Recent advances in rational structure design for nonlinear optical crystals: leveraging advantageous templates.

Nonlinear optical (NLO) crystals that can expand the spectral range of laser outputs have attracted significant attention for their optoelectronic applications. The research progress from the discovery of new single crystal structures to the realization of final device applications involves many key steps and is very time consuming and challenging. Consequently, exploring efficient design strategies to shorten the research period and accelerate the rational design of novel NLO materials has become imperative to address the pressing demand for advanced materials. The recent shift in paradigm toward exploring new NLO crystals involves significant progress from extensive "trial and error" methodologies to strategic approaches. This review proposes the concept of rational structure design for nonlinear optical crystals leveraging advantageous templates. It further discusses their optical characteristics, promising applications as second-order NLO materials, and the relationship between their structure and performance, and highlights urgent issues that need to be addressed in the field of NLO crystals in the future. The review aims to provide ideas and driving impetus to encourage researchers to achieve new breakthroughs in the next generation of NLO materials.

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 new A 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.

Two Hydroxyfluorooxoborates Achieving Deep-Ultraviolet Cutoff Edges and Moderate Birefringence by Assembling Multi-Anionic Groups.

Assembling multi-anionic groups is conducive to utilizing respective advantage to achieve the enhancement of optical performance. Two new hydroxyfluorooxoborates, Ama2-Rb2B3O3F4(OH) and K8Cs2B15O14(OH)7F20 ⋅ H2O with [B3O3F4(OH)] six-membered rings were synthesized for the first time. The title compounds exhibit short ultraviolet cutoff edges (<200 nm) and K8Cs2B15O14(OH)7F20 ⋅ H2O possesses a moderate experimental refractive index difference of 0.051@546 nm.

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.

Cd8(BO3)4SiO4: Metal Cation Inducing the Formation of Isolated [BO3] and [SiO4] Units in Borate Silicate.

The first compound of cadmium-borate silicate Cd8(BO3)4SiO4, crystallizing in space group P42/n (no. 86), has been successfully synthesized by the conventional high-temperature solution method and melts congruently. The zero-dimensional anionic groups of Cd8(BO3)4SiO4 are isolated [BO3] triangles and isolated [SiO4] tetrahedra which are filled in the framework formed by [CdO6] polyhedra. It has a moderate birefringence (Δn = 0.053 at 546 nm), which is measured by experiment and evaluated by first-principles calculations; meanwhile, the source of birefringence is revealed through the response electronic distribution anisotropy method. The UV-vis-NIR diffuse reflectance spectrum indicates that Cd8(BO3)4SiO4 possesses a wide optical transparency range, with a UV cutoff edge at about 254 nm. This work enriches the structure chemistry of borate silicates, and we discussed the possible methods for the exploration and synthesis of novel optical crystals possessing zero-dimensional anionic groups in the borate silicate system.

(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.

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.

Sulfate Derivatives with Heteroleptic Tetrahedra: New Deep-ultraviolet Birefringent Materials in which Weak Interactions Modulate Functional Module Ordering.

Deep-ultraviolet (UV) birefringent materials are urgently needed to facilitate light polarization in deep-UV lithography. Maximizing anisotropy by regulating the alignment of functional modules is essential for improving the linear optical performance of birefringent materials. In this work, we proposed a strategy to design deep-UV birefringent materials that achieve functional module ordering via weak interactions. Following this strategy, four compounds CN4H7SO3CF3, CN4H7SO3CH3, C(NH2)3SO3CH3, and C(NH2)3SO3CF3 were identified as high-performance candidates for deep-UV birefringent materials. The millimeter-sized crystals of CN4H7SO3CF3, CN4H7SO3CH3, and C(NH2)3SO3CH3 were grown, and the transmittance spectra show that their cutoff edges are below 200 nm. CN4H7SO3CF3 exhibits the largest birefringence (0.149 @ 546 nm, 0.395 @ 200 nm) in the deep-UV region among reported sulfates and sulfate derivatives. It reveals that the hydrogen bond can modulate the module ordering of the heteroleptic tetrahedra and planar π-conjugated cations, thus greatly enhancing the birefringence. Our study not only discovers new deep-UV birefringent materials but also provides an upgraded strategy for optimizing optical anisotropy to achieve efficient birefringence.

Structure-Prediction-Oriented Synthesis of Thiophosphates as Promising Infrared Nonlinear Optical Materials.

Oriented synthesis of functional materials is a focus of attention in material science. As one of the most important function materials, infrared nonlinear optical materials with large second harmonic generation effects and broad optical band gap are in urgent need. In this work, directed by the theoretical structure prediction, the first series of non-centrosymmetric (NCS) alkali-alkaline earth metal [PS4]-based thiophosphates LiCaPS4 (Ama2), NaCaPS4 (P21), KCaPS4 (Pna21), RbCaPS4 (Pna21), CsCaPS4 (Pna21) were successfully synthesized. Comprehensive characterizations reveal that ACaPS4 could be regarded as promising IR NLO materials, exhibiting wide band gap (3.77-3.86 eV), moderate birefringence (0.027-0.064 at 1064 nm), high laser-induced damage threshold (LIDT, ~10×AGS), and suitable phase-matching second harmonic generation responses (0.4-0.6×AGS). Structure-properties analyses illustrate that the Ca-S bonds show non-ignorable covalent feature, and [PS4] together with [CaSn] units play dominant roles to determine the band gap and SHG response. This work indicates that Li-, Na- and K- analogs may be promising infrared nonlinear optical material candidates, and this is the first successful case of "prediction to synthesis" involving infrared (IR) nonlinear optical (NLO) crystals in the thiophosphate system and may provide a new avenue to the design and oriented synthesis of high-performance function materials in the future.

Where do the Fluorine Atoms Go in Inorganic-Oxide Fluorinations? A Fluorooxoborate Illustration under Terahertz Light.

The substitution of fluorine atoms for oxygen atoms/hydroxyl groups has emerged as a promising strategy to enhance the physical and chemical properties of oxides/hydroxides in fluorine chemistry. However, distinguishing fluorine from oxygen/hydroxyl in the reaction products poses a significant challenge in existing characterization methods. In this study, we illustrate that terahertz (THz) spectroscopy provides a powerful tool for addressing this challenge. To this end, we investigated two fluorination reactions of boric acid, utilizing MHF2 (M=Na, C(NH2)3) as fluorine reagents. Through an interplay between THz spectroscopy and solid-state density functional theory, we have conclusively demonstrated that fluorine atoms exclusively bind with the sp3-boron but not with the sp2-boron in the reaction products of Na[B(OH)3][B3O3F2(OH)2] (NaBOFH) and [C(NH2)3]2B3O3F4OH (GBF2). Based on this evidence, we have proposed a reaction pathway for the fluorinations under investigation, a process previously hindered due to structural ambiguity. This work represents a step forward in gaining a deeper understanding of the precise structures and reaction mechanisms involved in the fluorination of oxides/hydroxides, illuminated by the insights provided by THz spectroscopy.

Breaking the Inherent Interarrangement of [B3O6] Clusters for Nonlinear Optics with Orbital Hybridization Enhancement.

The [B3O6] group as a prime functional unit provides borates with intrinsic properties that are modified by coordination to cations. Inherent [B3O6] cluster structures in borates exclusively made of them have a near-plane configuration, with more than 90% of them having a maximum dihedral angle of zero and the remaining ones being less than 13°. Although such an inherent configuration can produce considerable birefringence for good phase-matching ability, this is not conducive to obtaining high conversion efficiency and beam quality due to the walk-off effects in the nonlinear optical process. In this article, two new borate halides Ca2B3O6X (X = Cl and Br) were reported, in which the confinement effects of distorted halogen-centered secondary building blocks compress the existence space of [B3O6] primitives, resulting in the nonparallel arrangement between [B3O6] clusters in this series. Both compounds show large second harmonic generation effects, and more importantly, the broken inherent interarrangement of [B3O6] clusters makes them a moderate birefringence and small walk-off angle. Their moderate birefringence is due to the large angular alignment between [B3O6] clusters, resulting from the orbital hybridization between the Ca s and the O p orbitals of the terminal O atoms on [B3O6] clusters. Our model supports this viewpoint and offers guidelines for rearranging [B3O6] clusters' arrangements in borates.

NaRb6(B4O5(OH)4)3(BO2) Featuring Noncentrosymmetry, Chirality, and the Linear Anionic Group BO2.

Noncentrosymmetric (NCS) structures are of particular interest owing to their symmetry-dependent physical properties, e.g., pyroelectricity, ferroelectricity, piezoelectricity, and nonlinear optical (NLO) behavior. Among them, chiral materials exhibit polarization rotation and host topological properties. Borates often contribute to NCS and chiral structures via their triangular [BO3] and tetrahedral [BO4] units and their numerous superstructure motifs. However, no chiral compound with the linear [BO2] unit has been reported to date. Herein, an NCS and chiral mixed-alkali-metal borate, NaRb6(B4O5(OH)4)3(BO2), with a linear BO2- unit in the structure was synthesized and characterized. The structure features a combination of three types of basic building units (BBUs), [BO2], [BO3], and [BO4] with sp-, sp2-, and sp3-hybridization of boron atoms, respectively. It crystallizes in the trigonal space group R32 (No. 155), one of the 65 Sohncke space groups. Two enantiomers of NaRb6(B4O5(OH)4)3(BO2) were found, and their crystallographic relationships are discussed. These results not only expand the small family of NCS structures with the rare linear BO2- unit but also prompt recognition to the fact that NLO materials have generally overlooked the existence of two enantiomers in achiral Sohncke space groups.

Zn2 HgP2 S8 : A Wide Bandgap Hg-Based Infrared Nonlinear Optical Material with Large Second-Harmonic Generation Response.

Hg-based chalcogenides, as good candidates for the exploration of high-performance infrared (IR) nonlinear optical (NLO) materials, usually exhibit strong NLO effects, but narrow bandgaps. Herein, an unprecedented wide bandgap Hg-based IR NLO material Zn2 HgP2 S8 (ZHPS) with diamond-like structure is rationally designed and fabricated by a tetrahedron re-organization strategy with the aid of structure and property predictions. ZHPS exhibits a wide bandgap of 3.37 eV, which is the largest one among the reported Hg-based chalcogenide IR NLO materials and first breaks the 3.0 eV bandgap "wall" in this system, resulting in a high laser-induced damage threshold (LIDT) of ≈2.2 × AgGaS2 (AGS). Meanwhile, it shows a large NLO response (1.1 × AGS), achieving a good balance between bandgap (≥3.0 eV) and NLO effect (≥1 × AGS) for an excellent IR NLO material. DFT calculations uncover that, compared to normal [HgS4 ]n , highly distorted [HgS4 ]d tetrahedral units are conducive to generating wide bandgap, and the wide bandgap in ZHPS can be attributed to the strong s-p hybridization between Hg─S bonding in distorted [HgS4 ]d , which gives some insights into the design of Hg-based chalcogenides with excellent properties based on distorted [HgS4 ]d tetrahedra.

Finding a Deep-UV Borate BaZnB4 O8 with Edge-sharing [BO4 ] Tetrahedra and Strong Optical Anisotropy.

The polarization modulation of deep-UV light is an important process that incorporates functionality to selectively respond to light-mater interaction. Typically, optical anisotropy is foremost to the use efficiency of deep-UV birefringent crystals. Herein, a new congruently melting polyborate with extremely large birefringence (Δn(001) =0.14@589.3 nm) and band gap (6.89 eV) is discovered as a high performance birefringent crystal, which breaks the current deadlock of deep-UV polyborates that usually show small birefringence. The rigid tetrahedra, including [ZnO4 ] and edge-sharing [BO4 ] tetrahedra, make all the planar [BO3 ] triangles in the lattice adopt preferential arrangement and thereby lead to an extraordinary large birefringence that is larger than all the deep-UV borates with experimentally measured values. Structural analyses with the additional theoretical calculations were used to study the origin of strong optical anisotropy in BaZnB4 O8 .

Stereochemically Active Tin(II)-Induced Enhancement of Birefringence in SnII SnIV (PO4 )2 and SrSn(PO4 )PO2 (OH)2.

Two new tin(II) phosphates, SnII SnIV (PO4 )2 and SrSn(PO4 )PO2 (OH)2 , were synthesized by the high-temperature solution method and hydrothermal method, respectively. Theoretical study indicates that by introducing tin(II) with stereochemical activity lone pairs (SCALP) in metal phosphates, the birefringence was enhanced, 0.048@1064 nm for SnII SnIV (PO4 )2 and 0.080@1064 nm for SrSn(PO4 )PO2 (OH)2 .

Rational Design of the First Ammonium Magnesium Borate with Deep-Ultraviolet Cutoff Edge and Moderate Birefringence and Further Investigation into the Nature of Ammonium in the Borate System.

Through the rational design of the experimental method, the first combination of ammonium and magnesium in the borate system was successfully achieved. In this paper, a case of ammonium magnesium borate, (NH4)2{Mg(H2O)2[B6O7(OH)6]2}·2H2O, was successfully synthesized by a mild hydrothermal method at a relatively low temperature. A brief review was performed to show the participation of NH4+ in the recent development of optical materials. By discussing the optimum synthesis method of ammonium-containing borates and the main factors affecting the dimensionality of B-O anionic groups in their structures, the design strategy for synthesizing ammonium-containing borate and adjusting its structure has been put forward. Relevant experimental measurement results and the first-principles calculation results show that the title compound has a deep-UV cutoff edge (<200 nm) and moderate birefringence (Δncal. = 0.064 @546 nm), which indicates its potential application in the deep-UV region.

Two Ultraviolet Optical Crystals K6B12O19F4 and K12B28O48: The Effects of Metal Cations Size and the F Ions on the Structure.

Borates and fluorooxoborates are advanced optical materials with great potential applications in the ultraviolet (UV) and deep ultraviolet (DUV) regions. Herein, two new UV optical crystals K6B12O19F4 and K12B28O48 were synthesized. Among them, K6B12O19F4 has a rare disorder of BO3 and BO4 units, which is the first time this disordered type has been found in fluorooxoborates. In this paper, the properties of K6B12O19F4 and K12B28O48 were tested and calculated, and their crystal structures and structural evolution were carefully analyzed. In addition, the effects of metal cations size and F ions on the crystal structure were analyzed. This research enriches the structural chemistry of borates and fluorooxoborates, and provides experience for the design of new UV optical crystals.

Borates: A Rich Source for Optical Materials.

The primary goal of this review is to present a clear chemical perspective of borates in order to stimulate and facilitate the discovery of new borate-based optical materials. These materials, which exhibit structures as varied as they are complex, are needed to meet the urgent technological milestones. In the current period of rapid sociotechnological breakthroughs, the need for the rational design and discovery of novel borates with superior performance is greater than ever before. Through the sustained efforts of chemists and material scientists, more than 3900 boron-containing compounds, including borate minerals and synthetic borates, have been documented in the scientific literature. This review provides a survey of all the reported anhydrous borates and an analysis of their complex structural chemistry. State-of-the-art progress related to technological advances in borate-based nonlinear optical, birefringent, and self-frequency-doubling materials is surveyed, with special emphasis on the relationships between structural architectures and optical properties. More importantly, this review serves both as a scientific introduction for graduates and post-doctoral researchers to the chemical richness of solid-state borates and as a comprehensive reference for researchers interested in borate-based optical materials.

Aluminoborates as Nonlinear Optical Materials.

Ultraviolet (UV) and Deep-UV (DUV, λ<200 nm) nonlinear optical (NLO) materials play a significant role to convert the wavelength of laser for producing UV/DUV coherent light in the laser industry. The Al [Ne]3s2 3p1 atom with empty d orbitals can form sp3 , sp3 d, and sp3 d2 hybrid orbitals when bonding with O/F atoms, which leads to the construction of [AlOm Fn ] (m+n=4, 5, 6) units. Therein, aluminoborates have received intensive attention due to the effective structural and functional adjustment effects of the diverse Al-O/F units. Accordingly, numerous aluminoborates as candidates for the next generation of UV/DUV NLO materials were discovered. In this review, recently discovered aluminoborates are overviewed and analyzed, including their syntheses, crystal structure features, and second-order NLO performances. We aim to provide the latest progress and outlook on the crystal chemistry of aluminoborates and boost the finding of the next NLO candidates with high performances.

Chain-like [Sx ] (x=2-6) Units Realizing Giant Birefringence with Transparency in the Near-Infrared for Optoelectronic Materials.

Birefringent crystals are requisite optical devices in laser and modern opto-electronic fields. Development of excellent birefringent materials is still challenging. Herein, the linear or chain-like [Sx ] (x=2-6) species were theoretically proved to be the origin of the large birefringence, and could be regarded as birefringent genes. Besides, the metal polysulfide family was first proposed to be rich birefringent materials source, among which Cs2 S6 realizes giant birefringence 0.58@1064 nm together with a wide band gap of 1.70 eV (based on the generalized gradient approximation). Moreover, the first dual-anion group polysulfide Na4 Ba3 (S2 )4 S3 was obtained, showing wide infrared transmission range (0.5-6.2 µm), wide band gap (2.3 eV), and large birefringence (0.37 at 1064 nm). This work provides a new guiding thought for exploring large birefringence crystals in the future.