English

• 0.   Introduction

  Nonlinear optical (NLO) crystals can convert laser frequency and have been widely used in semiconductor manufacturing, lithography, attosecond pulse generation, and advanced instruments^[1-5]. So far, a large number of inorganics, organics, organometallics, inorganic-organic hybrid compounds, polymers, and inorganic glasses have been found to have second-harmonic generation (SHG) activity. Currently, only a few NLO crystals have been commercially applied, including KBe₂BO₃F₂^[6-8], β-BaB₂O₄^[9], LiB₃O₅^[10], CsB₃O₅^[11], KH₂PO₄ (KDP)^[12], KTiOPO₄^[13], and AgGaS₂^[14]. However, these crystals all have some inherent drawbacks and cannot be called “perfect” crystals. Therefore, it will still take a long time to explore more promising crystals.

  NLO crystals can be divided into four groups according to their transparent wavelength range: deep ultraviolet (DUV, < 200 nm), ultraviolet (UV, 200-400 nm), visible near-infrared (Vis-NIR, 400 nm-2.5 µm), middle and far-infrared (MFIR, 2.5-20 µm) ^[15-16]. UV NLO crystals are the key component of frequency conversion to generate short-wavelength laser light that is intrinsic in cutting-edge laser technology and funda-mental science^[17]. For a long time, the exploration of UV NLO crystals has mainly focused on π-conjugated systems, such as BO₃^3-[18], CO₃^2-[19], and NO₃^-[20]. Based on these excellent π-conjugated NLO functional units, a large number of crystals with excellent NLO performance were reported, such as NH₄B₄O₆F^[21], NaB₄O₆F^[22], RbMgCO₃F^[23], NaZnCO₃(OH)^[24], Sr₂(OH)₃NO₃^[25], and Rb₂Na(NO₃)₃^[26]. In recent years, UV NLO crystals based on non-π-conjugated systems (PO₄^3- and SO₄^2-) have gradually gained attention. Wherein, PO₄^3- unit is usually conducive to the formation of short UV cut-off edges and tends to connect with other elements or units to form 0D to 3D crystal structures, such as chain, layer, and network structures. Many UV NLO phosphate crystals have been discovered successfully, including Ba₃P₃O₁₀X^[27], RbBa₂(PO₃)₅^[28], Rb₂Ba₃(P₂O₇)₂^[28], and LiCs₂PO₄^[29]. Compared to the PO₄^3- unit, theoretically speaking, the SO₄^2- unit possesses larger microscopic polarizability and polarizability anisotropy, which is more conducive to generating large macroscopic NLO effects. In addition, SO₄^2- has an optical band gap equivalent to PO₄^3-, which can promote the transmission of sulfate NLO crystals in UV and even DUV. In recent years, NH₄NaLi₂(SO₄)₂^[30], (NH₄)₂Na₃Li₉(SO₄)₇^[30], Li₂KRb(SO₄)₂^[31], and MF₂(SO₄) (M=Zr, Hf) ^[32] were reported with excellent NLO performance. However, PO₄^3- and SO₄^2- are both isotropic tetrahedral building units with almost nonpolar symmetry, which limits their contribution to SHG coefficient and birefringence. In addition, it is reported that the heteroatomic tetrahedra have some obvious advantages over the regular tetrahedra in NLO hyperpolarization and optical anisotropy, which helps to achieve a comprehensive balance between the large band gap, high SHG response, and proper birefringence of NLO crystals. These heteroatomic tetrahedra include BO_xF_4-x (x=1-3)^{[22, 33]}, PO _x F_4-x (x= 1-3)^[34-35], and SiO_x F_6-x (x=1-5)^[36-37]. In 2021, Ye et al.^[38] discovered a new non-π-conjugated unit NH₂SO₃^- through first-principles calculation, and successfully synthesized two excellent NLO crystals Sr(NH₂SO₃)₂ and Ba(NH₂SO₃)₂. The calculation results show that NH₂SO₃^- has a more obvious polarizability anisotropy, greater hyperpolarizability (about 1.92 times that of SO₄^2-), and a wider optical band gap (8.17 eV) than SO₄^2-. It provides a new way to explore non-π-conjugated systems.

  Based on the above considerations, we introduced alkali metal cations into the NH₂SO₃^- unit, and successfully obtained two alkali-metal sulfamates NLO crystals, Li(NH₂SO₃) and Na(NH₂SO₃), through the facile evaporation method. Although the structures of these two compounds have been reported, the physicochemical properties of these two compounds have not been reported, so far. Herein, we provide a detailed report on the physicochemical and NLO properties of these two compounds. The structure-function relationships of these two crystals have also been studied by theoretical calculations.

  1.   Experimental

  1.1   Reagents

  Sulfamic acid (NH₂SO₃H, 99.99%, Xiya Reagent), sodium carbonate (Na₂CO₃, 99.80%, Xiya Reagent), lithium carbonate (Li₂CO₃, 99.00%, Xiya Reagent) and deionized water were commercially available and used as received without further purification.

  1.2   Synthesis of A(NH₂SO₃) (A=Li, Na)

  Li₂CO₃ (1.0 mmol, 0.074 g) and NH₂SO₃H (2.0 mmol, 0.194 g) were mixed and poured into 3 mL deionized water, fully stirred at room temperature, and then placed in the fume hood. After 2-3 d, colorless rod-shaped crystals of Li(NH₂SO₃) could be obtained, with a yield of about 73% (based on the amount of Li used).

  The preparation of Na(NH₂SO₃) was the same as that of Li(NH₂SO₃) except that Li₂CO₃ was replaced by Na₂CO₃ (1.0 mmol, 0.106 g). Colorless rod-shaped crystals were obtained with a yield of about 70% (based on the amount of Na used).

  1.3   Single-crystal and powder X-ray diffraction

  Single-crystal X-ray diffraction data collection of Li(NH₂SO₃) and Na(NH₂SO₃) was carried out on a Bruker D8 VENTURE CMOS X-ray source with Mo Kα radiation (λ=0.071 073 nm) at room temperature. APEX Ⅱ software was applied to collect and reduce data. For Li(NH₂SO₃), in a range of 2.505° < θ < 27.469°, a total of 6 728 reflections were collected and 1548 were independent with R_int=0.039 8, of which 1 026 were observed with I > 2σ(I). For Na(NH₂SO₃), in a range of 2.598° < θ < 27.096°, a total of 9 539 reflections were collected and 2 372 were independent with R_int=0.030 2, of which 1 282 were observed with I > 2σ(I). Semi-empirical absorption corrections based on equivalent reflections were applied for both data sets using the APEX Ⅱ program. The structure was solved by direct methods and refined on F² by full-matrix least-squares methods using the OLEX2 software package^[39]. All non-hydrogen atoms were refined anisotropically. The detailed crystal data and structural refinement informa-tion for Li(NH₂SO₃) and Na(NH₂SO₃) are summarized in Table S1 (Supporting information). Selected bond distances (nm) and angles (°) are given in Tables S2 and S3, while atomic coordinates and equivalent isotropic displacement parameters as well as bond valence sums are collected in Tables S4 and S5. The powder X-ray diffraction data of each sample were collected on a Bruker D8 X-ray diffractometer equipped with Cu Kα radiation (λ =0.154 18 nm) in a 2θ range of 5°-80° with a step size of 0.02° at room temperature, the working voltage and current were 40 kV and 40 mA respectively (Fig.S1).

  1.4   IR and UV-Vis-NIR diffuse reflectance spectra

  The IR spectra were recorded on a Nicolet iS10 Fourier transform IR spectrometer (resolution 4 cm^-1, spectral range 400-4 000 cm^-1). An optical diffuse reflectance spectrum was obtained using a Cary 5000 UV-Vis-NIR spectrophotometer at room temperature. A BaSO₄ plate was used as a standard (100% reflectance). The absorption spectra of A(NH₂SO₃) (A=Li, Na) were calculated from the reflectance spectrum using the Kubelka-Munk function α/S = (1-R)²/(2R), where α is the absorption coefficient, S is the scattering coefficient that is essentially wavelength-independent when the particle size is larger than 5 µm, and R is the reflectance^[40].

  1.5   Thermogravimetric analysis (TGA)

  A Netzsch STA 409PC thermal analyzer was used to analyze the thermal stabilities of A(NH₂SO₃) (A=Li, Na). The samples were heated from 30 to 800 ℃ with a heating rate of 20 ℃·min^-1 in nitrogen gas.

  1.6   Powder SHG measurements

  The SHG intensities of A(NH₂SO₃) (A=Li, Na) were measured according to the modified method of Kurtz and Perry^[41]. A Q-switched Nd∶YAG laser with 1 064 nm radiation was employed for the visible SHG study. Because the SHG efficiency is related to the particle size, the polycrystalline samples of Li(NH₂SO₃) and Na(NH₂SO₃) were ground and sieved into several particle size ranges (0-26 µm, 26-50 µm, 50-74 µm, 74-105 µm, 105-150 µm, 150-200 µm). Crystalline KDP with the same particle size ranges was used as references.

  1.7   Theoretical calculations

  First-principles calculations on A(NH₂SO₃) (A=Li, Na) were performed using the CASTEP package^[42], a total energy package based on pseudopotential density functional theory (DFT) ^[43]. The correlation-exchange terms in the Hamiltonian were described by the functional developed by Perdew, Burke, and Ernzerhof in the generalized gradient approximation form^[44-45]. Optimized norm-conserving pseudopotentials^[46] in the Kleinman-Bylander form were adopted to model the effective interaction between the valence electrons and atom cores, which allows the choice of a relatively small plane-wave basis set without compromising the computational accuracy. A kinetic energy cutoff of 850 eV and dense Monkhorst-Pack^[47] k-point meshes spanning less than 0.000 015 nm³ in the Brillouin zone were chosen.

  2.   Results and discussion

  2.1   Crystal structure

  Li(NH₂SO₃) crystallizes into an orthorhombic polar space group Pca2₁. There are sixteen independent atoms in one asymmetric unit of the crystal, including two Li atoms, two S atoms, two N atoms, six O atoms, and four H atoms (Fig. S2a). Each Li atom coordinates with four O atoms from four NH₂SO₃^- units through corner-sharing, forming [LiO₄]^7- tetrahedra with Li—O distances in a range of 0.188 3(8)-0.196 0(9) nm. Each S atom forms four single bonds with three O atoms and one N atom to form NH₂SO₃^- tetrahedra with the S—O and S—N bond lengths varying from 0.144 2(3) to 0.146 8(4) nm and from 0.161 6(6) to 0.162 4(6) nm, respectively, and the O—S—O bond angles are in a range of 110.1(2)°-113.9(2)°, the O—S—N bond angles are in a range of 103.8(3)°-112.1(2)°. The [LiO₄]^7- unit is connected with NH₂SO₃^- units to form a 3D net-work structure. The Li atoms are connected by the O—S—O bonds of the three and four NH₂SO₃^- units to form the [Li₄O₈S₄] ring and the [Li₃O₆S₃] ring, respectively (Fig. 1a and 1c). The [Li₄O₈S₄] ring and the [Li₃O₆S₃] ring grow infinitely in the ac and ab planes, respectively (Fig. 1b and 1d). The NH₂SO₃^- units act as interlayer linkers connecting Li atoms, thus forming the 3D network structure (Fig. 1e).

  Figure 1

  

  Figure 1.  Crystal structure of Li(NH₂SO₃): (a)[Li₄O₈S₄] ring; (b) [Li₄O₈S₄]_∞ layer parallel to the ac plane; (c) [Li₃O₆S₃] ring; (d) [Li₃O₆S₃]_∞ layer parallel to the ab plane; (e) 3D framework structure of Li(NH₂SO₃)

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  Although Na(NH₂SO₃) shares a similar chemical formula as Li(NH₂SO₃), it crystallizes into orthorhombic polar space group P2₁2₁2₁. One asymmetric unit of Na(NH₂SO₃) contains twenty-four independent atoms, including three Na atoms, nine O atoms, three N atoms, three S atoms, and six H atoms (Fig. S2b). There are three different coordination modes of Na atoms, and all of them are six-coordinated with six O atoms from six NH₂SO₃^- units through corner-sharing, forming three twisted [NaO₆]^11- octahedral with Na—O distances in a range of 0.230 7(6)-0.239 6(5) nm, and then three different [NaO₆]^11- units connect by corner-sharing, form-ing the [Na₃ O₁₁] unit (Fig. 2a and 2b). For NH₂SO₃^- tetrahedra, the S—O and S—N bond lengths vary from 0.144 0(5) to 0.145 8(5) nm, and the O—S—O bond angles are in a range of 101.7(2)°-13.5(3)°. It can be observed that [Na₃O₁₁] units are tightly aligned together to form a chain structure by sharing O atoms (Fig. 2c). The NH₂SO₃^- units act as interlayer linkers connecting Na atoms, thus forming the 3D network structure (Fig. 2d).

  Figure 2

  

  Figure 2.  (a) Arrangement of NH₂SO₃^- units in Na(NH₂SO₃); (b) Coordination environment of Na atoms and the [Na₃O₁₁] unit; (c) Chain structure of [Na₃O₁₁] units; (d) 3D framework of Na(NH₂SO₃)

  Symmetry codes: #1: 0.5+x, 0.5-y, 1-z; #2:-0.5+x, 0.5-y, 1-z; #3: x, y, z; #4:-1+x, y, z; #5:-x, -0.5+y, 1.5-z.

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  2.2   Thermal stability

  To investigate the thermal stability of Li(NH₂SO₃) and Na(NH₂SO₃), TGA was performed (Fig. 3). The TG curves showed that two crystalline samples Li(NH₂SO₃) and Na(NH₂SO₃) were stable up to 50 and 250 ℃, respectively. Li(NH₂SO₃) underwent two steps of weight loss when the temperature increased. The first step started at 50 ℃ and ended at 200 ℃, which corresponds to the loss of 0.5 molecules of N₂ and 1 molecule of H₂. The observed weight loss of 15.39% is very close to the calculated value of 15.53% for Li(NH₂SO₃). The second weight loss (84.86%) occurred between 200 and 670 ℃, which is attributable to the elimination of 0.5 SO₂ and 0.5 Li₂SO₄ molecules (Calcd. 84.47%). The decomposition reaction is Li(NH₂SO₃)→ 0.5N₂+H₂ +0.5SO₂ +0.5Li₂SO₄. Na(NH₂SO₃) also underwent two steps of weight loss when the temperature increased. The first step occurred between 250 and 400 ℃, corresponding to the loss of 0.5 molecules of N₂ and 1 molecule of H₂. The observed weight loss of 11.17% is close to the calculated 13.44%. The second step occurred between 400 and 450 ℃, corresponding to the loss of 0.5 SO₂ molecules. The observed weight loss of 32.75% is consistent with the calculated 26.89%. The remaining part of the 0.5 Na₂SO₄ molecules gradually lost weight as the temperature increased until the sample lost all weight (due to the maximum temperature set for testing at 800 ℃, subsequent curves cannot be made). The decomposition reaction is Na(NH₂SO₃)→0.5N₂+H₂+0.5SO₂+0.5Na₂SO₄.

  Figure 3

  

  Figure 3.  TG curves of (a) Li(NH₂SO₃) and (b) Na(NH₂SO₃)

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  2.3   IR spectra

  The IR spectra for the compounds Li(NH₂SO₃) and Na(NH₂SO₃) were measured in a wavenumber range of 400-4 000 cm^-1 at room temperature (Fig. 4). The IR spectra of the two compounds were very similar. Taking Li(NH₂SO₃) as an example, the absorption bands occurring at 3 356 and 3 277 cm^-1 are attributable to the stretching vibration of the N—H bond, and the absorption band occurring at 1 573 cm^-1 is ascribed to the in-plane bending vibration of the N—H bond, the absorption bands occurring at 738 and 673 cm^-1 are ascribed to the out of plane bending vibration of the N—H bond. The absorption bands observed at 1 241, 1 156, and 1 058 cm^-1 are ascribed to S—O vibrations. The absorption band at 557 cm^-1 is ascribed to the vibration of the Li—O bond.

  Figure 4

  

  Figure 4.  IR spectra of (a) Li(NH₂SO₃) and (b) Na(NH₂SO₃)

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  2.4   UV-Vis-NIR diffuse reflectance spectra

  The UV-Vis-NIR diffuse reflectance spectra of Li(NH₂SO₃) and Na(NH₂SO₃) are shown in Fig. 5. The spectra indicated that the optical band gaps for compounds Li(NH₂SO₃) and Na(NH₂SO₃) were ca. 5.25 and 4.81 eV, respectively, corresponding to absorption edg-es of 236 and 258 nm, respectively.

  Figure 5

  

  Figure 5.  UV-Vis-NIR diffuses reflectance spectra of (a) Li(NH₂SO₃) and (b) Na(NH₂SO₃)

  The inset shows the corresponding band gap.

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  2.5   Powder SHG response

  Based on the polar structure with a space group of Pca2₁ and P2₁2₁2₁, the powder SHG response was measured by the Kurtz-Perry method. The SHG intensities increased with the increasing of particle size in a range of 26-280 µm (Fig. 6a), indicating both Li(NH₂SO₃) and Na(NH₂SO₃) are phase-match crystals at the 1 064 nm laser. As shown in Fig. 6b, Li(NH₂SO₃) and Na(NH₂SO₃) showed SHG intensities of about 0.32 times and 0.31 times that of KDP in the particle size range of 105-150 µm, respectively. The SHG response of crystals correlates with the induced dipole moments reflecting the ease of electron motion.

  Figure 6

  

  Figure 6.  (a) Phase-matching curve of Li(NH₂SO₃) and Na(NH₂SO₃) with 1064 nm laser radiation; (b) Oscilloscope traces of the SHG signals for powders of Li(NH₂SO₃) and Na(NH₂SO₃) and KDP in the same particle size range of 105-150 µm

  KDP was used as a reference for the SHG measurement at 1 064 nm.

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  2.6   Theoretical calculation

  To further understand the structure-function relationship of Li(NH₂SO₃) and Na(NH₂SO₃), theoretical calculations using the CASTEP program based on DFT were performed. The band structure diagram (Fig. 7a and 7b) shows that the minimum conduction band (CBM) and maximum valence band (VBM) of Li(NH₂SO₃) and Na(NH₂SO₃) were located at the G point, with direct band gaps of 5.47 and 4.99 eV, respectively, which is greater than the experimental value of 5.25 and 4.81 eV. The discrepancy can be attributed to the limitations of the exchange-correlation function GGA-PBE^[48-49]. Fig. 7c and 7d show the total density of states (DOS) and partial density of states (PDOS) diagrams of Li(NH₂SO₃) and Na(NH₂SO₃). Due to their similarity, we will take Li(NH₂SO₃) as an example to provide a detailed introduction. Observing the DOS of Li(NH₂SO₃), it is evident that the conduction band located between-10 and 0 eV is mainly occupied by O2p, N2p, and S3p orbitals, along with smaller contributions from Li1s, Li2p, H1s, and S3s orbitals. The valence band from 0 to 10 eV is mainly composed of S3p, Li2p, and O2p orbitals, as well as some Li1s, S3s, H1s, and N2s orbitals. The above calculations indicate that Li, O, N, and S atoms significantly affect the band structure and optical properties of Li(NH₂SO₃), which are mainly attributed to the synergistic effect of the [LiO₄]^7- polyhedra and the NH₂SO₃^- tetrahedra. Compared with Na(NH₂SO₃), the difference in band gaps between the two crystals is mainly due to the difference between the [LiO₄]^7- polyhedra and the [NaO₆]^11- octahedra.

  Figure 7

  

  Figure 7.  Band structure diagram of (a) Li(NH₂SO₃) and (b) Na(NH₂SO₃); DOS and PDOS diagrams of (c) Li(NH₂SO₃) and (d) Na(NH₂SO₃)

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  3.   Conclusions

  In summary, we have successfully synthesized two alkali-metal sulfamates nonlinear optical crystals, Li(NH₂SO₃) and Na(NH₂SO₃), through the facile evaporation method. Li(NH₂SO₃) and Na(NH₂SO₃) crystallize in the space group Pca2₁ and P2₁2₁2₁, respectively. The UV-Vis-NIR spectra demonstrated that Li(NH₂SO₃) and Na(NH₂SO₃) possessed large optical band gaps of 5.25 and 4.81 eV, respectively. Powder SHG measurements showed that the SHG intensity of Li(NH₂SO₃) and Na(NH₂SO₃) were 0.32 times and 0.31 times that of KDP, respectively. This research demonstrates that Li(NH₂SO₃) and Na(NH₂SO₃) are two promising UV NLO materials.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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  Figure 1  Crystal structure of Li(NH₂SO₃): (a)[Li₄O₈S₄] ring; (b) [Li₄O₈S₄]_∞ layer parallel to the ac plane; (c) [Li₃O₆S₃] ring; (d) [Li₃O₆S₃]_∞ layer parallel to the ab plane; (e) 3D framework structure of Li(NH₂SO₃)

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  Figure 2  (a) Arrangement of NH₂SO₃^- units in Na(NH₂SO₃); (b) Coordination environment of Na atoms and the [Na₃O₁₁] unit; (c) Chain structure of [Na₃O₁₁] units; (d) 3D framework of Na(NH₂SO₃)

  Symmetry codes: #1: 0.5+x, 0.5-y, 1-z; #2:-0.5+x, 0.5-y, 1-z; #3: x, y, z; #4:-1+x, y, z; #5:-x, -0.5+y, 1.5-z.

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  Figure 3  TG curves of (a) Li(NH₂SO₃) and (b) Na(NH₂SO₃)

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  Figure 4  IR spectra of (a) Li(NH₂SO₃) and (b) Na(NH₂SO₃)

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  Figure 5  UV-Vis-NIR diffuses reflectance spectra of (a) Li(NH₂SO₃) and (b) Na(NH₂SO₃)

  The inset shows the corresponding band gap.

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  Figure 6  (a) Phase-matching curve of Li(NH₂SO₃) and Na(NH₂SO₃) with 1064 nm laser radiation; (b) Oscilloscope traces of the SHG signals for powders of Li(NH₂SO₃) and Na(NH₂SO₃) and KDP in the same particle size range of 105-150 µm

  KDP was used as a reference for the SHG measurement at 1 064 nm.

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  Figure 7  Band structure diagram of (a) Li(NH₂SO₃) and (b) Na(NH₂SO₃); DOS and PDOS diagrams of (c) Li(NH₂SO₃) and (d) Na(NH₂SO₃)

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