Constructing Ultraviolet Nonlinear Optical Crystals with Superior Thermal Stability Based on Organic π-Conjugated [HCOO] Groups

In recent years, more and more organic π-conjugated moieties have been screened out to create excellent ultraviolet (UV) and deep-UV nonlinear optical (NLO) crystal materials. However, NLO crystals with organic groups usually exhibit poor thermal stability compared with those of traditional inorganic NLO materials. Herein, the organic group [HCOO], which is similar to traditional planar trigonal π-conjugated anionic groups, was employed to successfully build three UV NLO crystals RE(HCOO)3 (RE = Eu, Gd, and Dy) and two centrosymmetric compounds RE(HCOO)2(OH) (RE = Eu, and Gd) with superior thermal stability. Their structures and properties were further studied and characterized, especially the structure evolutions between noncentrosymmetric RE(HCOO)3 (RE = Eu, Gd, and Dy) and centrosymmetric RE(HCOO)2(OH) (RE = Eu, and Gd), as well as the NLO properties of the three crystals RE(HCOO)3 (RE = Eu, Gd, and Dy).

LiNa2Ca8B12O24F6Cl and Li1.2Na2.8B6O11: A Case of Cation-Induced Birefringence Enhancement via Dimensional Changes of Highly Polymerized [B12O24] Motifs

Borates, due to their structural chemistry diversity and exceptional performance, are premier material systems for investigating UV optical crystals. The B-O anionic groups with high polymerization (B ≥ 6) are much less in the borate-based system, which is worthy of further research. Herein, cations with different radii and proportions are introduced to borate system, and two new highly polymerized borates, LiNa2Ca8B12O24F6Cl (LNCBFC) and Li1.2Na2.8B6O11 (LNBO) were designed and synthesized successfully. LNCBFC possesses commonly isolated high-symmetry [B12O24] groups, while the structure of LNBO contains an unprecedented 1∞[B12O22] chain constructed by [B12O24] groups. Owing to the orientation of the functional motifs in the chain structure, LNBO displays an enhanced birefringence, which is about 25 × higher than that of LNCBFC and retains a short UV cutoff edge (< 200 nm). Even more significantly, a discussion of the cationic modulation of [B12O24]-based compounds and the patterns of isolated [BnO2n] motifs consisting of B-O rings was carried out by reviewing previous studies and existing borates. This work puts forward a decent structure design and property regulation strategy for highly polymerized borates.

A Rare-Earth-Based Borate Optical Crystal with Enlarged Dihedral Angles of Distinct [BO3] Groups Exhibiting a Wide UV Transparent Range

Borates, as advanced optical materials, have garnered wide interest due to their diverse structural configurations and great potential for applications in the ultraviolet (UV) regions. Herein, we synthesized a new rare-earth borate crystal, namely, K2NaYB2O6, which is classified as one of the ABReB2O6 compounds, where A and B represent alkali metal and Re denotes rare-earth metal. K2NaYB2O6 adopts in the monoclinic space group P21/c (No. 14), showcasing a three-dimensional (3D) framework composed of a planar triangular configuration of [BO3] units and distortive [YO7] polyhedra. Notably, both dihedral angles between distinct [BO3] units reach 79.6°, which represents an unprecedented structural feature in monoclinic ABReB2O6-type crystals. Moreover, the compound has a short UV absorption edge at around 204 nm, corresponding to a wide band gap of approximately 5.67 eV. Additionally, it possesses a moderate birefringence of 0.028 at 1064 nm. Further analysis utilizing theoretical calculations suggests that the optical behaviors of K2NaYB2O6 are mainly governed by its basic structural unit [BO3] triangles and distorted [YO7] polyhedra. These findings enrich the structure chemistry of rare-earth borates and offer valuable insights for the design of optical crystals in the UV wavelength range.

Bi(SO4)F·H2O and Bi(SO4)(NO3)·3H2O: Chemical Substitution-Induced Birefringence Enhancement in Bismuth Sulfates

Two new Bi(III)-based sulfates, namely, Bi(SO4)F·H2O (BSOF) and Bi(SO4)(NO3)·3H2O (BSNO), have been successfully synthesized through aliovalent replacement of partial [SO4]2- groups with F- and [NO3]- anions, respectively, in the parent structure of Bi2(SO4)3. Such chemical replacement altered the coordination environment of Bi3+ cations, facilitating changes in the structure and optical properties. Notably, the birefringence values of BSOF and BSNO are found to be 4.4 and 15.5 times that of parent Bi2(SO4)3. Further investigation into the structure-property relationship revealed that the birefringence enhancement in BSOF and BSNO is attributed to the improvement of the polarizability anisotropy of Bi3+-centered polyhedra in BSOF and BSNO compared to that of Bi2(SO4)3. In addition, the existence and optimized arrangement of planar [NO3]- groups are also indispensable for further birefringence improvement of the BSNO compound.

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.

Nonlinear Optical Effects of Hybrid Antimony(III) Halides Induced by Stereoactive 5s2 Lone Pairs and Trimethylammonium Cations

Organic-inorganic hybrid metal halides have unique optical and electronic properties, which are advantageous in the study of nonlinear optical materials. To investigate the effect of stereoactive lone pair electrons and the induction of organic cations on the structure of hybrid antimony(III) halides on nonlinear optics, we synthesize two noncentrosymmetric hybrid antimony(III)-based halide single crystals (TMA)3Sb2X9 (TMA = NH(CH3)3+, X = Cl, Br) by a room-temperature slow evaporation method, and their single-crystal structures, phase transition, X-ray photoelectron spectroscopy, and energy-band structure calculations are studied. More importantly, second-harmonic generation results of (TMA)3Sb2X9 (X = Cl, Br) are about 0.7 and 0.8 × KH2PO4(KDP), respectively. Interestingly, (TMA)3Sb2Cl9 single crystals undergo a reversible structural transition from Pc (No. 7) at room temperature to P21/c (No. 14) at 400 K, while the (TMA)3Sb2Br9 single crystals belong to the noncentrosymmetric space group R3c (No. 161), which clarifies the previous results. This work not only deepens the understanding of the role in lone pair electrons and organic cations in the structural induction in antimony-based halide perovskite materials but also provides guidance for subsequent nonlinear optical explorations.

A Sharp Improvement of Sulfate's Birefringence Induced by the Synergistic Effect of Heteroleptic and Dimeric Strategies

As for tetrahedron-based ultraviolet nonlinear optical crystals, it is so difficult to achieve a sufficient birefringence to satisfy the phase-matching condition. Thereover, it is necessary to greatly increase the polarizability anisotropy of tetrahedra. Meanwhile, the tetrahedra should be arranged as uniformly as possible. Based on these ideas, a new strategy that dimerizes the heteroleptic tetrahedra with heteroleptic atoms as bridges is proposed and implemented, resulting in a new compound, Na2S3O6, which crystallizes in a centrosymmetric monoclinic phase (phase-I) and a non-centrosymmetric orthorhombic one (phase-II). In the phase-I, a large birefringence of 0.103 @ 546 nm comparable to those of many triangle-based crystals is confirmed experimentally, indicating the ability of [S3O6] group in improving the birefringence. Besides, the phase-II exhibits an improved birefringence of 0.056 @ 546 nm and a moderate second harmonic generation response of 1.4 × KH2PO4. In particular, the phase-II exhibits a typical phase-matching behavior under the irradiation of 532 nm laser. Moreover, the excellent optical properties of [S3O6] group are further verified by theoretical calculations. These results completely illustrate the validity of the design strategy. Therefore, this work lights a new route for exploring novel ultraviolet nonlinear optical crystals.

AIB3 IIC2 IIIQ6 VIXVII: A Thioborate Halide Family for Developing Wide Bandgap Infrared Nonlinear Materials by Coupling Planar [BS3] and Polycations

Developing high-performance infrared (IR) nonlinear optical (NLO) materials is urgent but challenging due to the competition between NLO coefficient and bandgap in one compound. Herein, by coupling NLO-active [BS3] planar units and halide-centered polycations, six new metal thioborate halides ABa3B2S6X (A = Rb, Cs; X = Cl, Br, I) composed of zero-dimensional [XBamRbn/Csn] polycations and [BS3] units, belonging to a newA I B 3 II C 2 III Q 6 VI X VII ${\mathrm{A}}^{\mathrm{I}}{\mathrm{B}}_{3}^{\mathrm{II}}{\mathrm{C}}_{2}^{\mathrm{III}}{\mathrm{Q}}_{6}^{\mathrm{VI}}{\mathrm{X}}^{\mathrm{VII}}$ family, are rationally designed and fabricated. The compounds show an interesting structural transition from Pbcn (ABa3B2S6Cl) to Cmc21 (ABa3B2S6Br and ABa3B2S6I) driven by the clamping effect of polycationic frameworks. ABa3B2S6Br and ABa3B2S6I are the first series metal thioborate halide IR NLO materials, and the introduction of [BS3] unit effectively widens the bandgap of planar unit-constructed chalcogenides. ABa3B2S6Br and ABa3B2S6I, exhibiting wide bandgaps (3.55-3.60 eV), high laser-induced damage thresholds (≈ 6 × AgGaS2), and strong SHG effects (0.5-0.6 × AgGaS2) with phase-matching behaviors, are the promising IR NLO candidates for high-power laser applications. The results enrich the chemical and structural diversity of boron chemistry and give some insights into the design of new IR NLO materials with planar units.

Extending Unique 1D Double-Chains to 2D Layers with Birefringent Gain by Introducing a Hydroxyl Group

It remains a significant hurdle for discovering birefringent materials in the deep ultraviolet (DUV, λ < 200 nm). It is well-known that the OH anions are recognized for their capability to eliminate the dangling bonds from terminal oxygen atoms, promoting the ultraviolet (UV) cutoff edge blueshift and regulating the crystal structure. Here, two new barium hydroxyborates, Ba3B11O18(OH)3(H2O) (BaBOH) and Na2BaB10O16(OH)2(H2O)2 (NaBaBOH), were designed and synthesized while displaying different dimensions. Remarkably, BaBOH presents novel one-dimensional (1D) [B22O37(OH)6]∞ double-chains formed by a new fundamental building block (FBB)[B11O21(OH)3]. NaBaBOH possesses a 2D [B10O16(OH)2]∞ layer with a less common FBB [B10O19(OH)2]. They enrich the structural diversity of hydroxyborates. Moreover, NaBaBOH exhibits a broad transparent window within the DUV spectral range (<190 nm) and possesses a favorable birefringence of 0.064. Furthermore, detailed summaries and structural comparisons have been implemented for all hydroxyborates containing alkali and alkaline-earth metals. This reveals that the OH group modulation strategy can be appropriately employed for the structural design.

Two Short-Wave UV Antimony(III) Sulfates Exhibiting Large Birefringence

In the present work, we have successfully obtained two new UV antimony-based sulfates, NH4Sb(SO4)2 and Ca2Sb2O(SO4)4, by a conventional hydrothermal method. Interestingly, both compounds share similar structural building blocks, such as SbO4 seesaws and SO4 tetrahedra, yet they endow discrepant birefringence values measured at 546 nm with values of 0.150 and 0.114, respectively, owing to the different distortions of the SbO4 groups with SCALP electrons. Moreover, both compounds display large band gaps (4.32 and 4.43 eV, respectively), so they can be used as short-wavelength UV birefringent materials. Moreover, NH4Sb(SO4)2 is a noncentrosymmetric compound, showing a frequency doubling effect of 0.2 × KDP. Detailed structural analyses and calculations confirm the source of superior optical performance and the reasons for the different birefringence of the two compounds. This work provides ideas for the following discovery of antimony-based optical materials with excellent properties.

Ca2 Ln(BS3 )(SiS4 ) (Ln = La, Ce, and Gd): Mixed Metal Thioborate-Thiosilicates as Well-Performed Infrared Nonlinear Optical Materials

The first examples of thioborate-thiosilicates, namely Ca2 Ln(BS3 )(SiS4 ) (Ln = La, Ce, and Gd), are synthesized by rationally designed high-temperature solid-state reactions. They crystalize in the polar space group P63 mc and feature a novel three-dimensional crystal structure in which the discrete [BS3 ]3- and [SiS4 ]4- anionic groups are linked by Ca2+ and Ln3+ cations occupying the same atomic site. Remarkably, all three compounds show comprehensive properties required as promising infrared nonlinear optical materials, including phase-matchable strong second harmonic generation (SHG) responses at 2.05 µm (1.1-1.2 times that of AgGaS2 ), high laser-induced damage thresholds (7-10 times that of AgGaS2 ), wide light transmission range (0.45-11 µm), high thermal stabilities (>800 °C), and large calculated birefringence (0.126-0.149 @1064 nm), which justify the material design strategy of combining [BS3 ]3- and [SiS4 ]4- active units. Theoretical calculations suggest that their large SHG effects originate mainly from the synergy effects of the LnS6 , BS3 , and SiS4 groups. This work not only broadens the scope of research on metal chalcogenides but also provides a new synthetic route for mixed anionic thioborates.

(C(NH2)3)2(I2O5F)(IO3)(H2O) and C(NH2)3IO2F2: Two Guanidine Fluorooxoiodates with Wide Band Gap and Large Birefringence

Enhancing anisotropy through an effective synergistic arrangement of anionic and cationic groups is crucial for improving the birefringence optical properties of materials. In this work, by transforming I-O into I-F through the fluorination strategy, two metal-free guanidine fluorooxoiodates (C(NH2)3)2(I2O5F)(IO3)(H2O) and C(NH2)3IO2F2 and one guanidine iodate C(NH2)3IO3 were successfully synthesized using the hydrothermal method. An unprecedented dimer [I2O5F] formed by [IO3F] and [IO3] in (C(NH2)3)2(I2O5F)(IO3)(H2O) was found, which greatly enriches the structural diversity of fluorooxoiodates. All three compounds feature a relatively large birefringence (Δn = 0.068, 0.110 and 0.075 at 546 nm) and a short ultraviolet cutoff edge. The theoretical calculation was carried out to understand the electronic structures and linear optical properties.

Mg(C3 O4 H2 )(H2 O)2 : A New Ultraviolet Nonlinear Optical Material Derived from KBe2 BO3 F2 with High Performance and Excellent Water-Resistance

Acquiring high-performance ultraviolet (UV) nonlinear optical (NLO) materials that simultaneously exhibit a strong second harmonic generation (SHG) coefficients, as short as possible SHG phase-matching (PM) wavelength and non-hygroscopic properties has consistently posed a significant challenge. Herein, through multicomponent modification of KBe2 BO3 F2 (KBBF), an excellent UV NLO crystal, Mg(C3 O4 H2 )(H2 O)2 , was successfully synthesized in malonic system. This material possesses a unique 2D NLO-favorable electroneutral [Mg(C3 O4 H2 )3 (H2 O)2 ]∞ layer, resulting in the rare coexistence of a strong SHG response of 3×KDP (@1064 nm) and short PM wavelength of 200 nm. More importantly, it exhibits exceptional water resistance, which is rare among ionic organic NLO crystals. Theoretical calculations revealed that its excellent water-resistant may be originated from its small available cavity volumes, which is similar to the famous LiB3 O5 (LBO). Therefore, excellent NLO properties and stability against air and moisture indicate it should be a promising UV NLO crystal.

A2Hgx(SeO3)y (A = K, Rb, Cs): Three Alkali Metal Mercury Selenites Featuring Unique 1D [HgOm(SeO3)n]∞ Chains

Herein, three alkali metal mercury selenites, K2Hg2(SeO3)3, Rb2Hg2(SeO3)3, and Cs2Hg3(SeO3)4, were successfully obtained by a hydrothermal method. The three compounds featured same one-dimensional (1D) [HgOm(SeO3)n]∞ chain structure that consisting of distorted Hg-O polyhedra and SeO3 triangular pyramids with stereochemically active lone pair (SCALP) electrons. Interestingly, the rich coordination environment of Hg atoms and the size difference of alkali metal cations lead to diverse arrangement of SeO3 groups, which makes them exhibit different birefringence. The band gaps of the three compounds indicate that they are potential ultraviolet (UV) optical materials. Detailed theoretical calculations demonstrate that the combined effects of SeO3 triangular pyramids and Hg-O polyhedra are responsible for the optical characteristics of the reported compounds.

Strategy for a Rational Design of Deep-Ultraviolet Nonlinear Optical Materials from Zeolites

Deep-ultraviolet (DUV) nonlinear optical (NLO) materials play a crucial role in cutting-edge laser technology. In order to solve the serious layered growth tendency of the sole commercial DUV NLO crystal KBe2BO3F2 (KBBF), developing alternative systems of compounds with bulk crystal habits has become an urgent task for practical applications. Herein, a novel strategy was developed by applying non-centrosymmetric (NCS) cancrinite (CAN)-type zincophosphates {Na6(OH)2(H2O)2}Cs2[ZnPO4]6 with bulk-crystal habits as the prototype to design new DUV NLO crystals. Two new anhydrous alkali zincophosphates, namely, {(Li6 -xNaxO)A2}[(ZnPO4)6] (A = Cs, Rb; x = 2-3) crystallizing in the NCS hexagonal space group P63 (no. 173) with a CAN-type framework, were successfully synthesized via a modified fluoro-solvo-hydrothermal method by applying triethylamine (TEA) and concentrated NaF solution as a co-solvent. Interestingly, the rigidity of the NCS CAN-type framework acting as the host ensures the non-centrosymmetry of the resulting new compounds. Meanwhile, the replacement of water molecules by guest cationic species in the channels or cages can greatly improve the thermal stability of the resultant crystal and tune its NLO properties. The synergetic effect of the host framework and the guest species makes the two compounds transparent down to the DUV region (<200 nm) and exhibit SHG effects. Therefore, the proposed rational design strategy of applying the known zeotype NCS frameworks as prototypes together with the modified fluoro-solvo-hydrothermal method opens a great avenue for highly effectively exploring new DUV NLO materials.

d6versus d10, Which Is Better for Second Harmonic Generation Susceptibility? A Case Study of K2TGe3Ch8 (T = Fe, Cd; Ch = S, Se)

Two acentric chalcogenide compounds, K₂CdGe₃S₈ and K₂CdGe₃Se₈, were synthesized via conventional high-temperature solid-state reactions. The crystal structures of K₂CdGe₃S₈ and K₂CdGe₃Se₈ were accurately determined by single-crystal X-ray diffraction and crystallize in the K₂FeGe₃S₈ structure type. K₂CdGe₃S₈ is isostructural to K₂FeGe₃S₈ with superior nonlinear optical properties. For the second harmonic generation (SHG) response, K₂CdGe₃S₈ is 18× K₂FeGe₃S₈ for samples of particle size of 38-55 μm. The superior nonlinear optical properties of K₂CdGe₃S₈ over K₂FeGe₃S₈ are mainly contributed by the chemical characteristics of Cd compared with Fe, which are elucidated by nonlinear optical property measurements, electronic structure calculations, and density functional theory calculations. The [CdS₄] tetrahedra within K₂CdGe₃S₈ exhibit a higher degree of distortion and larger volume compared to the [FeS₄] tetrahedra in K₂FeGe₃S₈. This study possesses a good platform to investigate how d-block elements contribute to the SHG response. The fully occupied d¹⁰-elements are better for SHG susceptibility than d⁶-elements in this study. K₂CdGe₃S₈ is a good candidate as an infrared nonlinear optical material of high SHG response (2.1× AgGaS₂, samples of particle size of 200-250 μm), type-I phase-matching capability, high laser damage threshold (6.2× AgGaS₂), and good stability.

M(NH2SO3)2·xH2O (M = Ca, Pb, x = 0, 1, 4): Effect of Hydrogen Bonding on Structural Transformations and Second Harmonic Generation of Metal Sulfamates

Nonlinear optical (NLO) crystals are very important for laser technology, but the performances of available NLO crystals are still insufficient for increasing demand. Recently, the exploration of new NLO crystals in non-π-conjugated systems with the heteroatomic tetrahedra is attracting a lot of interest. In this work, we systematically explore the metal sulfamates containing [NH₂SO₃] groups and four metal sulfamates, namely, Ca(NH₂SO₃)₂·4H₂O, Ca(NH₂SO₃)₂·H₂O, Pb(NH₂SO₃)₂·H₂O, and Pb(NH₂SO₃)₂ were synthesized by aqueous solution and hydrothermal methods. Notably, these metal sulfamates exhibit different crystal structures and optical properties owing to the diverse arrangement of the functional groups in their structures. In addition, due to hydrogen bond regulation, the centrosymmetric (CS) compound Ca(NH₂SO₃)₂·4H₂O can transform into noncentrosymmetric (NCS) Ca(NH₂SO₃)₂·H₂O, leading to NLO activity. Experimental characterizations and theoretical analysis reveal that these metal sulfamates are ultraviolet transparent and suitable for developing new NLO materials.

Postsynthetic Modification of NU-1000 for Designing a Polyoxometalate-Containing Nanocomposite with Enhanced Third-Order Nonlinear Optical Performance

For the advancement of laser technologies and optical engineering, various types of new inorganic and organic materials are emerging. Metal-organic frameworks (MOFs) reveal a promising use in nonlinear optics, given the presence of organic linkers, metal cluster nodes, and possible delocalization of π-electron systems. These properties can be further enhanced by the inclusion of solely inorganic materials such as polyoxometalates as prospective low-cost electron-acceptor species. In this study, a novel hybrid nanocomposite, namely, SiW₁₂@NU-1000 composed of SiW₁₂ (H₄SiW₁₂O₄₀) and Zr-based MOF (NU-1000), was assembled, completely characterized, and thoroughly investigated in terms of its nonlinear optical (NLO) performance. The third-order NLO behavior of the developed system was assessed by Z-scan measurements using a 532 nm laser. The effect of two-photon absorption and self-focusing was significant in both NU-1000 and SiW₁₂@NU-1000. Experimental studies suggested a much superior NLO performance of SiW₁₂@NU-1000 if compared to that of NU-1000, which can be assigned to the charge-energy transfer between SiW₁₂ and NU-1000. Negligible light scattering, good stability, and facile postsynthetic fabrication method can promote the applicability of the SiW₁₂@NU-1000 nanocomposite for various optoelectronic purposes. This research may thus open new horizons to improve and enhance the NLO performance of MOF-based materials through π-electron delocalization and compositing metal-organic networks with inorganic molecules as electron acceptors, paving the way for the generation of novel types of hybrid materials for prospective NLO applications.

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.

(N2H6)[HPO3F]2: maximizing the optical anisotropy of deep-ultraviolet fluorophosphates

Although phosphates are a rich source of deep-ultraviolet optical materials, the realization of large optical anisotropy in them still remains a challenge because of the small polarizability anisotropy of [PO₄] units. Inspired by the fluoridation strategy and hydrogen bond interaction, a new metal-free monofluorophosphate, (N₂H₆)[HPO₃F]₂, was synthesized, which exhibits a large birefringence (cal. 0.077) and wide band gap (∼6.51 eV). Such a large birefringence in (N₂H₆)[HPO₃F]₂ sets a new record among available fluorophosphates, and the [HPO₃F] unit is theoretically confirmed to be a new birefringence-active unit.

CsY(SO4)2·4H2O: A Deep-Ultraviolet Birefringent Crystal Induced by an Edge-Sharing Connection

SO₄ tetrahedral groups have weak polarization anisotropy, which thus results in the small birefringence of sulfates. Here, we report new sulfate CsY(SO₄)₂·4H₂O with unprecedented birefringence among deep-ultraviolet (deep-UV) sulfates. Its single crystal (10 mm × 3.5 mm × 1.5 mm) was simply grown by an aqueous solution evaporation technique, and it features a rare layered structure composed of YO₉ polyhedra, SO₄ tetrahedra, and H₂O molecules. Interestingly, each SO₄ group donates two oxygen atoms to edge-share with one adjacent YO₉ polyhedron and thus causes severe distortion of these groups. The characteristic edge-sharing mode gives CsY(SO₄)₂·4H₂O a large birefringence of ∼0.045@546 nm, which is the maximum among deep-UV sulfates and phosphates with similar non-π-conjugated anionic groups. The ultraviolet-visible-near-infrared diffuse reflection and transmission spectra, infrared spectrum, thermal stability, and theoretical calculations are also presented. The fascinating results will improve our understanding of sulfates and may provide useful insights into the exploration of deep-UV sulfates with large birefringence.

Ba6(CuxZy)Sn4S16 (Z = Mg, Mn, Zn, Cd, In, Bi, Sn): High Chemical Flexibility Resulting in Good Nonlinear-Optical Properties

Seven acentric sulfides Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi, Sn) were grown by a high-temperature salt flux method. The crystal structures of the Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi, Sn) compounds were determined by single-crystal X-ray diffraction with the aid of solid-state NMR spectroscopy. The Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi) compounds are isostructural and crystallize in the Ba₆Ag₄Sn₄S₁₆ structure type. The Sn-containing compound exhibits high structural similarity to Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi) with the presence of an interstitial atomic position partially occupied by Sn atoms. The chemical bonding characteristics of Ba₆(Cu_2.9Sn_0.4)Sn₄S₁₆ were understood with electron localization function calculations coupled with crystal orbital Hamilton population calculations. The Ba-S and Cu-S interactions are dominantly ionic, but the Sn-S interactions consist of strong covalent bonding characteristics in Ba₆(Cu_2.9Sn_0.4)Sn₄S₁₆. The monovalent Cu atoms, mixed with certain metals with various oxidation states, significantly shift the optical properties of the Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi) compounds. This results in a good balance between the second-harmonic-generation (SHG) response and laser damage threshold (LDT). Ba₆(Cu_1.9Zn_1.1)Sn₄S₁₆ possesses a high SHG response and a high LDT of 2.8 × AGS and 3 × AGS, respectively. A density functional theory calculation revealed that CuS₄ and SnS₄ tetrahedra significantly contribute to the SHG response in Ba₆(Cu₂Mg)Sn₄S₁₆, which also confirmed that CuS₄ tetrahedra are crucial for the stability and optical properties of the Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi, Sn) compounds revealed by electronic structure analysis.

Na3B6O10(HCOO): an ultraviolet nonlinear optical sodium borate-formate

A new sodium borate-formate, which displays a pcu-type open framework and ultraviolet nonlinear optical properties, has been synthesized under surfactant-thermal conditions.

(C3N6H7)2SiF6·H2O: an ultraviolet birefringent crystal exceeding the intrinsic energy gap of an organic reagent

A new organic–inorganic hybrid compound (C3N6H7)2SiF6·H2O, in which [C3N6H7]+ groups are uniformly arranged under the regulation of SiF6 octahedra, shows highly polarization anisotropy and a breakthrough in the energy gap.

Performance of optical materials with derivative planar π-conjugated groups: Recent advances and future prospects

Planar π-conjugated groups which possess not only large hyperpolarizability but also optical anisotropy are proven to be a good functional motif for optical materials with outstanding nonlinear optics and/or birefringence....

A2P2S6 (A=Ba, Pb): A Good Platform to Study Polymorph Effect and Lone Pairs Effect to Form Acentric Structure

Three ternary thiophosphates α-Ba2P2S6, β-Ba2P2S6, and Pb2P2S6, were synthesized via a high temperature salt flux method or an I2 transport reaction. β-Ba2P2S6 and Pb2P2S6 were previously structurally characterized without investigating...

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,...

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

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

PbBe2B2O6: an ultraviolet nonlinear-optical crystal with unprecedented π–π interacting BeBO5 group

Ultraviolet crystal PbBe2B2O6 with an unprecedented π–π interacting BeBO5 group, has wide optical-transparency, whole-spectrum phase-matching, and large nonlinear-optical effects.

The Rise of Infrared Nonlinear Optical Pnictides: Advances and Outlooks

Infrared (IR) nonlinear optical (NLO) materials are the core devices to realize IR laser output, which are of vital importance in civilian and military fields. Non-centrosymmetric chalcogenide and pnictide compounds have already been widely accepted as favorable systems for IR-NLO materials. Compared to the extensively investigated IR-NLO chalcogenides during the past few decades, the research of non-centrosymmetric phosphides as IR-NLO materials is relatively scarce. In this frontier article, the recent progress of pnictides as emerging IR-NLO candidates has been highlighted based on the perspective of new crystal exploration. These IR-NLO pnictides recently reported were divided into three groups from binary to quaternary according to their chemical compositions. The synthetic methods, structural chemistry, and structure-activity relationships are analyzed and summarized in detail. Finally, current problems and the future development of this field are also proposed.

Chalcogen Bond Involving Zinc(II)/Cadmium(II) Carbonate and Its Enhancement by Spodium Bond

Carbonate MCO3 (M = Zn, Cd) can act as both Lewis acid and base to engage in a spodium bond with nitrogen-containing bases (HCN, NHCH2, and NH3) and a chalcogen bond with SeHX (X = F, Cl, OH, OCH3, NH2, and NHCH3), respectively. There is also a weak hydrogen bond in the chalcogen-bonded dyads. Both chalcogen and hydrogen bonds become stronger in the order of F > Cl > OH > OCH3 > NH2 > NHCH3. The chalcogen-bonded dyads are stabilized by a combination of electrostatic and charge transfer interactions. The interaction energy of chalcogen-bonded dyad is less than -10 kcal/mol at most cases. Furthermore, the chalcogen bond can be strengthened through coexistence with a spodium bond in N-base-MCO3-SeHX. The enhancement of chalcogen bond is primarily attributed to the charge transfer interaction. Additionally, the spodium bond is also enhanced by the chalcogen bond although the corresponding enhancing effect is small.

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.

Strain-Driven Polarized Electric Field-Promoted Photocatalytic Activity in Borate-Based CsCdBO3 Bulk Materials

Piezoelectrically polarized electric field can provide a strong driving force for the separation of the photoinduced charge carriers that has attracted a wide attention in the field of photocatalysis. In this paper, a new type of piezoelectric borate material CsCdBO₃ exhibits a high efficiency for the degradation of typical organic pollutants under the synergistic effects of strain and light conditions. The oxidation rate constant of the synergistic effect is 0.653 min^-1, which is 3.77 times that of just under visible light irradiation. Further, the material shows a higher efficiency when treated both under the clockwise stirring direction and a high stirring speed. A characteristic piezoresponse hysteresis loop was detected using the piezoresponse force microscopy (PFM) approach. The strain-driven polarized electric field facilitates to promote the photoinduced electron-hole pair separation, thus enhancing the photocatalytic activity. The present work provides a new direction of the borate with a noncentrosymmetric structure in the environmental remediation.

Centrosymmetric or Noncentrosymmetric? Transition Metals Talking in K2TGe3S8(T = Co, Fe)

Two new quaternary sulfides K₂TGe₃S₈(T = Co, Fe) have been synthesized by a high-temperature solid-state routine and flux growth method. The crystal growth process of K₂TGe₃S₈(T = Co, Fe) was elucidated by in situ powder X-ray diffraction and DSC thermal analysis. The millimeter-sized crystals of K₂TGe₃S₈(T = Co, Fe) were grown. K₂CoGe₃S₈ crystallizes in a new structure type in centrosymmetric space group P1 (no. 2) with unit cell parameters of a = 7.016(1) Å, b= 7.770(1) Å, c = 14.342(1) Å, α = 93.80(1)°, β = 92.65(1)°, γ = 114.04(1)°. K₂FeGe₃S₈ crystallizes in the K₂FeGe₃Se₈ structure type and the noncentrosymmetric space group P2₁ (no. 4) with unit cell parameters of a = 7.1089(5)Å, b = 11.8823(8) Å, c = 16.7588(11) Å, β = 96.604(2)°. There is a high structural similarity between K₂CoGe₃S₈ and K₂FeGe₃S₈. The larger volume coupled with higher degrees of distortion of the [FeS₄] tetrahedra compared to the [CoS₄] tetrahedra accounts for the structure's shift from centrosymmetric to noncentrosymmetric. The theory simulation confirms that [TS₄]_{T= Co or Fe} tetrahedra play a crucial role in controlling the structure and properties of K₂TGe₃S₈(T = Co, Fe). The measured optical bandgaps of K₂CoGe₃S₈ and K₂FeGe₃S₈ are 2.1(1) eV and 2.6(1) eV, respectively. K₂FeGe₃S₈ shows antiferromagnetic ordering at 24 K while no magnetic ordering was detected in K₂CoGe₃S₈. The magnetic measurements also demonstrate the divalent nature of transition metals in K₂TGe₃S₈(T = Co, Fe).

Recent progress in oxychalcogenides as IR nonlinear optical materials

This Frontiers article discusses the recent progress and challenges of non-centrosymmetric oxychalcogenides in the IR nonlinear optical field.

Synergism of multiple functional chromophores significantly enhancing the birefringence in layered non-centrosymmetric chalcohalides

Compared with one or two functional chromophores materials, Hg₃AsQ₄X (Q = S, Se; X = Cl, Br, I) with multiple ones generate extremely large birefringence due to the synergism of the d¹⁰ cation Hg²⁺, lone pair layer of As³⁺ and mixed anions Q²⁻/X⁻.

Ba2B7O12F with novel FBB [B7O16F] and deep-ultraviolet cut-off edge

A new fluorooxoborate Ba₂B₇O₁₂F features novel FBB [B₇O₁₆F] and two different sizes of channels in its anionic structure, which possesses a deep-ultraviolet cutoff edge of 180 nm and a large bandgap of 6.67 eV.

A3Mn2Sb3S8 (A = K and Rb): a new type of multifunctional infrared nonlinear optical material based on unique three-dimensional open frameworks

A new type of multifunctional IR-NLO material, A₃Mn₂Sb₃S₈ (A = K and Rb), with unique 3D open frameworks has been developed using a facile surfactant–thermal method.

Ba2Ge2Te5: a ternary NLO-active telluride with unusual one-dimensional helical chains and giant second harmonic-generation tensors

The linear-optical and NLO properties of a ternary NLO-active telluride, Ba2Ge2Te5, were investigated systematically at the experimental and theoretical levels for the first time.

BaTi(BO3)2: an excellent birefringent material with highly coplanar isolated [BO3] groups

The birefringence of BaTi(BO₃)₂ is equal to or larger than 0.169 at 546 nm (Δn_(cal.) = 0.185 at 546 nm), which mainly originates from the high-density and parallel arrangement of the isolated [BO₃] triangles in the unit cell.

Bi32Cd3P10O76: a new congruently melting nonlinear optical crystal with a large SHG response and a wide transparent region

Large size single crystal Bi₃₂Cd₃P₁₀O₇₆ with a large SHG response (4 × KDP) and a wide transmission range covering 0.36–4.9 μm.

Pb2.28Ba1.72B10O19 featuring a three-dimensional B–O anionic network with edge-sharing [BO4] obtained under ambient pressure

The unique borate Pb_2.28Ba_1.72B₁₀O₁₉ containing a three-dimensional B–O anionic network with edge-sharing [BO₄] was obtained under ambient pressure, featuring an extremely large B–B interatomic distance (d_B–B) and small internal O–B–O angles (∠_{in(O–B–O)}).