₂₄, was successfully synthesized by a high-temperature solution method. It crystallizes in the monoclinic space group P2₁/c (no. 14) and features a fundamental building block (FBB) [B₁₄O₃₀] which is composed of eight [BO₃] and six [BO₄] units. The FBBs further condense to form an unwonted infinite 2D double layer extended in the bc plane. The UV-Vis-NIR diffuse reflectance spectrum shows that the cutoff edge of Sr₃B₁₄O₂₄ is less than 200 nm, which indicates its potential use in the deep UV region. In addition, the first principles theoretical study was carried out to better understand the relationship between the structure and performance.

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.

Structure and Origin of the Second-Harmonic Generation Response of Nonlinear Optical Material Sr2Be2B2O7

Sr₂Be₂B₂O₇ (SBBO) has long been considered as one of the most promising deep-ultraviolet nonlinear optical materials, but its crystal structure described by space group P6̅c2 in previous studies has remained questionable. On the basis of first-principles calculations coupled with the high-throughput crystal structure prediction method, we found three energetically favorable structures for SBBO with space groups Cm, Pm, and P6̅. These structures and a superstructure of space group Pm-S derived from the Cm structure were refined by the Rietveld method using the available powder X-ray diffraction data. These analyses show that the Pm-S structure is the best one, but its parent Cm structure is almost equally good and has the advantage of having higher symmetry. Via atom response theory analysis, we resolved the cause for the second-harmonic generation (SHG) responses of SBBO at the atomic and orbital level to elucidate the importance of local inversion symmetry in reducing the SHG response.

From β-Na2 B6 O10 to Na3 AlB8 O15 and Na3 Al2 B7 O15 : Structural Tuning of Anionic-Group Architectures by Substitution of [BO4 ] by [AlO4 ] Covalent Tetrahedra

Two new sodium aluminum borates, Na₃ AlB₈ O₁₅ and Na₃ Al₂ B₇ O₁₅ , have been successfully synthesized by the high-temperature solution method. They crystallize in the different space groups, P2₁ /c and P2/c, respectively. The B-O configurations of β-Na₂ B₆ O₁₀ , Na₃ AlB₈ O₁₅ and Na₃ Al₂ B₇ O₁₅ are compared to feature complicated different dimensional open-framework structures caused by the substitution of [BO₄ ] by [AlO₄ ] covalent tetrahedra. Moreover, the experimental results indicate that Na₃ AlB₈ O₁₅ and Na₃ Al₂ B₇ O₁₅ have short ultraviolet (UV) cutoff edges (<187 nm). The first-principles calculations show that Na₃ AlB₈ O₁₅ and Na₃ Al₂ B₇ O₁₅ have moderate birefringence (0.075 and 0.041@1064 nm, respectively).

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.

Sn2 PO4 I: An Excellent Birefringent Material with Giant Optical Anisotropy in Non π-Conjugated Phosphate

Exploring non π-conjugated phosphate birefringent crystal with a large birefringence has been a great challenge. Herein, based on the unique two-dimensional layered structure in KBe₂ BO₃ F₂ (KBBF), two new compounds, Sn₂ PO₄ I and Sn₂ BO₃ I, were designed and synthesized successfully, maintaining the layer structural feature and enhancing the optical anisotropy of crystals. In particular, the birefringence of Sn₂ PO₄ I is larger than or equal to 0.664 @546 nm, which is largest among the reported borates and phosphates, even surpassing commercial birefringent crystals YVO₄ and TiO₂ . This work indicates that a breakthrough in birefringence of inorganic compound was achieved. Also, it provides a guiding idea for exploring large birefringence materials in the future.

Thermal anomalies and phase transitions in Pb2Sc2Si2O9 and Pb2In2Si2O9

Pb2Sc2Si2O9 and Pb2In2Si2O9, respectively, the scandium and indium containing structural analogues of the mineral kentrolite are grown by spontaneous crystallization from a PbO flux. The corresponding polycrystalline powder samples are synthesized by conventional solid-state approach. The compounds are thoroughly characterized using temperature-dependent single crystal and powder X-ray diffraction, heat capacity measurements, second harmonic generation experiments and Raman spectroscopy. At ambient conditions, both compounds crystallize in the non-centrosymmetric Pna21 space group and undergo phase transitions to the centrosymmetric Pbcn space group at elevated temperatures. The Pbcn into Pna21 phase transitions are complemented by the signals of the temperature-dependent second harmonic generation. The specific heat capacity exhibits distinct cusp, supporting the λ-type second-order phase transition. The temperature dependency of some selective Raman modes further complements the findings, showing softening and hardening of the phonons across the phase transitions.

Strong SHG Responses in a Beryllium-Free Deep-UV-Transparent Hydroxyborate via Covalent Bond Modification

Deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals are key materials in creating tunable deep-UV lasers for frequency conversion technology. However, practical application of the sole usable crystal, KBe₂ BO₃ F₂ , has been hindered by the high toxicity of beryllium and its layering tendency in crystal growth. Herein, we report a beryllium-free deep-UV NLO material NaSr₃ (OH)(B₉ O₁₆ )[B(OH)₄ ] (NSBOH), synthesized by a covalent bond modification strategy under hydrothermal conditions. Moisture-stable NSBOH exhibits strong second-harmonic generation (SHG) at 1064 nm (3.3 × KH₂ PO₄ ) and 532 nm (0.55 × β-BaB₂ O₄ ), both amongst the largest powder SHG responses for a deep-UV borate, with good phase-matchability and a short wavelength cutoff edge (below 190 nm). NSBOH possesses a 3D covalent anionic [B₉ O₁₉ ]_∞ honeycomb-like framework with no layering. The Sr²⁺ and Na⁺ ions, residing in the cavities of the anionic framework, act as templates for the assembly and favorable alignment of NLO-active groups, resulting in an optimal balance between strong SHG activities and wide UV transparency. These merits indicate NSBOH is a very attractive candidate for deep-UV NLO applications.

Ba2.5Pb1.5B12O22: structural transformation from a centrosymmetric to a noncentrosymmetric space group by introducing Pb into Ba2B6O11

A new congruent-melting lead barium borate, Ba_2.5Pb_1.5B₁₂O₂₂, was synthesized via a high-temperature solid-state reaction under atmospheric pressure. It crystallizes in the noncentrosymmetric space group Cmc2₁ with a unit cell of a = 19.051(8) Å, b = 10.726(3) Å, c = 8.612(3) Å, and Z = 4. It features a new functional building block (FBB), [B₁₂O₂₆]^16-, that is made up of five [B₃O₈]^7- and one [BO₃]^3- connected by sharing vertex O atoms. It is derived by introducing Pb into Ba₂B₆O₁₁, which leads to a structural transformation from a centrosymmetric to a noncentrosymmetric space group. Its second harmonic generation response (SHG) is approximately 0.9 times that of KDP at the fundamental wavelength of 1064 nm and is also phase-matchable. It has a wide transparent region with a UV cutoff edge of about 250 nm according to the reflectance spectra and high thermal stability, which illustrates that it is a potential candidate for ultraviolet nonlinear optical materials.

[Ag(NH3 )2 ]2 SO4 : A Strategy for the Coordination of Cationic Moieties to Design Nonlinear Optical Materials*

Over recent decades, guided by the anionic group theory of nonlinear optical (NLO) materials, rational design strategies have been primarily focused on anionic moieties; consequently, structural modification and design of cationic moieties have long been neglected. Herein, we report a strategy for the coordination of cationic moieties that substantially enhances the optical properties of NLO materials. For an example with well-known crystal structure, [Ag(NH₃ )₂ ]₂ SO₄ , we demonstrate that the coordination of the Ag⁺ cation by the neutral ligand drives the formation of a noncentrosymmetric tetragonal P 4 ‾ 2₁ c structure as a positive uniaxial crystal. The bending of the [Ag(NH₃ )₂ ]⁺ cationic moiety parallel to the z-axis generates an anisotropic arrangement of the dipoles, i.e., a dipole of 0.12 D along the z-direction, yet zero dipole in the xy-plane, which interacts anisotropically with the incident light oscillating electromagnetic wave, leading to optical anisotropy with a large birefringence. The incident beam of 589.3 nm normal to the (110) crystal plane measures Δn_obv. =0.08, and [Ag(NH₃ )₂ ]₂ SO₄ also exhibits a phase-matching NLO response 1.4 times that of KH₂ PO₄ (KDP) (obv. 1.4×KDP @1064 nm; cal. d₃₆ =1.50 pm V^-1 ).

Co-crystal AX·(H3C3N3O3) (A = Na, Rb, Cs; X = Br, I): a series of strongly anisotropic alkali halide cyanurates with a planar structural motif and large birefringence

Birefringent crystals with strong anisotropy are important components in modern optical devices. The newly discovered planar π-conjugated cyanurate group (H_xC₃N₃O₃)^x-3 (x = 0-3) has been demonstrated as an effective functional motif for improving birefringence in the ultraviolet region. Here, single co-crystals of alkali halide cyanurates, RbBr·(H₃C₃N₃O₃) (I), RbI·(H₃C₃N₃O₃) (II), and CsBr·(H₃C₃N₃O₃) (III) were synthesized by the ethanol solution method, and NaBr·(H₃C₃N₃O₃) (IV) was obtained via the solvent-drop grinding method. These four co-crystals feature a planar (H₃C₃N₃O₃) arrangement and exhibit wide band gaps (> 4.90 eV), tunable birefringence (Δn_exp∼ 0.124-0.256), and high thermal stability (156 °C-349 °C). In addition, first principles calculations were also carried out to evaluate the relationship between molecule density, spatial arrangement and optical birefringence, and suggested a great tailoring effect of the alkali metal and halogen species on regulating the optical anisotropy of co-crystal cyanurates.

Fluorine-Driven Enhancement of Birefringence in the Fluorooxosulfate: A Deep Evaluation from a Joint Experimental and Computational Study

Understanding and exploring the functional modules (FMs) consisting of local atomic groups can promote the development of the materials with functional performances. Oxygen-containing tetrahedral modules are popular in deep-ultraviolet (DUV) optical materials, but their weak optical anisotropy is adverse to birefringence. Here, the fluorooxosulfate group is proved as a new birefringence-enhanced FM for the first time. The birefringence of fluorooxosulfates can be 4.8-15.5 times that of sulfates with the same metal cations while maintaining a DUV band gap. The polarizing microscope measurement confirms the birefringence enhancement by using the millimeter crystals experimentally. The theoretical studies from micro and macro levels further reveal a novel universal strategy that the fluorine induced anisotropic electronic distribution in fluorooxo-tetrahedral group is responsible for the enhancement of birefringence. This study will guide the future discovery of DUV optical materials with enlarged birefringence.

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.

Two Tellurium(IV)-Based Sulfates Exhibiting Strong Second Harmonic Generation and Moderate Birefringence as Promising Ultraviolet Nonlinear Optical Materials

Two tellurium(IV)-based sulfate nonlinear optical (NLO) materials, Te₂O₃(SO₄) and Te(OH)₃(SO₄)·H₃O, were successfully synthesized via the mild hydrothermal method. Te₂O₃SO₄ has a two-dimensional (2D) structure consisting of [Te₆O₁₂]_∞ layers as well as [SO₄] groups. Te(OH)₃(SO₄)·H₃O features a simple 0D structure made up of an isolated [TeO₃] pyramid and a [SO₄] tetrahedra. Both of them are phase-matching materials and show remarkable powder second harmonic generation (SHG) efficiencies about 6 and 3 times that of KH₂PO₄ (KDP), respectively, for Te₂O₃SO₄ and Te(OH)₃(SO₄)·H₃O. Especially for Te(OH)₃(SO₄)·H₃O, in addition to a large SHG response, it possesses a short UV cutoff edge (∼233 nm) as well as moderate birefringence (0.052@546.1 nm). Furthermore, theoretical calculations confirmed that their strong SHG effects are due to the synergistic effect of the [TeO₃] pyramid and [SO₄] tetrahedra.

Cs4 B4 O3 F10 : First Fluorooxoborate with [BF4 ] Involving Heteroanionic Units and Extremely Low Melting Point

Herein, a new congruently melting mixed-anion compound Cs₄ B₄ O₃ F₁₀ has been characterized as the first fluorooxoborate with [BF₄ ] involving heteroanionic units. Compound Cs₄ B₄ O₃ F₁₀ possesses two highly fluorinated anionic clusters and therefore its formula can be expressed as Cs₃ (B₃ O₃ F₆ ) ⋅ Cs(BF₄ ). The influence of [BF₄ ] units on micro-symmetry and structural evolution was discussed based on the parent compound. More importantly, Cs₄ B₄ O₃ F₁₀ shows the lowest melting point among all the available borates and thus sets a new record for such system. This work is of great significance to enrich and tailor the structure of borates using perfluorinated [BF₄ ] units.

Be2 (BO3 )(IO3 ): The First Anion-mixed Van der Waals Member in the KBe2 BO3 F2 Family with a Very Strong Second Harmonic Generation Response

To obtain new nonlinear optical (NLO) materials with a large second harmonic generation (SHG) effect has always been a great challenge. We have synthesized the first metal borate-iodate NLO crystal, Be₂ (BO₃ )(IO₃ ) (BBIO), by multicomponent modification of KBe₂ BO₃ F₂ (KBBF), in which the structural features of KBBF were maintained and (IO₃ )^- groups were connected to honeycomb [Be₂ BO₃ O₂ ]_∞ layers. As the first KBBF family member with mixed anionic groups, BBIO benefited from the synergistic effect of (IO₃ )^- , (BO₃ )^3- and (BeO₄ )^6- groups, and exhibited a very strong SHG response of ≈7.2×KH₂ PO₄ (KDP, @1064 nm) and a large birefringence (Δn) of 0.172 at 1064 nm. BBIO may, unexpectedly, optimize growth habits via van der Waals forces. This study presents borate-iodate as a new NLO material and it demonstrates opportunities in KBBF structural engineering.

Sr2Pb(BeB5O10)(BO3): An Excellent Ultraviolet Nonlinear-Optical Beryllium Borate by the Pb-Modified Construction of a Conjugated System and Lone-Pair Effect

The design of material by chemical and/or crystalline modification of a classic structure model benefits not only the optimized physical properties but also the controllability and efficiency. Herein, a new nonlinear-optical (NLO) beryllium borate crystal, Sr₂Pb(BeB₅O₁₀)(BO₃) (SPBBO), is successfully designed and synthesized by chemical and crystalline modification of the perovskite-like K₃B₆O₁₀Cl NLO crystal. SPBBO displays a 3D BeB₅O₁₀^3- open-framework structure composed of interconnecting BeB₅O₁₃ groups with filled cationic Sr/Pb and anionic BO₃ groups, which exhibits the striking enhancement of the second-harmonic-generation (SHG) response (8 × KDP) and birefringence (0.10) compared to the parent model. Replacement of K by Sr and Pb with a lone pair and replacement of Cl by conjugated BO₃ result in the synergistic conjugation of Pb with host BeB₅O₁₀^3- and filled BO₃ groups, contributing to the striking enhancement of the SHG and birefringence. Single-crystal measurements show that SPBBO has a short UV absorption edge of 280 nm with a wide energy band gap of 4.35 eV and an outstanding laser-induced resistant behavior with a remarkably high laser-induced damage threshold of 2100 MW cm^-2. The excellent properties indicate that the SPBBO crystal is a very promising UV NLO functional material.

Tin Chloride Sulfates A3Sn2(SO4)3-xCl1+2x (A = K, Rb, Cs; x = 0, 1) as Multifunctional Optical Materials

The series of alkali-metal tin chloride sulfates A₃Sn₂(SO₄)_3-xCl_1+2x (A = K, Rb, Cs; x = 0, 1), K₃Sn₂(SO₄)₃Cl, Rb₃Sn₂(SO₄)₂Cl₃, and Cs₃Sn₂(SO₄)₂Cl₃, were successfully synthesized through an improved mild hydrothermal method. Interestingly, in addition to the cation size effect, the structure-directing effect of anions induces different symmetries in the three title compounds, with K₃Sn₂(SO₄)₃Cl being noncentrosymmetric, while Rb₃Sn₂(SO₄)₂Cl₃ and Cs₃Sn₂(SO₄)₂Cl₃ are centrosymmetric. Powder second-harmonic generation (SHG) measurements indicate that K₃Sn₂(SO₄)₃Cl is a nonlinear optical material that is type I phase matchable with a weak SHG response (0.1× KDP). Photoluminescence tests reveal that the three title compounds emit strong greenish yellow, orange, and salmon light, respectively, under UV excitation, indicating that they are promising inorganic solid fluorescent materials. Simultaneously, a detailed structural analysis of all the known tin(II) halide sulfates has been performed, which will guide the systematic exploration of high-performance tin(II)-based functional materials in the future.

Discovery of First Magnesium Fluorooxoborate with Stable Fluorine Terminated Framework for Deep-UV Nonlinear Optical Application

The generated light can be tuned to cover almost the entire spectral range from deep-ultraviolet to terahertz wavelengths by utilizing the nonlinear optical crystals with a simple frequency doubling process. Among them, the discovery of novel candidates for the production of deep-ultraviolet light is by extension a great challenge toward realizing the vast potential. Actually, the availability for this process mainly depends on whether the critical performance can be well coexisted in one practical crystal. Herein, the first magnesium fluorooxoborate MgB₅ O₇ F₃ was synthesized as a new competitive candidate for deep-ultraviolet nonlinear optical application. It has a sufficiently large nonlinearity and a deep-ultraviolet phase matching wavelength, indicating that it holds great potential for the production of coherent light below 200 nm. The critical performance enhancement of MgB₅ O₇ F₃ when compared with its isomorphic phases was demonstrated and discussed. More importantly, we proposed that fluorooxoborate system with the general formula of MB₅ O₇ F₃ (M=divalent metal) possesses stable fluorine terminated framework, which makes them tend to retain their crystallized space groups unchanged.

π-Conjugated Trigonal Planar [C(NH2)3]+ Cationic Group: A Superior Functional Unit for Ultraviolet Nonlinear Optical Materials

Employing π-conjugated anionic groups in molecular construction has been proven to be an effective strategy to find superior ultraviolet (UV) nonlinear optical (NLO) crystals over the decades. Herein, unlike the traditional π-conjugated anionic groups, we identify that a π-conjugated cationic group, viz., [C(NH2)3]+, is also an excellent UV NLO-active functional group in theory. Furthermore, we identify a [C(NH2)3]+-containing compound, C(NH2)3ClO4, as a promising UV NLO candidate due to its short UV cutoff edge (200 nm), remarkable second-harmonic generation effect (∼3 × KDP), and moderate birefringence of 0.076@1064 nm. Additionally, C(NH2)3ClO4 has excellent ferroelectric properties and reversal of domains, which also enables it to produce ultraviolet coherent light as short as 200 nm by a quasi-phase matching technique with a periodically poling method. Our study may provide not only a promising UV NLO crystal but also a new π-conjugated functional unit, [C(NH2)3]+, which will open a path to finding new classes of high-performance UV NLO crystals.

Two Lead Halides with Strong SHG Response Obtained by the Isovalent Substitutions of Alkali Metal Cation and Halogen Anion

The substitution of alkali metal cation or halogen anion based on nonlinear crystals is an effective strategy to exploit new optical materials. The strategy has been successfully expanded to discover two new lead halides, Rb₃Pb₂(CH₃COO)₂X₅ (X = Br, Cl). The substitution of the Cs⁺ cation with a Rb⁺ cation can not only increase the local dipole moment of the distorted [PbBr₄O₂] polyhedron but also reduce the cell unit, resulting in a large net macroscopic polarization. Therefore, Rb₃Pb₂(CH₃COO)₂Br₅ possesses a strong second-harmonic generation (SHG) response (6 × KDP) and a large birefringence (0.18@1064 nm). Furthermore, by the substitution of the Br^- anion with a Cl^- anion, Rb₃Pb₂(CH₃COO)₂Cl₅ exhibits a high laser damage threshold (LDT, 84 × AgGaS₂) and a short UV cutoff edge of 287 nm, as well as moderate SHG response (3 × KDP) and birefringence (0.11@1064 nm). Detailed theory calculations elucidate the origin of the linear and nonlinear optical properties of these compounds.