Nonpolar Na ₁₀ Cd(NO ₃ ) ₄ (SO ₃ S) ₄ Exhibits a Large Second-Harmonic Generation

Abstract

Open AccessCCS ChemistryCOMMUNICATION1 Feb 2022Nonpolar Na10Cd(NO3)4(SO3S)4 Exhibits a Large Second-Harmonic Generation Youchao Liu, Youquan Liu, Zheshuai Lin, Yanqiang Li, Qingran Ding, Xin Chen, Lina Li, Sangen Zhao, Maochun Hong and Junhua Luo Youchao Liu State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Google Scholar More articles by this author , Youquan Liu Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author , Zheshuai Lin Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author , Yanqiang Li State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of the Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Qingran Ding State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of the Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Xin Chen State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Google Scholar More articles by this author , Lina Li State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou, Fujian 350108 Google Scholar More articles by this author , Sangen Zhao *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of the Chinese Academy of Sciences, Beijing 100049 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou, Fujian 350108 Google Scholar More articles by this author , Maochun Hong State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou, Fujian 350108 Google Scholar More articles by this author and Junhua Luo State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of the Chinese Academy of Sciences, Beijing 100049 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou, Fujian 350108 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202000758 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Conventional wisdom says that nonpolar structures do not favorably produce strong second-harmonic generation (SHG) responses since the polarization in their microscopic functional groups are counteracted. Herein, we report the first nonlinear optical thiosulfate, Na10Cd(NO3)4(SO3S)4, which crystallizes in a nonpolar space group of P 4 − . However, this thiosulfate exhibits a strong SHG response of about 4.2 times that of the benchmark KH2PO4, which is larger than those of polar sulfates. According to first-principles calculations and a flexible dipole model, the SHG enhancement is mainly ascribed to the larger flexibility of the S=S bond in the SO3S tetrahedra in comparison with the S–O bond in the SO4 tetrahedra. These findings indicate that constructing flexible bonds is an effective strategy to design high-performance nonlinear optical materials regardless of polarity. Download figure Download PowerPoint Introduction Nonlinear optical (NLO) crystals1–3 are vital materials for portable all solid-state laser devices, which have been widely applied in modern laser micromachining, resource exploration, and laser communication.4 Second-harmonic generation (SHG) response is the most important criteria for a superior NLO crystal. Based on anionic group theory,5 it is well known that the total optical nonlinearity of a crystal is the geometrical superposition of the microscopic second-order susceptibility (polarization) of NLO-active anionic groups. Consequently, conventional wisdom is that nonpolar structures do not favorably produce strong SHG response as their microscopic NLO-active anionic groups are essentially counter packing to cancel out their polarization. Notably, this could only be a statistical experience, while actually, SHG responses are caused by the second-order nonlinear susceptibilities of the induced polarization in materials with non-centrosymmetric structures, rather than caused by the spontaneous polarization (permanent dipole moment) in materials with polar structures. Nevertheless, the search for new NLO materials especially focuses on how to construct polar structures. A variety of polar NLO materials have been discovered or predesigned, such as BaTeM2O9 (M = Mo and W),6 Li2ZrTeO6,7 Rb2CdBr2I2,8 ABi2(IO3)2F5 (A = K, Rb, and Cs),9 LiM(IO3)3 (M = Zn and Cd),10 Na2Be4B4O11,11 BaHgSe2,12 BaF2TeF2(OH)2,13 CsSbF2SO4,14 RbNaMgP2O7,15,16 Pb18O8Cl15I5,17 and Pb13O6Cl9Br5.18 In comparison, there are much fewer nonpolar NLO materials with strong SHG responses.19,20 Elemental substitution is a common but effective strategy to design novel NLO-active anionic groups and new NLO materials with enhanced SHG responses. For example, the substitution of oxygen by the much more electronegative fluorine gives rise to novel oxyfluoride NLO-active units, such as BO3F,21–23 BO2F2,24 PO3F,25,26 as well as the emerging SO3F,27 and SiO5F,28 resulting in polar NLO materials, such as NH4B4O6F,23 MB5O7F3 (M = Ca and Sr),21,22 (NH4)2PO3F,25,26 C(NH2)3SO3F,27 and CsSiP2O7F.28 Recently, the substitution of oxygen by the less electronegative sulfur also led to several polar NLO materials, such as CaCoSO,29 SrZn2S2O,30 and Sr6Cd2Sb6O7S10.31 In particular, Sr6Cd2Sb6O7S10 exhibits a strong SHG response owing to the highly polarizable Sb/O/S groups. In our previous works,32,33 we found that sulfates have very wide transparency windows to the deep-UV spectral region; nevertheless, the NLO-active SO4 tetrahedra possess rather small microscopic SHG coefficients. Guided by the aforementioned results, we expected that the substitution of oxygen by the less electronegative sulfur would help to enhance the SHG responses. In this work, we successfully synthesized the first NLO thiosulfate, Na10Cd(NO3)4(SO3S)4 ( I), by a facile solution-evaporation method. Unexpectedly, I crystallizes in a nonpolar structure but exhibits a strong SHG response up to 4.2 times that of KH2PO4 (KDP), which is larger than that of polar sulfates. Results and Discussion Rod-like single crystals of I (Figure 1a) were obtained by a slow evaporation method from water solution with a mixture of Cd(NO3)2·4H2O (0.771 g, 2.5 mmol) and Na2S2O3 (1.581 g, 10 mmol). The crystal structure was determined by single-crystal X-ray diffraction (XRD) analysis. The phase purity of I was confirmed by powder XRD ( Supporting Information Figure S1). Energy dispersive X-ray (EDX) spectroscopy measurements revealed the presence of Na/Cd/N/S elements and gave the molar ratio of Na/Cd/N/S of 9.58:1.06:4:8.21 ( Supporting Information Figure S2), which is consistent with the compositions determined by single-crystal XRD analysis. Figure 1 | The crystals and structure of I. (a) Single crystals of I. (b) Structure of I viewed along the c axis. (c) Isolated Cd(SO3S)4 unit. Download figure Download PowerPoint I crystallizes in a nonpolar non-centrosymmetric tetragonal space group of P 4 − ( Supporting Information Tables S1–S4). The structure of I is composed of isolated Cd(SO3S)4 units and NO3 anionic groups (Figure 1b). The central S6+ atom coordinates with three O atoms and one terminal S2− atom to form a SO3S tetrahedron. As illustrated in Figure 1c, every Cd2+ cation connects with four S2− ions to form CdS4 tetrahedra, which further share corners to four adjacent SO3S tetrahedra to construct isolated Cd(SO3S)4 units. Data in Supporting Information Table S2 show that the S–O bonds range from 1.448(4) to 1.462(4) Å and the S=S bond is 2.019(2) Å, both of which fall in the reasonable range.34,35 The Cd–S distances are 2.5285(15) Å, which are comparable with the previous report.36–38 The N–O bond lengths range from 1.228(6) to 1.252(6) Å. The Cd(SO3S)4 units and NO3 anionic groups also operate themselves through the 4 ¯ axis at (0, 0, 0) along the c axis and eventually crystallize in the P 4 − symmetry. Meanwhile, Na+ serves as a charge-balancing counter cation. To understand the nonpolarity of I, we calculated the local dipole moments for the CdS4, SO3S, and NO3 groups using a bond-valence approach proposed by Poeppelmeier et al.39,40 Detailed calculation results are shown in Supporting Information Table S5, which summarizes the direction and magnitude of the dipole moments. It is notable that the SO3S tetrahedra show very large dipole moments of 14.60 Debye ( Supporting Information Table S5). Clearly, the substitution of oxygen by sulfur leads to very large distortion for SO3S tetrahedra. In addition, the NO3 groups show a small dipole moment of 0.54 Debye. The magnitude of CdS4 tetrahedral dipole moment is zero owing to their regular tetrahedral configuration. As shown in Supporting Information Figure S3, the dipole moments of SO3S and NO3 groups cancel out themselves, thereby resulting in nonpolarity for the unit cell of I. The thermogravimetric (TG) and differential thermal analysis (DTA) curves ( Supporting Information Figure S4) reveal that I could be stable up to 540 K. The UV–vis–NIR diffuse reflectance spectrum for I was carried out with the PerkinElmer Lamda-950 UV/vis/NIR spectrophotometer. The UV/vis/NIR diffuse reflectance spectrum indicated that I is optically transparent from 1100 nm to 332 nm with a band gap of 3.74 eV ( Supporting Information Figure S5). This result coincides with the calculated direct band gap of 2.95 eV ( Supporting Information Figure S6). The relatively large band gap implies that I is likely to possess a high laser damage threshold. We measured the laser damage threshold of I with samples in the particle size of 150–212 μm by a solid-state laser (1064 nm, 10 ns, 1 Hz) with AgGaS2 as the reference.3 The measured laser damage threshold value of I was 144.4 MW cm−2, which is about 65 times larger than that of the reference AgGaS2 (2.2 MW cm−2). The powder SHG measurements were carried out using the Kurtz–Perry method41 on a 1064 nm Q-switched Nd:YAG laser and sieved KDP polycrystals as the standard sample. Figure 2a exhibits the curve obtained by the plot of SHG signals as a function of particle size of I. The SHG signals become larger with increasing particle sizes, which indicate that I is Type-I phase-matchable. As demonstrated in Figure 2b, I exhibits a relatively large SHG efficiency, which is approximately 4.2 times that of KDP in the same particle size of 180–212 μm at the wavelength of 1064 nm. Bright green light can be observed on the sample (see the insert picture of Figure 2b), which confirms that the oscilloscope signals are ascribed to the SHG responses. Notably, the SHG responses of I are larger than those of the polar sulfates, such as Li8NaRb3(SO4)6·2H2O (0.5 × KDP),33 NH4NaLi2(SO4)2 (1.1 × KDP),32 (NH4)2Na3Li9(SO4)7 (0.5 × KDP),32 CsSbF2SO4 (3.0 × KDP),14 and RbSbF2SO4 (0.96 × KDP).42 Figure 2 | NLO properties of I. (a) SHG intensity vs particle sizes at λ = 1064 nm for I. (b) SHG signals of I as compared with the KDP reference. Bright green light can be observed on the sample (the insert picture in b). Download figure Download PowerPoint According to the conventional wisdom, the counter-packing of NLO-active groups is unfavorable for producing strong SHG effects. To deeply investigate the relationships between excellent NLO properties and the crystal structure of I, we performed the first-principles calculations using the planewave pseudopotential method43,44 based on the density functional theory (DFT).45 Since the space group of I is P 4 − , there are three independent SHG coefficient tensors (d15, d14, and d24) under the restriction of Kleinman symmetry.46 The calculated SHG tensor components of I are given in Table 1, and the largest tensor (d15) is 1.69 pm V−1 @ 1064 nm, in good agreement with the experimental result (given d36(KDP) = 0.39 pm V−1).47 The calculated birefringence is approximately 0.01 at λ = 1064 nm ( Supporting Information Figure S7). The electronic density of states (DOS) and partial DOS of the respective species (Figure 3a) reveal that (1) the orbitals on central S6+ in the SO3S groups occupy the relatively deep region (−10.0 to −5.0 eV), while the terminal S2− is located at a high region near the valence band maximum (−5.0 to 0.0 eV). (2) The S2− and S6+ show large extents of hybridization in valence band low parts (−15.0 to −5.0 eV), demonstrating the formation of relatively strong covalent S=S bonds in the SO3S tetrahedra. (3) In the −8.5 and −6.4 eV, Cd 4d orbitals show some degree of hybridization with the orbitals of S2− due to the relatively strong electronegativity of Cd. More importantly, the upper regions in the valence band are dominantly composed of S2− 3p orbitals and O 2p orbitals, while the bottoms of the conduction band are dominantly composed of N 2p orbitals and O 2p orbitals. In comparison, the contribution from Na+ and Cd2+ cations to these electronic states is neglectably small. Since the optical properties are mainly associated with the electronic transition near the forbidden band edge, the optical properties of I are mainly determined by the SO3S tetrahedra and NO3 units. Figure 3 | Theoretical calculation of I. (a) DOS and partial DOS in I. (b) SHG-weighted densities of the occupied state in the virtual electron process. Download figure Download PowerPoint Figure 3b presents SHG-weighted densities of the occupied state in the virtual electron process, which visualizes the SHG contributions of every group/ion. Clearly, the SHG contributions of I are concentrated on the terminal S2− atoms. It means that the S=S covalent bonds of SO3S tetrahedra play a more important role in SHG responses than the S–O bonds and NO3 groups. The specific SHG contributions from the respective groups/ions were calculated by the real-space atom-cutting method.48–50 As shown in Table 1, the contributions from NO3, SO3S, and Cd groups/ions to the largest SHG coefficient d15 are 17.9%, 73.4%, and 8.6%, respectively. It indicates that the SHG effect is determined by the SO3S tetrahedra. To confirm this conclusion, we further adopted a flexible dipole model51 to calculate the flexibility index (F) in the SO3S tetrahedra. The flexibility index represents the flexibility of the electronic motion within the bond (or the compliance with the dipole moment). To perform a direct comparison, we constructed a hypothetical crystal Na10Cd(NO3)4(SO4)4 ( II), in which the terminal S2− anions in the Na10Cd(NO3)4(SO3S)4 structure were replaced by O2− ions to form the SO4 tetrahedra. As shown in Table 1, the S=S bonds in SO3S tetrahedra possess an evidently larger flexibility index (F = 0.195) as compared with S–O bonds (F = 0.140) in SO4 tetrahedra, indicating that S=S bonds are much easier than S–O bonds to form an induced dipole moment when the electrons are subjected to the external perturbation. Therefore, it is reasonable that the nonpolar I exhibits a stronger SHG response than polar sulfates. Further first-principles calculations (also see Table 1) show that the microscopic SHG response of SO3S tetrahedra (d15 = 1.25 pm V−1) is about 4.8 times that of SO4 tetrahedra (d15 = −0.26 pm V−1). These results unambiguously demonstrate that the substitution of one oxygen atom by a sulfur atom leads to flexible S=S bonds in SO3S tetrahedra, which is responsible to the enhanced SHG responses regardless of the nonpolar structure of I. Table 1 | The Calculated SHG Tensor Components for the Cd, SO3S, and NO3 Groups, and Total SHG Tensor Components in I. The SO4 Group and Total SHG Tensor in Na10Cd(NO3)4(SO4)4 (II), The Flexibility Index F of S=S, N–O, S–O Bonds Were Also Calculated d15 (pm V−1) d14 (pm V−1) d24 (pm V−1) F SO3S 1.25 −0.03 −1.25 FS=S = 0.195 Cd 0.15 −0.08 −0.15 — NO3 0.31 −0.22 −0.31 FN–O = 0.117 I 1.69 0.46 −1.69 — SO4 0.03 −0.26 −0.03 FS–O = 0.140 II 0.14 −0.43 −0.14 — Conclusions The first NLO thiosulfate Na10Cd(NO3)4(SO3S)4 has been synthesized by a facile method. It is nonpolar but exhibits a strong SHG response of 4.2 × KDP. According to first-principles calculations and a flexible dipole model, the substitution of oxygen by sulfur atom leads to flexible S=S bonds in SO3S tetrahedra and is responsible for the enhanced SHG responses of I. These findings demonstrate that the substitution of oxygen by the less electronegative sulfur to construct flexible bonds is favorable to predesign and synthesize new NLO materials with enhanced SHG responses even though they are nonpolar. Supporting Information Supporting Information is available and includes experimental details, single-crystal X-ray diffraction data, PXRD patterns, UV–vis–NIR diffuse reflectance spectroscopy, TG and DTA curves, EDX spectroscopy, calculated electronic band structure, and birefringence. CCDC number: 2008207. Conflict of Interest The authors declare no competing financial interest. Funding Information This work is financially supported by the National Natural Science Foundation of China (nos. 21833010, 61975207, 21921001, 21971238, 51872297, and 51890864), the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (no. ZDBS-LY-SLH024), the Strategic Priority Research Program of the Chinese Academy of Sciences (nos. XDB20010200 and XDB20000000), Fujian Institute of Innovation (no. 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