Zinc-based halide ultraviolet nonlinear optical crystal material as well as preparation method and application thereof

Abstract

The invention relates to a zinc-based halide ultraviolet nonlinear optical crystal material and a preparation method and application thereof, the chemical formula of the zinc-based halide ultraviolet nonlinear optical crystal material is one of the following two types: type A: (C5H6N2O2) (C5H5N2O2) ZnX, the crystal belongs to a monoclinic system, and the space group is Cc; the type B is (C5H6N2O2) ZnX2, the crystal belongs to a triclinic system, and the space group is P1; wherein C5H6N2O2 represents 2-(1-imidazolyl) acetic acid, C5H5N2O2 represents 2-(1-imidazolyl) acetate radical, and X represents Cl or Br. According to the zinc-based halide ultraviolet nonlinear optical crystal material, the problem that the birefringence of the UVNLO crystal introduced with the organic pi conjugate plane group is too large is solved, and the provided zinc-based halide ultraviolet nonlinear optical crystal material has a large SHG coefficient (0.4 * KDP, 3.5 * KDP and 8.2 * KDP), a short ultraviolet cut-off edge (band gaps are 5.20 eV, 5.19 eV and 5.05 eV respectively), optimal birefringence (delta n is 0.048, 0.063 and 0.071 respectively) and excellent thermal stability (250-300 DEG C).

Description

Technical Field

[0001] The present invention relates to the technical field of nonlinear optical materials, and in particular to a zinc-based halide ultraviolet nonlinear optical crystal material, a preparation method thereof, and applications thereof. Background Art

[0002] Ultraviolet nonlinear optical (UVNLO) crystals, as one of the core materials in modern optical technology, exhibit significant application potential in cutting-edge fields such as optical communications, laser processing, optical sensing, and quantum optics due to their excellent frequency conversion capabilities. However, currently reported UVNLO crystals generally face a key performance bottleneck: the difficulty in simultaneously achieving high second-harmonic generation (SHG) response (>1×KDP), a short UV cutoff edge (<280nm), and a suitable birefringence (Δn). Traditional approaches often achieve progress in one performance metric at the expense of others, severely limiting their comprehensive functionality and practical applications.

[0003] In recent years, organic π-conjugated planar groups have been widely considered as ideal functional units for UVNLO crystals due to their large microscopic second-order hyperpolarizability and strong optical anisotropy, which can significantly enhance the SHG response and birefringence. However, researchers often overlook a key issue: the parallel arrangement of these π-conjugated planar groups often leads to excessive birefringence (Δn > 0.11 @ 1064 nm). For UVNLO crystals, excessive birefringence leads to walk-off effects, reducing SHG conversion efficiency, hindering high-power output, and ultimately limiting practical applications. A typical example is the commercial UVNLO crystal BBO (β-BaB2O4), which has an SHG response five times higher than that of potassium dihydrogen phosphate (KDP) but suffers from output limitations due to excessive birefringence (Δn = 0.113 @ 1064 nm). Therefore, the rational introduction of organic π-conjugated planar groups to simultaneously achieve a large SHG coefficient, a short UV cutoff edge, and optimal birefringence (Δn = 0.05–0.1) is urgently needed, which is a key requirement for advancing next-generation UVNLO materials. Summary of the Invention

[0004] In order to solve the problem of excessive birefringence caused by the introduction of π-conjugated planar organic groups while maintaining a large SHG effect and a wide bandgap, the present invention provides a zinc-based halide ultraviolet nonlinear optical crystal material, a preparation method and an application thereof.

[0005] The technical solutions provided by the present invention are as follows: The first aspect of the present invention provides a zinc-based halide ultraviolet nonlinear optical crystal material, the chemical formula of which is one of the following two types: Type A: (C5H6N2O2)(C5H5N2O2)ZnX, the crystal belongs to the monoclinic system, the space group isC c; Type B: (C5H6N2O2)ZnX2, the crystal belongs to the triclinic system, the space group is P 1; Wherein, C5H6N2O2 represents 2-(1-imidazolyl)acetic acid, C5H5N2O2 represents 2-(1-imidazolyl)acetic acid ion, and X represents Cl or Br.

[0006] In some embodiments of the present invention, the chemical formula of type A is (C5H6N2O2)(C5H5N2O2)ZnCl, and the unit cell parameters are: a=12.7832(12)Å, b=9.9453(9)Å, c=11.0607(12)Å, α=90°, β=111.113(4)°, γ=90°, Z=4, V=1311.8(2)A³, Zn 2+ The ions are coordinated with two oxygen atoms, one nitrogen atom, and one chlorine atom to form mirror-image overlapping [Zn1NO2Cl] and [Zn2NO2Cl] tetrahedra, which are connected with organic rings to form a two-dimensional layered structure, with the layers alternately stacked to form circular pores.

[0007] In some embodiments of the present invention, the chemical formula of type B is (C5H6N2O2)ZnCl2, and the unit cell parameters are: a=4.7731(4)Å, b=6.6216(6)Å, c=7.2920(6)Å, α=97.132(3)°, β=95.455(3)°, γ=99.130(3)°, Z=1, V=224.23(3)ų, each unit cell contains a zinc atom, two Cl atoms and a [ZnO2Cl2] tetrahedron formed by C5H6N2O2, each [ZnO2Cl2] tetrahedron is connected by the carboxyl group of the organic ring C5H6N2O2 to form a one-dimensional chain extending infinitely along the a-axis; the one-dimensional chains are interwoven with each other through hydrogen bonds and are evenly arranged in space.

[0008] In some embodiments of the present invention, the chemical formula of type B is (C5H6N2O2)ZnBr2, and the unit cell parameters are: a=4.7733(2) Å, b=6.9005(4) Å, c=7.5507(4) Å, α=98.736(2)°, β=95.035(2)°, γ=99.367(2)°, Z=1, and each unit cell contains a zinc atom, two Br atoms and a [ZnO2Br2] tetrahedron formed by C5H6N2O2, and each [ZnO2Br2] tetrahedron is connected by the carboxyl group of the organic ring C5H6N2O2 to form a one-dimensional chain extending infinitely along the a-axis; the one-dimensional chains are interwoven with each other by hydrogen bonds and are evenly arranged in space.

[0009] In some embodiments of the present invention, the π-conjugated plane of the carboxylate in 2-(1-imidazolyl)acetic acid forms a dihedral angle of 81° to 84° with the plane of the imidazole ring.

[0010] A second aspect of the present invention provides a method for preparing a zinc-based halide ultraviolet nonlinear optical crystal material, comprising the following steps: mixing 2-(1-imidazolyl)acetic acid and ZnX2 in a molar ratio of 1:0.8~1.2 in an alcohol solution containing an organic acid, heating at 140±10°C for at least 6 days and then slowly cooling to room temperature, washing the product with deionized water and drying to obtain a zinc-based halide ultraviolet nonlinear optical crystal material with a chemical formula of (C5H6N2O2)(C5H5N2O2)ZnX, wherein C5H6N2O2 represents 2-(1-imidazolyl)acetic acid, C5H5N2O2 represents 2-(1-imidazolyl)acetic acid, and X represents Cl or Br.

[0011] In some preferred embodiments, the organic acid is one of formic acid, acetic acid, and propionic acid; and the alcohol solution is methanol and / or ethanol.

[0012] A third aspect of the present invention provides a method for preparing a zinc-based halide ultraviolet nonlinear optical crystal material, comprising the following steps: dissolving C5H6N2O2 and ZnX2 in a molar ratio of 1:0.8~1.2 in deionized water, slowly evaporating the water, and obtaining a zinc-based halide ultraviolet nonlinear optical crystal material with a chemical formula of (C5H6N2O2)ZnX2, wherein C5H6N2O2 represents 2-(1-imidazolyl)acetic acid and X represents Cl or Br.

[0013] The method of slowly evaporating the water is to leave it in the open at room temperature.

[0014] After dissolving C5H6N2O2 and ZnX2 in a molar ratio of 1:0.8-1.2 in deionized water, the method further includes: using a filter to remove undissolved matter.

[0015] A fourth aspect of the present invention provides an application of a zinc-based halide ultraviolet nonlinear optical crystal material, the application comprising: (1) Manufacturing ultraviolet laser frequency conversion devices for converting infrared light or visible light into ultraviolet light; (2) Fabrication of optical parametric oscillators for wide-tunable laser output in the 200-800 nm band; (3) Manufacturing nonlinear optical elements in high-power laser systems for frequency doubling, sum frequency or difference frequency conversion.

[0016] A fifth aspect of the present invention provides an ultraviolet nonlinear optical element, comprising the above-mentioned zinc-based halide ultraviolet nonlinear optical crystal material, for converting infrared light or visible light into ultraviolet light.

[0017] A sixth aspect of the present invention provides an ultraviolet nonlinear optical device, comprising the above-mentioned ultraviolet nonlinear optical element.

[0018] Compared with the prior art, the present invention has the following beneficial effects: The zinc-based halide ultraviolet nonlinear optical crystal material with a π-conjugated planar organic group provided by the present invention has a large SHG coefficient (compound I: 0.4×KDP, compound II: 3.5×KDP, compound III: 8.2×KDP), a short ultraviolet cutoff edge (the band gaps of compounds I, II and III are 5.20, 5.19 and 5.05 eV, respectively) and optimal birefringence (compound I: Δn=0.048, compound II: Δn=0.063, compound III: Δn=0.071). BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 .Crystal photograph of (C5H6N2O2)(C5H5N2O2)ZnCl(I).

[0020] Figure 2 .Crystal photograph of (C5H6N2O2)ZnCl2(II).

[0021] Figure 3 .Crystal photograph of (C5H6N2O2)ZnBr2(III).

[0022] Figure 4 Experimental and calculated XRD patterns of (C5H6N2O2)(C5H5N2O2)ZnCl(I).

[0023] Figure 5 .Experimental and calculated XRD patterns of (C5H6N2O2)ZnCl2(II).

[0024] Figure 6 .Experimental and calculated XRD patterns of (C5H6N2O2)ZnBr2(III).

[0025] Figure 7 Scanning electron microscope image of (C5H6N2O2)(C5H5N2O2)ZnCl(I) and its elemental distribution.

[0026] Figure 8 Scanning electron microscope image of (C5H6N2O2)ZnCl2(II) and its elemental distribution.

[0027] Figure 9 Scanning electron microscope image of (C5H6N2O2)ZnBr2(III) and its elemental distribution.

[0028] Figure 10Two polyhedral structures of (C5H6N2O2)(C5H5N2O2)ZnCl(I).

[0029] Figure 11 .Pore structure of (C5H6N2O2)(C5H5N2O2)ZnCl(I).

[0030] Figure 12 .IR spectrum of (C5H6N2O2)(C5H5N2O2)ZnCl(I).

[0031] Figure 13 .IR spectrum of (C5H6N2O2)ZnCl2(II).

[0032] Figure 14 .IR spectrum of (C5H6N2O2)ZnBr2(III).

[0033] Figure 15 UV-Vis diffuse reflectance spectrum of (C5H6N2O2)(C5H5N2O2)ZnCl(I) (the inset is the experimental band gap calculated using the Kubelka-Munk method).

[0034] Figure 16 UV-Vis diffuse reflectance spectrum of (C5H6N2O2)ZnCl2(II) (inset is the experimental band gap calculated using the Kubelka-Munk method).

[0035] Figure 17 UV-Vis diffuse reflectance spectrum of (C5H6N2O2)ZnBr2(III) (inset is the experimental band gap calculated using the Kubelka-Munk method).

[0036] Figure 18 .Thermogravimetric analysis of (C5H6N2O2)(C5H5N2O2)ZnCl(I).

[0037] Figure 19 .Thermogravimetric analysis of (C5H6N2O2)ZnCl2(II).

[0038] Figure 20 .Thermogravimetric analysis of (C5H6N2O2)ZnBr2(III).

[0039] Figure 21 .Calculated band structure of (C5H6N2O2)(C5H5N2O2)ZnCl(I).

[0040] Figure 22 .Calculated band structure of (C5H6N2O2)ZnCl2(II).

[0041] Figure 23 .Calculated total density of states and partial density of states of (C5H6N2O2)(C5H5N2O2)ZnCl(I).

[0042] Figure 24 .Calculated total density of states and partial density of states of (C5H6N2O2)ZnCl2(II).

[0043] Figure 25 Theoretical SHG coefficient of (C5H6N2O2)(C5H5N2O2)ZnCl(I).

[0044] Figure 26 .Theoretical SHG coefficient of (C5H6N2O2)ZnCl2(II).

[0045] Figure 27 .Electron density distribution of (C5H6N2O2)(C5H5N2O2)ZnCl(I).

[0046] Figure 28 .Electron density distribution of (C5H6N2O2)ZnCl2(II).

[0047] Figure 29 SHG-weighted density of (a) VEOCCP and (b) VEUNOCCP in (C5H6N2O2)(C5H5N2O2)ZnCl(I).

[0048] Figure 30 SHG-weighted density of (a) VEOCCP and (b) VEUNOCCP in (C5H6N2O2)ZnCl2(II).

[0049] Figure 31 (a) Calculated band structure, (b) total density of states and partial density of states, (c) theoretical SHG coefficient, (d) electron density distribution, (e) VEOCCP and (f) VEUNOCCP SHG weighted density of (C5H6N2O2)ZnBr2(III).

[0050] Figure 32 . Crystal structures of compound I (a), (b), (c) and compound II (d), (e), (f).

[0051] Figure 33 (a) Phase matching curves of compounds I, II, III, and KDP under 1064 nm laser irradiation; (b) Powder SHG (280–450 μm) intensity comparison of compounds I, II, III, and KDP; (c) Statistical comparison of SHG intensities of compounds I, II, III and other reported zinc-based halides.

[0052] Figure 34 .(a), (b), and (c) are birefringence photographs of the test crystals of compounds I, II, and III (from left to right: original crystal photograph, complete extinction photograph, and thickness photograph); (d), (e), and (f) are theoretically calculated birefringence curves of compounds I, II, and III; (g) is the birefringence statistical data of compounds I, II, and III and reported zinc-based halides and imidazole ring-containing compounds.

[0053] Figure 35 Comparative analysis of the synthesis methods, structures, and properties of compounds I, II, and III. (a) Coordination mode switching strategy; (b) Polarizability control; (c) Dihedral angle modulation.

[0054] Figure 36 .(a) Band structure diagram of compound III; (b) total density of states and orbital-resolved partial density of states of compound III; (c) theoretical SHG coefficient of compound III; (d) electron density distribution diagram of compound III; (e) SHG-weighted density of VEOOC in compound III; (f) SHG-weighted density of VEUNOOC in compound III.

[0055] Figure 37 Comparison of SHG, band gap and birefringence of compounds I, II and III. DETAILED DESCRIPTION

[0056] To date, researchers have reported the birefringence of compounds containing more than 30 types of π-conjugated planar organic groups, including cyanuric acid [(C3N3O3H x ) x−3 (x=0–3)], trithiocyanuric acid [(C3N3S3H x ) x−3 (x=0–3)], cyanuric acid [(C3N6H 6+x ) x+ (x=0–2)], pyridine and its derivatives such as [(C5NOH 5+x ) x+ (x=0–1), (C5N2H7) + , (C6N2H5) + etc.], pyrimidine and its derivatives such as [(C4N3H 5+x ) x+ , (C4N3OH6) + ], imidazole [(C3N2H5)⁺], guanidine [C(NH2)3⁺] and quinoline derivatives such as [(C 10 H 10 NO2)⁺, (C9H6BrN) etc.].

[0057] This study statistically analyzed the birefringence of 255 reported compounds containing π-conjugated planar organic groups, categorizing the data into five groups: A (cyanuric acid), B (trithiocyanuric acid, melamine, and their derivatives), C (pyridine rings and their derivatives), D (guanidine groups), and E (other π-conjugated planar organic groups). The vast majority (86.7%) of these crystalline materials exhibited excessive birefringence (>0.1), with an average value exceeding 0.15. Only 11.8% of these materials fell within the optimal birefringence range (Δn = 0.05-0.1), severely limiting their practical applications as UVNLO crystals. Therefore, addressing the excessive birefringence caused by the introduction of π-conjugated planar organic groups while maintaining a robust SHG effect and wide bandgap is key to developing UVNLO crystals with superior overall performance.

[0058] Based on this analysis, the present invention proposes a "triple control coordination engineering" strategy that integrates dihedral angle modulation, coordination mode switching, and polarizability control to design and synthesize UVNLO crystal materials with large second harmonic generation (SHG), wide bandgap, and suitable birefringence. Leveraging the dynamic switching of ligand modes, this switching induces an ordered arrangement of polyhedra, effectively enhancing the second harmonic generation (SHG) performance. Furthermore, halogen substitution further enhances the SHG effect by controlling the polarizability.

[0059] This study discovered a unique organic ligand, 2-(1-imidazolyl)acetic acid, which contains two π-conjugated planes: an imidazole (C3H4N2) and a carboxylate (COO⁻), which are connected to a methylene group (methylene) in a tetrahedral configuration via C-C and C-N single bonds. This crucial tetrahedral configuration acts as a flexible "fulcrum," allowing varying degrees of internal rotation around adjacent single bonds. This internal rotation imparts dynamic properties to the conjugated group.

[0060] The tunability allows them to flexibly adapt within the crystal structure, thereby controlling the birefringence to an appropriate level. The core of this strategy lies in adjusting the angle between the two π-conjugated planes in 2-(1-imidazolyl)acetic acid to maintain the appropriate birefringence.

[0061] The present invention synthesizes three compounds by precisely controlling pH value and temperature and adopting halogen substitution: (C5H6N2O2)(C5H5N2O2)ZnCl( C c, Compound I), (C5H6N2O2)ZnCl2 ( P 1, compound II), and (C5H6N2O2)ZnBr2 ( P1, compound III). From compound I to compound II, the coordination pattern of 2-(1-imidazolyl)acetic acid changes, resulting in a complete alignment of the [ZnO₂Cl₂] polyhedra in compound II. Subsequently, replacing Cl with the more polarizable Br in compound III gradually enhanced the SHG effect, from 0.4×KDP in compound I to 3.5×KDP in compound II, and ultimately reaching 8.2×KDP in compound III. This represents a continuous improvement and breaks the record for SHG effects in zinc-based halide and imidazolate metal complexes.

[0062] At the same time, the π-conjugated carboxylic acid trigonal plane and the imidazole ring plane form a large dihedral angle (81°~84°), controlling the birefringence of the three compounds in the range of 0.04-0.08, thereby achieving the co-optimization of the (C5H6N2O2)ZnBr2(III) SHG effect (8.2×KDP) and birefringence (0.071@1064nm). In addition, all three materials exhibit wide band gaps (all greater than 5.0eV) and high thermal stability (thermal decomposition temperatures ranging from 250 to 300°C).

[0063] Therefore, the present invention has developed a promising UVNLO crystal candidate (C5H6N2O2)ZnBr2(III) with excellent comprehensive properties. This work not only solves the key performance bottleneck of current UVNLO crystal materials, but also provides a new method and approach for the design and development of high-performance UVNLO materials.

[0064] Compound I (CCDC: 2446806) was prepared by conventional solvothermal method ( Figure 1 ) was obtained in a yield of about 86% (based on zinc) as light grey transparent block crystals. Single crystals of compound II (CCDC: 2446805) and III (CCDC: 2446804) were obtained by a simple solvent evaporation method. Compound II was colorless plate-like crystals, and compound III was colorless needle-like crystals ( Figure 2 、 Figure 3 ), with yields of approximately 63% and 52% (based on zinc), respectively. Detailed crystallographic data, atomic coordinates, and chemical bond information for the three structures are shown in Tables 1 to 17. The phase purity of the three compounds was confirmed by powder X-ray diffraction (XRD) ( Figures 4 to 6 ), the presence and uniform distribution of C, N, O, Zn, Cl, and Br were verified by field emission scanning electron microscopy (FESEM) analysis ( Figures 7 to 9 ).

[0065] The crystal structure of compound I was determined by single crystal X-ray diffraction. The compound crystallized in the monoclinic system, polar space group Cc (number 9), with the following unit cell parameters: a=12.7832(12)A, b=9.9453(9)A, c=11.0607(12)A, α=γ=90°, β=111.113(4)°, Z=4, V=1311.8(2)A³. In the smallest asymmetric unit, it contains two organic rings C5H6N2O2 and (C5H5N2O2)⁻, a chlorine atom, and two partially disordered zinc atoms (Zn1 accounts for 12.4(4)%, and Zn2 accounts for 87.5(4)%). The structure is as follows Figure 32 As shown in a. The Zn²⁺ ion is coordinated with two oxygen atoms, one nitrogen atom and one chlorine atom to form [Zn1NO2Cl] and [Zn2NO2Cl] tetrahedra. The bond lengths are as follows: Zn1-N is 1.984(12)A, Zn1-O is 2.186(12)A and 1.952(12)A, Zn1-Cl is 2.122(9)A, Zn2-N is 1.999(8)A, Zn2-O is 1.943(7)A and 2.009(7)A, and Zn2-Cl is 2.224(2)A ( Figure 10 ). [Zn1NO2Cl] and [Zn2NO2Cl] tetrahedra mirror-image overlap and connect with organic rings C5H6N2O2 or (C5H5N2O2⁻) to form a unique V-shaped structure. Each V-shaped structure is connected to each other on the ab plane and extends infinitely to form a two-dimensional layer ( Figure 32 b). These layers are stacked alternately along the c-axis, eventually forming a spatial structure with circular pores ( Figure 32 c and Figure 11 ).

[0066] The crystal structures of compounds II and III were also determined by single crystal X-ray diffraction. Both compounds crystallized in the noncentrosymmetric triclinic P1 space group (number 1). Since they are isomers, the crystal structure of compound II is described in detail as an example. The unit cell parameters of compound II are as follows: a=4.7731(4)A, b=6.6216(6)A, c=7.2920(6)A, α=97.132(3)°, β=95.455(3)°, γ=99.130(3)°, Z=1, V=224.23(3)A³ (detailed parameters of compounds II and III are provided in the Supplementary Information). In its unit cell, it is composed of one zinc atom, two chlorine atoms and one C5H6N2O2 (intramolecular proton transfer, in which the hydrogen proton is transferred from the oxygen atom on the carboxyl group to the nitrogen atom in the imidazole ring), forming a distorted [ZnO2Cl2] tetrahedron, with zinc-oxygen bond lengths of 1.995(6)A and 1.996(6)A, respectively, and zinc-chloride bond lengths of 2.216(3)A and 2.2366(19)A, respectively. Figure 32 d). Each [ZnO2Cl2] tetrahedron is connected by the carboxyl group of the organic ring C5H6N2O2 to form a one-dimensional chain that extends infinitely along the a-axis ( Figure 32 e). Subsequently, these one-dimensional chains are intertwined with each other through hydrogen bonds and evenly arranged in space, forming the unique spatial structure of compound II ( Figure 32 f). Under the synergistic effect of weak interactions, due to hydrogen bonding and π-π stacking of imidazole rings, the spatial arrangement of each [ZnO2Cl2] tetrahedron and each C5H6N2O2 organic ring is exactly the same, which is an important reason why compounds II and III can produce large SHG effects.

[0067] In order to further verify the structural rationality of compounds I, II and III, infrared (IR) spectroscopy analysis was performed ( Figure 12-14 The specific attribution of the infrared absorption peak is consistent with the structure determined by single crystal X-ray diffraction. In addition, in order to explore the optical properties of these compounds, the present invention also conducted UV-visible diffuse reflectance spectroscopy analysis ( Figure 15-17 ). The band gaps of compounds I, II, and III were calculated using the Kubelka-Munk method to be 5.20, 5.19, and 5.05 eV, respectively. Surprisingly, despite the presence of bromine in compound III, its band gap remains above 5.0 eV. The band gaps of these three compounds are all greater than the 4.20 eV required for UVNLO crystals, indicating that they have potential for application in the ultraviolet region. In addition, the present invention also evaluates the thermal stability of these compounds. Compounds I, II, and III all exhibit good thermal stability and begin to decompose at temperatures exceeding 250°C ( Figures 18-20 ).

[0068] The SHG responses of compounds I, II, and III at 1064 nm were measured using the Kurtz-Perry powder SHG test method. The experimental results show that all three compounds exhibit SHG activity at 1064 nm. Furthermore, the SHG intensity of these compounds increases with particle size, reaching a plateau in the range of 280 to 450 µm, satisfying the phase matching condition ( Figure 33 a). Specifically, within this particle size range, the SHG intensities of compounds I, II, and III are approximately 0.4, 3.5, and 8.2 times that of KDP powder of the same particle size ( Figure 33 b). It is worth mentioning that compound III exhibits the largest SHG effect among the recently reported zinc-based halides. Figure 4 c shows the comparison of the SHG response intensity of compounds I, II, and III with other reported zinc-based halides (Table 18).

[0069] As mentioned above, the birefringence of UVNLO crystals must be moderate (0.05-0.1). Therefore, the present invention studied the birefringence of compounds I, II, and III in detail. Their birefringence was measured using a polarizing microscope, and the results are as follows: Figure 34 As shown in a-34c, the birefringence values of compounds I, II, and III at 1064 nm are 0.048, 0.063, and 0.071, respectively. The birefringence properties of these compounds were also theoretically calculated ( Figure 34 d. Figure 34 e. Figure 34 f). The calculated refractive index dispersion curves show that compound I is a positive biaxial crystal (na>nc>nb), while compounds II and III are negative biaxial crystals (nc>nb>na). The calculated birefringence values of compounds I, II, and III at 1064nm are 0.042, 0.043, and 0.062, respectively. The birefringence of compound III is within the appropriate range for UVNLO crystals. This birefringence characteristic is relatively rare in materials containing zinc halides and imidazole rings ( Figure 34 g, Table 19).

[0070] like Figure 35 As shown in Figure 1, with the same stoichiometric ratio of the starting materials, two different synthesis methods resulted in two different coordination modes for the zinc-based complexes I and II (III). This study analyzes why these two compounds have such significant differences in structure and properties. During the synthesis of compound I, acetic acid was added to lower the pH of the reaction system to approximately 2.0, which inhibited the dissociation of C5H6N2O2. The relatively high reaction temperature (140°C) was the cause of the Zn 2+ This provides sufficient energy to coordinate with the nitrogen and oxygen atoms in C5H6N2O2, ultimately forming a two-dimensional layered structure. In addition, the partially disordered arrangement of zinc atoms in compound I and the antiparallel arrangement of the two-dimensional layers lead to a significant cancellation of the dipole moments, resulting in a weaker SHG effect.

[0071] In contrast, the synthesis of compounds II and III was carried out in a simple and mild aqueous solution with a pH of approximately 4.0 and a low reaction temperature (25°C). Under these conditions, the electrostatic attraction between the zinc ion and the negatively charged carboxylate oxygen ion (COO⁻) dominates, promoting the coordination of the carboxylate of C5H6N2O2 with the zinc ion. The dissociated proton is transferred to the nitrogen atom at the 3rd position of the imidazole ring, forming a one-dimensional chain structure. Subsequently, through hydrogen bond interactions and π-π stacking of the imidazole ring, the [ZnO2X2] tetrahedron and the imidazole ring form a rare perfect arrangement. This arrangement enables the microscopic dipole moments of compounds II and III to be superimposed most effectively, resulting in their SHG response being significantly higher than that of compound I ( Figure 35 a).

[0072] Due to the perfect tetrahedral arrangement in compounds II and III, any small change will be amplified several times. Therefore, when the chlorine atoms in compound III are replaced by more polarizable bromine atoms, its SHG response is significantly enhanced ( Figure 35 b). In addition, in the one-dimensional chains of compounds II and III, although the π-conjugated imidazole rings are completely aligned, the special structure of 2-(1-imidazolyl)acetic acid results in a large angle (>80°) between the imidazole ring plane and the carboxylate plane, which significantly offsets the polarization anisotropy and avoids excessive birefringence ( Figure 35 c).

[0073] In UVNLO crystals, birefringence is closely related to the anisotropic distribution of the electron cloud within the crystal structure. Through this dynamic regulation, the arrangement and orientation of the conjugated units can, to a certain extent, alter the anisotropic distribution of the electron cloud within the crystal, thereby adjusting the birefringence. Specifically, proper dynamic regulation can avoid excessively uniform parallel alignment of the conjugated units, which would lead to excessive birefringence. Conversely, by allowing some orientational dispersion of the conjugated units within the crystal, birefringence can be reduced to a moderate range, avoiding problems associated with excessive birefringence, such as the walk-off effect. The synergistic effect of all these factors ultimately enables compound III to achieve a large SHG response and appropriate birefringence.

[0074] The present inventors were very curious about the excellent overall performance of compound III (band gap > 5.0 eV, SHG = 8.2 × KDP, Δn = 0.071). Therefore, the present inventors used density functional theory (DFT) to perform systematic theoretical calculations on compounds I, II, and III. Figure 21 、 Figure 22 、 Figure 36 a) Showing that they all have direct band gaps, with I having a double band gap. The calculated values are 4.18 (2.69), 4.46, and 4.09 eV, respectively. The fact that the calculated values are smaller than the experimental values (5.20, 5.19, and 5.05 eV) is reasonable due to the discontinuity of the exchange correlation in the generalized gradient approximation function and the energy of the disordered structure.

[0075] The total density of states and orbital-resolved partial density of states of compound I ( Figure 23 ) shows that in the range of -2~0eV, the top of the valence band is mainly occupied by the C-2p, O-2p and N-2p orbitals of the organic ring C5H6N2O2; at the same time, from the conduction band minimum (CBM) to 6eV, it is mainly occupied by the H-1s, C-2p, N-2p, O-2p and Zn-4s orbitals. For compounds II and III, the total density of states and orbital-resolved partial density of states ( Figure 24 、 Figure 36b) shows that from -2 to 0 eV, the valence band top is primarily occupied by C-2p, O-2p, and Cl-3p / Br-4p orbitals; from the conduction band minimum (CBM) to 6 eV, it is primarily occupied by H-1s, C-2p, N-2p, O-2p, and Zn-4s orbitals. The zinc and halogen atoms in compounds I, II, and III contribute little to the CBM, so the introduction of halogens has little effect on the band gaps of these three compounds, which explains their large band gaps.

[0076] In addition, the present invention also calculated the SHG coefficients of these three compounds. Under the constraint of Clayman symmetry, compound I has six independent non-zero SHG coefficients ( Figure 25 Since compounds II and III crystallize in the P1 space group, which has no symmetry operation, they have ten independent non-zero SHG coefficients ( Figure 26 、 Figure 36 c). Their maximum SHG coefficients are d 32 =-0.304, d 22 = -2.197, and d 22 =3.308pm / V(KDP’s d 33 =0.39pm / V), these results are consistent with the experimental data.

[0077] In order to analyze the origin of SHG in these compounds, the present invention calculated their electron density distribution ( Figure 27 、 Figure 28 and Figure 36 d), and SHG-weighted electron density analysis was performed to intuitively display the electron cloud of the component that dominates the SHG response ( Figure 29 、 Figure 30 、 Figure 36 e and Figure 36 f). Combining analysis of the electron density distribution and the SHG weighted density map, the SHG responses of compounds I, II, and III all originate from the synergistic effect between the zinc polyhedron and the imidazole ring. Therefore, the SHG responses of fully aligned compounds II and III are much higher than those of compound I.

[0078] In summary, by rationally controlling the reaction temperature and pH and employing a halogen substitution strategy, the present invention achieves multi-level enhancement of the SHG response while maintaining a wide bandgap and moderate birefringence. This ultimately leads to the successful synthesis of the high-performance compound III. Through theoretical calculations, the present invention discovered that the SHG in compounds II and III primarily originates from the synergistic effect of the [ZnO2X2] polyhedron (where X = Cl, Br) and the imidazole ring. Therefore, the present invention calculated the instantaneous polarizability, polarizability, and hyperpolarizability of the polyhedron and organic ring in the dipole compounds II and III (Table 20, Figure 21). Compared with the single [ZnO2Cl2] polyhedron, the hyperpolarizability of the [ZnO2Br2] polyhedron is significantly increased. Due to the perfect arrangement of the [ZnO2X2] polyhedrons in compounds II and III, the microscopic hyperpolarizability is effectively accumulated, resulting in the SHG effect of compound III being more than twice that of II. In addition, since the halogen substitution in these three compounds has little effect on their band gaps, compound III is able to maintain a higher band gap. Finally, the dynamic conjugation regulation of 2-(1-imidazolyl)acetic acid prevents excessive birefringence caused by the introduction of π conjugated planes. The combined modulation of these three dimensions ultimately enables compound III to exhibit excellent overall performance ( Figure 37 ).

[0079] In summary, this study established a disruptive "triple-control coordination engineering" strategy, effectively resolving the long-standing performance trade-off in ultraviolet nonlinear optical materials. By leveraging the dynamic carbon-carbon-nitrogen bond rotation in 2-(1-imidazolyl)acetic acid, the present invention achieves precise control of the dihedral angle (81°-84°) between π-conjugated planes, effectively suppressing excessive optical anisotropy while maintaining matched birefringence (Δn = 0.048–0.071 @ 1064 nm). Systematically modulating pH and temperature-mediated coordination changes, combined with halogen substitution (Cl→Br), for polarizability engineering, significantly enhances SHG. Consequently, three zinc halides—[(C5H6N2O2)(C5H5N2O2)ZnCl (I), (C5H6N2O2)ZnCl2 (II), and (C5H6N2O2)ZnBr2 (III)]—were developed, exhibiting a stepwise amplification of the second harmonic generation response (0.4 → 3.5 → 8.2 × KDP). Compound III offers a record-breaking second harmonic generation coefficient among zinc-based halides, attributed to the synergistic effects of the fully aligned [ZnN2Br2] polyhedron and the high polarizability of bromine. Importantly, compound III maintains a wide bandgap (>5.0 eV) and excellent thermal stability (approximately 300°C), making it an outstanding candidate for next-generation high-power UV nonlinear optical applications. Overall, this proposed strategy not only provides a model for optimizing SHG, Δn, and Eg in UV nonlinear optical crystals but also paves the way for the future development of UV nonlinear optical materials with superior comprehensive properties. Example

[0080] 1. Reagents: H6N2O2, 99%, Adamas), zinc chloride (ZnCl2, 99%, Adamas), and zinc bromide (ZnBr2, 99%, Adamas) were used without further purification.

[0081] 2. Synthesis Compound (C5H6N2O2)(C5H5N2O2)ZnCl (I) was synthesized via a solvothermal method. C5H6N2O2 (0.252 g, 2 mmol), zinc chloride (0.272 g, 2 mmol), ethanol (2 mL), and acetic acid (2 mL) were mixed and placed in a 25 mL polytetrafluoroethylene-lined container. The container was then sealed in an autoclave and heated at 140°C for 6 days, followed by slow cooling to room temperature at a rate of 1.25°C / hour. The product was washed with deionized water and dried in air. I was obtained as light gray, transparent, blocky crystals with a yield of 86% (based on zinc).

[0082] Compounds (C5H6N2O2)ZnCl2 (II) and (C5H6N2O2)ZnBr2 (III) were synthesized via a simple solution evaporation method. C5H6N2O2 (0.272 g, 2 mmol) and ZnX2 (ZnCl2 = 0.272 g, 2 mmol, ZnBr2 = 0.450 g, 2 mmol) were added to a beaker. 3 mL of deionized water was added, and the mixture was stirred at room temperature for 0.5 hours. After filtration, a colorless, transparent solution was obtained. The solution was then slowly evaporated at room temperature for 7 days to afford colorless flake crystals II and colorless needle-shaped crystals III, respectively, in yields of 63% and 52% (based on zinc).

[0083] 3. Characterization:

[0084] Single crystal X-ray diffraction (SC-XRD) data for I were collected at 223 K on an XtaLABSynergyR equipped with a graphite monochromator using MoKα radiation (λ = 0.71073 Å). The crystal structure was solved by direct methods using Olex2 software and refined using the least-squares method with F², using anisotropic thermal parameters for all atoms. The crystal data and structural refinement information for I are summarized in Table 1. Atomic coordinates, equivalent isotropic displacement parameters, anisotropic displacement parameters, selected bond lengths, and bond angles are summarized in Tables 2 to 5. The hydrogen bonding interactions of I are listed in Table 6.

[0085] Single-crystal X-ray diffraction (SC-XRD) data were collected on an XtaLABSynergyR equipped with a graphite monochromator using Mo Kα radiation (λ = 0.71073 Å) at 223 K. The crystal structure was solved by direct methods using Olex2 software and refined using the least-squares method with F², taking into account anisotropic thermal parameters for all atoms. The crystallographic data and structural refinement information for II are summarized in Table 7. Atomic coordinates, equivalent isotropic displacement parameters, anisotropic displacement parameters, and selected bond lengths and angles are shown in Tables 8 to 11.

[0086] Single-crystal X-ray diffraction (SC-XRD) data were collected on an XtaLABSynergyR equipped with a graphite monochromator using Mo Kα radiation (λ = 0.71073 Å) at 193 K. The crystal structure was solved by direct methods using Olex2 software and refined using the least-squares method with F², accounting for anisotropic thermal parameters for all atoms. The crystallographic data and structural refinement information for III are summarized in Table 12. Atomic coordinates, equivalent isotropic displacement parameters, anisotropic displacement parameters, selected bond lengths, and bond angles are summarized in Tables 13 to 16. The hydrogen bonding interactions of III are listed in Table 17.

[0087] Powder X-ray Diffraction (PXRD): Powder X-ray diffraction data were recorded on an Advance diffractometer using an A temperature of 40 kV, 100 mA, Cu Kα1 radiation (Bruker D8, wave number 1.5406 Å), a scan rate of 10° / min, and a scan angle range of 10–70°. Phase purity was determined by powder X-ray diffraction.

[0088] EDS: Elemental mapping and microprobe elemental analysis were measured on a field emission scanning electron microscope (SEM, JSM-7800) equipped with an energy dispersive X-ray spectrometer (EDS, Oxford INCA).

[0089] Infrared Spectroscopy: IR spectra were collected in the wavelength range of 4000–400 cm⁻¹ using a Nicolet iS5 Fourier Transform Infrared Spectrometer at room temperature. The sample was ground into a pellet using a 1:100 weight ratio of dried potassium bromide.

[0090] UV-Vis Diffuse Reflectance Spectroscopy: UV-Vis Diffuse Reflectance Spectra were measured using a Varian Cary 5000 spectrophotometer at room temperature over a scan range of 200–800 nm. Pure barium sulfate (100% reflectance) was used as a reference, upon which the ground powder sample was coated.

[0091] Thermogravimetric analysis (TGA) was performed using a Netzsch STA 449 F5 analyzer. About 0.8 mg of sample was weighed, placed in a platinum crucible, and heated from room temperature to 800°C at a rate of 10 K / min under a nitrogen atmosphere.

[0092] Second-harmonic generation (SHG) measurements: Second-order NLO measurements of powder samples were performed at room temperature using a modified Kurtz and Perry method with a Q-switched 1064 nm Nd:YAG laser. Polycrystalline samples of the compound were sieved to different particle sizes (35-50, 50-74, 74-100, 100-154, 154-180, 180-280, and 280-450 μm) to investigate whether their SHG responses could be matched. Potassium dihydrogen phosphate (KDP) was used as a reference material for comparison of the SHG efficiency of the samples to assess the second-order NLO effect.

[0093] Computational methods: First-principles calculations of compounds I, II, and III were performed using the CASTEP module in the Materials Studio software package. Density functional theory (DFT) was used for system geometry optimization. The exchange-correlation functional was calculated using the generalized gradient approximation Heyd-Scuseria-Ernzerhof (HSE06).

[0094] For compound I, the cutoff energy was set to 830 eV and a Monkhorst-Pack k-point grid of 1×1×1 was selected in the first Brillouin zone to ensure the accuracy of the calculation results. The self-consistent iterative convergence (self-consistent field) was 5.0×10⁻ 7 eV / atom, the maximum displacement converges to 5.0×10⁻ 4 A, the internal stress is 0.02 GPa, the force on the atoms is 0.01 eV / A, and the energy converges to 5.0×10⁻ 6 eV / atom. All atomic ion-electron interactions are modeled using ultrasoft pseudopotentials, and the atomic electronic configurations are: C (2 s ²2 p ²), H (1 s ¹), N(2 s ²2 p ³), O(2 s ²2 p 4 ), Cl (3 s ²3 p 5 ), Zn(3 d ¹ 0 4 s ²).

[0095] For compound II, the cutoff energy was set to 750 eV and a Monkhorst-Pack k-point grid of 3×2×2 was selected in the first Brillouin zone to ensure the accuracy of the calculated results. The self-consistent iterative convergence (self-consistent field) was 5.0×10⁻ 7 eV / atom, the maximum displacement converges to 5.0×10⁻4 A, the internal stress is 0.02 GPa, the force on the atoms is 0.01 eV / A, and the energy converges to 5.0×10⁻ 6 eV / atom. The ion-electron interaction of all atoms adopts the ultrasoft pseudopotential model, and the atomic electronic configurations are: carbon (2 s ²2 p ²), hydrogen (1 s ¹), nitrogen (2 s ²2 p ³), oxygen (2 s ²2 p 4 ), chlorine (3 s ²3 p 5 ), zinc (3 d ¹ 0 4 s ²).

[0096] For compound III, the cutoff energy was set to 750 eV and a Monkhorst-Pack k-point grid of 3×2×2 was selected in the first Brillouin zone to ensure the accuracy of the calculated results. The self-consistent iterative convergence (self-consistent field) was 5.0×10⁻ 7 eV / atom, the maximum displacement converges to 5.0×10⁻ 4 A, the internal stress is 0.02 GPa, the force on the atoms is 0.01 eV / A, and the energy converges to 5.0×10⁻ 6 eV / atom. The ion-electron interaction of all atoms adopts the ultrasoft pseudopotential model, and the atomic electronic configurations are: carbon (2 s ²2 p ²), hydrogen (1 s ¹), nitrogen (2 s ²2 p ³), oxygen (2 s ²2 p 4 ), zinc (3 d ¹ 0 4 s ²), bromine (4 s ²4 p 5 ).

[0097] The following tables show the characterization results of the three compounds of the present invention: Table 1. Crystallographic data and structure refinement of (C5H6N2O2)(C5H5N2O2)ZnCl(I)

[0098] Table 2. Atomic coordinates and equivalent isotropic displacement parameters and occupancy of (C5H6N2O2)(C5H5N2O2)ZnCl(I)

[0099] Table 3. Anisotropic displacement parameters of (C5H6N2O2)(C5H5N2O2)ZnCl(I) (Ų×10³)

[0100] Table 4. Bond lengths of (C5H6N2O2)(C5H5N2O2)ZnCl(I)

[0101] 11 / 2+X,-1 / 2+Y,+Z; 2-1 / 2+X,1 / 2+Y,+Z; 3-1 / 2+X,-1 / 2+Y,+Z Table 5. Bond angles of (C5H6N2O2)(C5H5N2O2)ZnCl(I)

[0102] 11 / 2+X,-1 / 2+Y,+Z; 2-1 / 2+X,1 / 2+Y,+Z; 3-1 / 2+X,-1 / 2+Y,+Z; 41 / 2+X,1 / 2+Y,+Z.

[0103] Table 6. Hydrogen bonds of (C5H6N2O2)(C5H5N2O2)ZnCl(I)

[0104] Table 7. Crystallographic data and structure refinement of (C5H6N2O2)ZnCl2 (II)

[0105] Table 8. Atomic coordinates and equivalent isotropic displacement parameters and occupancy of (C5H6N2O2)ZnCl2(II)

[0106] Table 9. Anisotropic displacement parameters of (C5H6N2O2)ZnCl2(II) (Ų×10³)

[0107] Table 10. Bond lengths of (C5H6N2O2)ZnCl2(II)

[0108] 1 -1+X,+Y,+Z.

[0109] Table 11. Bond angles of (C5H6N2O2)ZnCl2(II)

[0110] 1 -1+X,+Y,+ Table 12. Crystallographic data and structure refinement of (C5H6N2O2)ZnBr2(III)

[0111] Table 13. Atomic coordinates and equivalent isotropic displacement parameters and occupancy of (C5H6N2O2)ZnBr2(III)

[0112] Table 14. Anisotropic displacement parameters of (C5H6N2O2)ZnBr2(III) (Ų×10³)

[0113] The anisotropic displacement factor exponential form is: -2π²[h²a ²U 11 + 2hka b*U 12 +…].

[0114] Table 15. Bond lengths of (C5H6N2O2)ZnBr2(III)

[0115] 1 + X, +Y, +Z.

[0116] Table 16. Bond angles of (C5H6N2O2)ZnBr2(III)

[0117] 1 + X, +Y, +Z; 2 -1 + X, +Y, +Z.

[0118] Table 17. Hydrogen bonds of (C5H6N2O2)ZnBr2(III)

[0119] 1 -1 + X, 1 + Y, 1 + Z; 2 + X, +Y, 1 + Z; 3 + X, 1 + Y, +Z.

[0120] Table 18. SHG statistics of zinc-based halides

[0121] Table 19. Birefringence statistics of zinc-based halide and imidazole materials

[0122] Table 20. Calculated dipole moment components of Zn1NO2Cl, Zn2NO2Cl, ZnO2Cl2, and ZnO2Br2

[0123] Table 21. Calculated hyperpolarizabilities and polarization anisotropies of compounds II and III

[0124] Z; 2 1+X,+Y,+Z.

[0125] The foregoing description is intended only to provide specific embodiments of the present invention, which will enable those skilled in the art to understand and implement the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not intended to be limited to the embodiments shown herein, but is intended to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A zinc-based halide ultraviolet nonlinear optical crystal material, characterized by: The chemical formula of the zinc-based halide ultraviolet nonlinear optical crystal material is one of the following two types: Type A: (C5H6N2O2)(C5H5N2O2)ZnX, the crystal belongs to the monoclinic system, the space group is C c; Type B: (C5H6N2O2)ZnX2, the crystal belongs to the triclinic system, the space group is P 1; Wherein, C5H6N2O2 represents 2-(1-imidazolyl)acetic acid, C5H5N2O2 represents 2-(1-imidazolyl)acetic acid ion, and X represents Cl or Br.

2. The zinc-based halide ultraviolet nonlinear optical crystal material according to claim 1, characterized in that: The chemical formula of type A is (C5H6N2O2)(C5H5N2O2)ZnCl, and the unit cell parameters are: a=12.7832(12)Å, b=9.9453(9)Å, c=11.0607(12)Å, α=90°, β=111.113(4)°, γ=90°, Z=4, V=1311.8(2)A³, Zn 2+ The ions are coordinated with two oxygen atoms, one nitrogen atom, and one chlorine atom to form mirror-image overlapping [Zn1NO2Cl] and [Zn2NO2Cl] tetrahedra, which are connected with organic rings to form a two-dimensional layered structure, with the layers alternately stacked to form circular pores.

3. The zinc-based halide ultraviolet nonlinear optical crystal material according to claim 1, characterized in that: The chemical formula of type B is (C5H6N2O2)ZnCl2, and the unit cell parameters are: a= 4.7731(4)Å, b=6.6216(6)Å, c=7.2920(6)Å, α=97.132(3)°, β=95.455(3)°, γ=99.130(3)°, Z=1, V=224.23(3)ų, each unit cell contains a zinc atom, two Cl atoms and a [ZnO2Cl2] tetrahedron formed by C5H6N2O2, each [ZnO2Cl2] tetrahedron is connected by the carboxyl group of the organic ring C5H6N2O2 to form a one-dimensional chain extending infinitely along the a-axis; the one-dimensional chain is interwoven with each other by hydrogen bonds and is evenly arranged in space.

4. The zinc-based halide ultraviolet nonlinear optical crystal material according to claim 1, characterized in that: The chemical formula of type B is (C5H6N2O2)ZnBr2, and the unit cell parameters are: a=4.7733(2) Å, b=6.9005(4) Å, c=7.5507(4) Å, α=98.736(2)°, β=95.035(2)°, γ=99.367(2)°, Z=1. Each unit cell contains a zinc atom, two Br atoms and a [ZnO2Br2] tetrahedron formed by C5H6N2O2. Each [ZnO2Br2] tetrahedron is connected by the carboxyl group of the organic ring C5H6N2O2 to form a one-dimensional chain extending infinitely along the a-axis; the one-dimensional chain is interwoven with each other through hydrogen bonds and is evenly arranged in space.

5. The zinc-based halide ultraviolet nonlinear optical crystal material according to any one of claims 1 to 4, characterized in that: The π-conjugated plane of the carboxylate in 2-(1-imidazolyl)acetic acid forms a dihedral angle of 81°~84° with the plane of the imidazole ring.

6. A method for preparing a zinc-based halide ultraviolet nonlinear optical crystal material, characterized in that: The following steps are involved: 2-(1-imidazolyl)acetic acid and ZnX2 are mixed in a molar ratio of 1:0.8-1.2 in an alcohol solution containing an organic acid, heated at 140±10°C for at least 6 days, and then slowly cooled to room temperature. The product is washed with deionized water and dried to obtain a zinc-based halide ultraviolet nonlinear optical crystal material with a chemical formula of (C5H6N2O2)(C5H5N2O2)ZnX, wherein C5H6N2O2 represents 2-(1-imidazolyl)acetic acid, C5H5N2O2 represents 2-(1-imidazolyl)acetic acid, and X represents Cl or Br.

7. A method for preparing a zinc-based halide ultraviolet nonlinear optical crystal material, characterized in that: The following steps are involved: C5H6N2O2 and ZnX2 in a molar ratio of 1:0.8-1.2 are dissolved in deionized water, stirred at room temperature for 30 minutes, filtered, and the filtrate is slowly evaporated to remove water, thereby obtaining a zinc-based halide ultraviolet nonlinear optical crystal material with a chemical formula of (C5H6N2O2)ZnX2, wherein C5H6N2O2 represents 2-(1-imidazolyl)acetic acid and X represents Cl or Br.

8. Use of the zinc-based halide ultraviolet nonlinear optical crystal material according to any one of claims 1 to 5, characterized in that: The applications include: (1) Manufacturing ultraviolet laser frequency conversion devices for converting infrared light or visible light into ultraviolet light; (2) Fabrication of optical parametric oscillators for wide-tunable laser output in the 200-800 nm band; (3) Manufacturing nonlinear optical elements in high-power laser systems for frequency doubling, sum frequency or difference frequency conversion.

9. An ultraviolet nonlinear optical element, characterized in that: The ultraviolet nonlinear optical element comprises the zinc-based halide ultraviolet nonlinear optical crystal material according to any one of claims 1 to 5, and the zinc-based halide ultraviolet nonlinear optical crystal material is used to convert infrared light or visible light into ultraviolet light.

10. An ultraviolet nonlinear optical device, characterized in that: The ultraviolet nonlinear optical device comprises the ultraviolet nonlinear optical element according to claim 9.

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