English

• 1. INTRODUCTION

  Ultraviolet (UV) (λ < 400 nm) nonlinear optical (NLO) crystals, which are the key components of the all-solid-state UV lasers, have played a unique and crucial role in many newly developed scientific and technical areas such as lithography technology, photoelectron spectroscopy, laser spectroscopy, biophysics, laser medicine, etc^[1-4]. Due to the faulty physicochemical properties or growth difficulties of existing UV NLO crystals and the higher requirements for the comprehensive performance of crystals in practical applications, UV NLO crystals with high conversion efficiency under high-intensity lasers are extremely rare^[5-8]. Thus, it is particularly urgent to develop new high-performance UV NLO materials to break through the currently existing performance bottlenecks.

  At present, it is generally accepted that a superior UV NLO material should meet these criterions as follows: 1) a large second harmonic generation (SHG) coefficient (d_ij > 0.39 pm/V); 2) a wide UV transparency range; 3) an appropriate birefringence (0.07~0.1 at 1064 nm) for achieving the phase matching in the UV region; 4) good physicochemical and mechanical properties; 5) the easy growth of high-quality large single crystals. Over the past few decades, through protracted and unremitting efforts of a number of scientists, borates have been discovered to mostly satisfy the above conditions, e. g. β-BaB₂O₄ (BBO)^[9], LiB₃O₅ (LBO)^[10], KBe₂BO₃F₂ (KBBF)^[11], and SrBe₂BO₇ (SBBO)^[12]. Subsequently, the material systems for UV NLO application were further expanded to carbonates, fluorooxoborates, fluoro-phosphates, nitrates, sulfates, etc^[13-20]. However, ''Mr Perfect'' in UV NLO materials doesn't exist. Especially, the practical applications of many phosphates are limited seriously because of their inappropriate birefringence, recently. Hence, how to strike a balance among these above-mentioned criterions in a material becomes the core challenge for designing and synthesizing new UV NLO materials. Herein, this work is to provide a selective overview of UV NLO crystal materials rather than a comprehensive review of the rich literatures. We will briefly discuss the recent progresses of UV NLO materials including the evolution of functional groups, the expansion of material systems and the development of synthetic strategies. Furthermore, we present an outlook on the prospects of the development of UV NLO materials.

  2. EVOLUTION OF FUNCTIONAL GROUPS IN UV NLO MATERIALS

  Generally speaking, the fundamental attributes of a compound are decided by its crystal structure more than its chemical composition. Thus, fundamental functional groups directly affect the physical and chemical properties of materials. For NLO materials, especially in UV region, anionic groups make the major contribution for the linear and nonlinear optical properties. In borates, the classical UV NLO material system, the fundamental building units, are the triangular BO₃ and tetrahedral BO₄. And complex polyanions could be formed via the interconnection of BO₃ and BO₄, such as B₃O₆ in BBO and B₃O₇ in LBO. Thereinto, the BO₃ group possessing the planar triangle structure with π-conjugated molecular orbitals has been proved to be the best NLO basic structural unit for UV light generation owing to the relatively large microscopic second-order susceptibility and moderate optical anisotropy. Moreover, the increase of polymerization degree of the coplanar BO₃ groups would enhance the anisotropic polarizability. Analogous to BO₃ group, CO₃ and NO₃ groups are expected to be good NLO micro-structural units. Recently, carbonates and nitrates have been proved to be the potential candidates for UV NLO materials^[21-28]. Subsequently, planar group containing delocalized π bonds has been extended to C₃N₃O₃. Cyanurates exhibit strong NLO coefficient and large birefringence which are promising UV NLO materials^[29]. Except for π-conjugated planar anionic groups, oxygen-containing tetrahedron anion units like PO₄, SO₄ and SiO₄ groups have also been employed to explore novel UV NLO materials. Though most of phosphates, sulfates and silicates exhibit short UV cut-off edges, the small birefringence caused by the nearly isotropic configuration of tetrahedron anion groups limits their practical application in UV region. Few types of traditional NLO functional groups result in a serious shortage of new high-performance UV NLO materials. Hence, change must be made. Recently, the introduction of fluorine into known functional groups opens a door for the exploration of new UV NLO materials. A series of new NLO-active functional groups have been discovered such as BO₃F, BO₂F₂, PO₃F and PO₂F₂ which display large anisotropic polarizability, resulting in the discovery of new excellent fluorooxoborates and fluorophosphates. Fig. 1 displayed some representative UV NLO-active functional groups.

  Figure 1

  

  Figure 1.  The reported representative UV NLO-active functional groups

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  At present, relatively single types of NLO functional groups fundamentally restrict the service performance of the materials. Therefore, it is urgent to carry out frequency doubling genetic engineering research, search for new frequency doubling genes, develop new NLO material systems, screen new high-performance NLO crystals, and then break through the performance bottleneck and continue to maintain China's leading position in UV NLO crystalline materials.

  3. EXPANSION OF UV NLO MATERIALS SYSTEMS

  So far, borates are still considered as the ideal candidates for linear and NLO materials in the UV region owing to their great superiorities including high probability of asymmetric structures, wide UV transparency windows, relatively strong second-order NLO coefficients and moderate birefringence. Chen and his collaborators developed consecutively a series of excellent UV NLO borates in the last 40 years. Some of them have been successfully commercialized such as BBO and LBO, resulting in that China gained the leadership in UV NLO crystals market. Tremendous efforts have been paid for the development of borates UV NLO crystals guided by the anionic group theory proposed by Chen, from the initial discovery of BBO, to LBO, CsB₃O₅ (CBO)^[30] and CsLiB₆O₁₀ (CLBO)^[31], then to KBBF, SBBO, K₂Al₂B₂O₇ (KABO)^[32], BaAl₂B₂O₇ (BABO), etc (Fig. 2). Among all the discovered borates, the KBBF crystal is a star material which could produce deep-UV coherent light through direct SHG methods. However, the strong layer habit in crystal growth and the high toxicity of raw materials severely restrict its practical application. After borates being systematically studied, carbonates and nitrates start to attract people's attention due to CO₃ and NO₃ groups being analogous to BO₃ group. In 2011, Ye's group discovered a family of fluoride carbonates, MNCO₃F (M = K, Rb, Cs; N = Ca, Sr, Ba)^[33] by introducing alkaline fluoride into carbonates which have been proved to be promising candidates for UV NLO application. Then they expanded the materials systems to nitrates and cyanurates.

  Figure 2

  

  Figure 2.  Representative borates UV NLO materials discovered by Professor Chuangtian Chen and his collaborators

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  Except for π-conjugated material systems, non π-conjugated systems have gradually caught people's attention very recently. Among them, phosphates were studied relatively systematically. Owing to the wide transmittance window, environmental friendliness, and favorable single-crystal growth habit, phosphates have emerged as potential UV NLO materials. A series of representative phosphates have been developed such as Ba₃P₃O₁₀X (X = Cl, Br)^[34], RbNaMgP₂O₇^[35] and so on. Subsequently, sulfates also showed their potential in UV NLO application^[36-40]. Unfortunately, the small polarizability anisotropy of the tetrahedral groups induces the generally small birefringence which would severely restrict their potential applications in UV region. For example, though the absorption edge of K₄Mg₄(P₂O₇)₃^[41] (170 nm) is far less than 200 nm, the actual SHG limit (494 nm) is far above 200 nm due to its tiny birefringence. In order to enhance the polarizability anisotropy of the tetrahedral groups, fluorine was integrated to produce novel superior functional groups such as BO₃F, BO₂F₂, PO₃F and PO₂F₂, resulting in the discovery of new UV NLO materials systems, eg.; fluorooxoborates and fluorophosphates. Pan's group systematically studied the fluorooxoborates. Series of novel fluorooxoborates have been successfully synthesized and confirmed to exhibit outstanding linear and nonlinear optical properties in UV region, especially in deep-UV region. Also fluorophosphates show potential in UV NLO application.

  4. DEVELOPMENT OF SYNTHETIC STRATEGIES FOR UV NLO MATERIALS

  At the beginning, the traditional synthetic method, "trial-and-error", was employed by scientists to explore new UV NLO materials, which is time-consuming and inefficient. Later several classical and semiclassical theoretical models have been proposed through analysis and summary of the structure-property relationships. Thereinto, the famous anionic group theory proposed by Chen^[42], that the macroscopic nonlinear coefficients result from the superposition of the microscopic second-order NLO polarizability of functional groups, was proved to be successful to guide the exploration of UV NLO materials, especially in borates, resulting in discovering a series of classical UV NLO crystals such as β-BaB₂O₄ (BBO), LiB₃O₅ (LBO), KBe₂BO₃F₂ (KBBF), and K₂Al₂B₂O₇ (KABO). It also works well in other materials systems with planar structure anionic groups such as carbonates, nitrates and cyanurates. However, it has certain limitation due to ignoring the contribution from cations.

  As we know, lacking a center of symmetry for materials is the necessary condition to exhibit UV NLO behavior. Hence, a general strategy has been proposed to design noncentrosymmetry (NCS) materials via the rational combination of different asymmetric functional groups. These asymmetric building units are as below: 1) distorted polyhedra with a d⁰ cation center resulting from second-order Jahn-Teller effect; 2) anionic groups with π-conjugate system; 3) polar displacement of a d¹⁰ cation center; 4) stereochemically active lone pair (SCALP) units. Although combining the asymmetric units during the synthesis steps significantly improved the possibility of macroscopic NCS, it would be ineffective in the exploration of new materials in the UV region for the redshift of UV absorption cutoff edges to visible region. Recently, an emerging highly effective method, the chemical substitution-oriented design, has been developed to discover high-performance UV NLO materials, where one or more fundamental building units in a prototype phase are replaced to produce a better property. The chemical substitution approach can be easily applied to tune and extend the desired properties of solid state materials, which includes monosite, dualsite, and multisite substitution. A series of excellent UV NLO materials like KBe₂BO₃F₂ (KBBF)-type, Ca₅(PO₄)₃(OH)-type, and KTiOPO₄ (KTP)-type phases have been successfully discovered using this method (Fig. 3)^[43-46].

  Figure 3

  

  Figure 3.  Structural comparison between prototype phase KTiOPO₄ (KTP) and KTP-type UV NLO material CsSbF₂SO₄ synthesized by chemical substitution-oriented design

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  5. PERSPECTIVE

  There is an endless race on making shorter wavelengths accessible as we go down the Moore's law road. Thus, new NLO crystals are urgently needed for laser technologies in the UV regions. In this article, we have summarized recent research progresses on UV NLO materials. In recent years, this area is undergoing rapid progress. Several important new materials systems have been developed and many excellent crystals have been synthesized. However, an important thing to note is that there are still many problems and huge opportunities though great efforts have been made in the design, prediction, screening and growth of new UV NLO materials in the past few decades.

  Material genes (functional groups) directly influence the properties of materials. It is critical to break through the confinement of the existing material genes, obtain effective material gene mutations and discover brand-new material genes for the exploration of new superior UV NLO materials. Two ways could achieve this goal: first, to explore new aggregation forms of known genes and expand their new functions, and second, to develop deep learning and data mining to design new material genes with commonalities based on establishing a large database of frequency-doubling material genes. After finding brand-new genes, how to establish controllable preparation methods and create a new system of inorganic UV NLO materials is another important consideration. Recently, aliovalent substitution based on a classical structure mode has been proven to be very effective compared with the time-consuming traditional trial and error method. The key is to search for more structure prototypes which present the remarkable tolerance toward structural distortion and chemical substitution in order to accommodate a large number of elements or functional units. Simultaneously, developing new crystal growth method is helpful to expand the UV NLO materials systems, for instance, the introduction of solvent-free synthesis and ionothermal synthesis methods could solve the crystal growth problem of easily hydrolytic materials systems. The greatest challenge in the current research of UV NLO materials is the growth of large size single crystals, especially for the practical applications. Though many new UV NLO materials have been discovered, the lack of large high quality single crystals hinders the further optical properties tests. Hence, the opportunities and challenges coexist for the ongoing study of UV NLO materials.

  
  
• 1. [1]

     Niu, S. Y.; Joe, G.; Zhao, H.; Zhou, Y. C.; Orvis, T.; Huyan, H. X.; Salman, J.; Mahalingam, K.; Urwin, B.; Wu, J. B.; Liu, Y.; Tiwald, T. E.; Cronin, S. B.; Howe, B. M.; Mecklenburg, M.; Haiges, R.; Singh, D. J.; Wang, H.; Kats, M. A.; Ravichandran, J. Giant optical anisotropy in a quasi-one-dimensional crystal. Nat. Photonics 2018, 12, 392–396. doi: 10.1038/s41566-018-0189-1

  2. [2]

     Ghosh, S.; Wang, W. H.; Mendoza, F. M.; Myers, R. C.; Li, X.; Samarth, N.; Gossard, A. C.; Awschalom, D. D. Enhancement of spin coherence using Q-factor engineering in semiconductor microdisc lasers. Nat. Mater. 2006, 5, 261–264. doi: 10.1038/nmat1587

  3. [3]

     Halasyamani, P. S.; Zhang, W. G. Viewpoint: inorganic materials for UV and deep-UV nonlinear-optical applications. Inorg. Chem. 2017, 56, 12077–12085. doi: 10.1021/acs.inorgchem.7b02184

  4. [4]

     Ok, K. M.; Chi, E. O.; Halasyamani, P. S. Bulk characterization methods for non-centrosymmetric materials: second-harmonic generation, piezoelectricity, pyroelectricity, and ferroelectricity. Chem. Soc. Rev. 2006, 35, 710–717. doi: 10.1039/b511119f

  5. [5]

     Guo, J. Y.; Tudi, A.; Han, S. J.; Yang, Z. H.; Pan, S. L. Sn₂B₅O₉Cl: a material with large birefringence enhancement activated via alkaline-earth metal substitution by tin. Angew. Chem., Int. Ed. 2019, 58, 17675–17678. doi: 10.1002/anie.201911187

  6. [6]

     Dong, X. H.; Long, Y.; Zhao, X. Y.; Huang, L.; Zeng, H. M.; Lin, Z. E.; Wang, X.; Zou, G. H. A₆Sb₄F₁₂(SO₄)₃ (A = Rb, Cs): two novel antimony fluoride sulfates with unique crown-like clusters. Inorg. Chem. 2020, 59, 8345–8352. doi: 10.1021/acs.inorgchem.0c00756

  7. [7]

     Wang, Q.; Yang, F.; Wang, X.; Zhou, J.; Huang, L.; Gao, D. J.; Bi, J.; Zou, G. H. Deep-ultraviolet mixed-alkali-metal borates with induced enlarged birefringence derived from the structure rearrangement of the LiB₃O₅. Inorg. Chem. 2019, 58, 5949–5955. doi: 10.1021/acs.inorgchem.9b00271

  8. [8]

     Wang, Q.; Lin, C. S.; Zou, G. H.; Liu, M. J.; Gao, D. J.; Bi, J.; Huang, L. K₂[B₃O₃(OH)₅]: a new deep-UV nonlinear optical crystal with isolated [B₃O₃(OH)₅]^2– anionic groups. J. Alloys Compd. 2018, 735, 677–683. doi: 10.1016/j.jallcom.2017.11.174

  9. [9]

     Jiang, A. D. A new-type ultraviolet SHG crystal — β-BaB₂O₄. Scientia. Sinica. Series B 1985, 28, 235–243.

  10. [10]

      Chen, C. T.; Wu, Y. C.; Jiang, A. D.; Wu, B. C.; You, G. M.; Li, R. K.; Lin, S. J. New nonlinear-optical crystal: LiB₃O₅. J. Opt. Soc. Am. B 1989, 6, 616–621. doi: 10.1364/JOSAB.6.000616

  11. [11]

      Mei, L. F.; Wang, Y.; Chen, C. T.; Wu, B. C. Nonlinear optical materials based on MBe₂-BO₃F₂ (M = Na, K). J. Appl. Phys. 1993, 74, 7014–7015. doi: 10.1063/1.355060

  12. [12]

      Chen, C. T.; Wang, Y. B.; Wu, B. C.; Wu, K. C.; Zeng, W. L.; Yu, L. H. Design and synthesis of an ultraviolet-transparent nonlinear optical crystal Sr₂Be₂B₂O₇. Nature 1995, 373, 322–324. doi: 10.1038/373322a0

  13. [13]

      Song, J. L.; Hu, C. L.; Xu, X.; Kong, F.; Mao, J. G. A facile synthetic route to a new SHG material with two types of parallel p-conjugated planar triangular units. Angew. Chem., Int. Ed. 2015, 54, 3679–3682. doi: 10.1002/anie.201412344

  14. [14]

      Dong, X. Y.; Jing, Q.; Shi, Y. J.; Yang, Z. H.; Pan, S. L.; Poeppelmeier, K. R.; Young, J.; Rondinelli, J. M. Pb₂Ba₃(BO₃)₃Cl: a material with large SHG enhancement activated by Pb-Chelated BO₃ groups. J. Am. Chem. Soc. 2015, 137, 9417–9422. doi: 10.1021/jacs.5b05406

  15. [15]

      Li, T.; Qi, X. J.; Li, J.; Zeng, H. M.; Zou, G. H.; Lin, Z. E. Using multifunctional ionic liquids in the synthesis of crystalline metal phosphites and hybrid framework solids. Inorg. Chem. 2018, 57, 14031–14034. doi: 10.1021/acs.inorgchem.8b02519

  16. [16]

      Luan, L. D.; Li, J.; Chen, C.; Lin, Z. E.; Huang, H. Solvent-free synthesis of crystalline metal phosphate oxalates with a (4, 6)-connected fsh topology. Inorg. Chem. 2015, 54, 9387–9389. doi: 10.1021/acs.inorgchem.5b01569

  17. [17]

      Dong, X. H.; Huang, L.; Hu, C. F.; Zeng, H. M.; Lin, Z. E.; Wang, X.; Ok, K. M.; Zou, G. H. CsSbF₂SO₄: an excellent ultraviolet nonlinear optical sulfate with a KTiOPO₄ (KTP)-type structure. Angew. Chem., Int. Ed. 2019, 58, 6528–6534. doi: 10.1002/anie.201900637

  18. [18]

      Dong, X. H.; Huang, L.; Liu, Q. Y.; Zeng, H. M.; Lin, Z. E.; Xu, D. G.; Zou, G. H. Perfect balance harmony in Ba₂NO₃(OH)₃: a beryllium-free nitrate as a UV nonlinear optical material. Chem. Commun. 2018, 54, 5792–5795. doi: 10.1039/C8CC03007C

  19. [19]

      Shi, G. Q.; Wang, Y.; Zhang, F. F.; Zhang, B. B.; Yang, Z. H.; Hou, X. L.; Pan, S. L.; Poeppelmeier, K. R. Finding the next deep-ultraviolet nonlinear optical material: NH₄B₄O₆F. J. Am. Chem. Soc. 2017, 139, 10645–10648. doi: 10.1021/jacs.7b05943

  20. [20]

      Wu, B. L.; Hu, C. L.; Mao, F. F.; Tang, R. L.; Mao, J. G. Highly polarizable Hg²⁺ induced a strong second harmonic generation signal and large birefringence in LiHgPO₄. J. Am. Chem. Soc. 2019, 141, 10188–10192. doi: 10.1021/jacs.9b05125

  21. [21]

      Zou, G. H.; Jo, H.; Lim, S. J.; You, T. S.; Ok, K. M. Rb₃VO(O₂)₂CO₃: a four-in-one carbonatoperoxovanadate exhibiting an extremely strong second-harmonic generation response. Angew. Chem., Int. Ed. 2018, 57, 8619–8622. doi: 10.1002/anie.201804354

  22. [22]

      Zou, G. H.; Lin, Z. E.; Zeng, H. M.; Jo, H.; Lim, S. J.; You, T. S.; Ok, K. M. Cs₃VO(O₂)₂CO₃: an exceptionally thermostable carbonatoperoxovanadate with an extremely large second-harmonic generation response. Chem. Sci. 2018, 9, 8957–8961. doi: 10.1039/C8SC03672A

  23. [23]

      Zhang, Y. T.; Long, Y.; Dong, X. H.; Wang, L.; Huang, L.; Zeng, H. M.; Lin, Z. E.; Wang, X.; Zou, G. H. Y₈O(OH)₁₅(CO₃)₃Cl: an excellent short-wave UV nonlinear optical material exhibiting an infrequent three-dimensional inorganic cationic framework. Chem. Commun. 2019, 55, 4538–4541. doi: 10.1039/C9CC00581A

  24. [24]

      Zou, G. H.; Lin, C. S.; Kim, H. G.; Jo, H.; Ok, K. M. Rb₂Na(NO₃)₃: a congruently melting UV-NLO crystal with a very strong second-harmonic generation response. Crystals 2016, 6, 42. doi: 10.3390/cryst6040042

  25. [25]

      Wang, L.; Yang, F.; Zhao, X. Y.; Huang, L.; Gao, D. J.; Bi, J.; Wang, X.; Zou, G. H. Rb₃SbF₃(NO₃)₃: an excellent antimony nitrate nonlinear optical material with a strong second harmonic generation response fabricated by a rational multi-component design. Dalton Trans. 2019, 48, 15144–15150. doi: 10.1039/C9DT03233A

  26. [26]

      Huang, L.; Wang, Q.; Lin, C. S.; Zou, G. H.; Gao, D. J.; Bi, J.; Ye, N. Synthesis and characterization of a new beryllium-free deep-ultraviolet nonlinear optical material: Na₂GdCO₃F₃. J. Alloys Compd. 2017, 724, 1057–1063. doi: 10.1016/j.jallcom.2017.07.120

  27. [27]

      Wang, Q.; He, F. F.; Huang, L.; Gao, D. J.; Bi, J.; Zou, G. H. Exploring potential beryllium-free, deep-ultraviolet optical crystals in the rare earth fluoride carbonate-water system. Cryst. Growth Des. 2018, 18, 3644–3653. doi: 10.1021/acs.cgd.8b00431

  28. [28]

      Li, Q. F.; Zou, G. H.; Lin, C. S.; Ye, N. Synthesis and characterization of CsSrCO₃F — a beryllium-free new deep-ultraviolet nonlinear optical material. New J. Chem. 2016, 40, 2243–2248. doi: 10.1039/C5NJ03059E

  29. [29]

      Lin, D. H.; Luo, M.; Lin, C. S.; Xu, F.; Ye, N. KLi(HC₃N₃O₃)·2H₂O: solvent-drop grinding method toward the hydro-isocyanurate nonlinear optical crystal. J. Am. Chem. Soc. 2019, 141, 3390–3394. doi: 10.1021/jacs.8b13280

  30. [30]

      Wu, Y. C.; Sasaki, T.; Nakai, S.; Yokotani, A.; Tang, H.; Chen, C. T. CsB₃O₅: a new nonlinear optical crystal. Appl. Phys. Lett. 1993, 62, 2614–2615. doi: 10.1063/1.109262

  31. [31]

      Tu, J. M.; Keszler, D. A. CsLiB₆O₁₀: a noncentrosymmetric polyborate. Mater. Res. Bull. 1995, 30, 209–215. doi: 10.1016/0025-5408(94)00121-9

  32. [32]

      Hu, Z. G.; Higashiyama, T.; Yoshimura, M.; Yap, Y. K.; Mori, Y.; Sasaki., T. A new nonlinear optical borate crystal K₂Al₂B₂O₇ (KAB). Jpn. J. Appl. Phys. 1998, 37, L1093–L1094. doi: 10.1143/JJAP.37.L1093

  33. [33]

      Zou, G. H.; Ye, N.; Huang, L.; Lin, X. S. Alkaline-alkaline earth fluoride carbonate crystals ABCO₃F (A = K, Rb, Cs; B = Ca, Sr, Ba) as nonlinear optical materials. J. Am. Chem. Soc. 2011, 133, 20001–20007. doi: 10.1021/ja209276a

  34. [34]

      Yu, P.; Wu, L. M.; Zhou, L. J.; Chen, L. Deep-ultraviolet non-linear optical crystals: Ba₃P₃O₁₀X (X = Cl, Br). J. Am. Chem. Soc. 2014, 136, 480–487. doi: 10.1021/ja411272y

  35. [35]

      Zhao, S. G.; Yang, X. Y.; Yang, Y.; Kuang, X. J.; Lu, F. Q.; Shan, P.; Sun, Z. H.; Lin, Z. S.; Hong, M. C.; Luo, J. H. Non-centrosymmetric RbNaMgP₂O₇ with unprecedented thermo-induced enhancement of second harmonic generation. J. Am. Chem. Soc. 2018, 140, 1592–1595. doi: 10.1021/jacs.7b12518

  36. [36]

      Yang, F.; Huang, L. J.; Zhao, X. Y.; Huang, L.; Gao, D. J.; Bi, J.; Wang, X.; Zou, G. H. An energy band engineering design to enlarge the band gap of KTiOPO₄ (KTP)-type sulfates via aliovalent substitution. J. Mater. Chem. C 2019, 7, 8131–8138. doi: 10.1039/C9TC02180A

  37. [37]

      He, F. F.; Wang, L.; Hu, C. F.; Zhou, J.; Li, Q.; Huang, L.; Gao, D. J.; Bi, J.; Wang, X.; Zou, G. H. Cation-tuned synthesis of the A₂SO₄·SbF₃ (A = Na⁺, NH₄⁺, K⁺, Rb⁺) family with nonlinear optical properties. Dalton Trans. 2018, 47, 17486–17492. doi: 10.1039/C8DT04400G

  38. [38]

      Wang, Q.; Wang, L.; Zhao, X. Y.; Huang, L.; Gao, D. J.; Bi, J.; Wang, X.; Zou, G. H. Centrosymmetric K₂SO₄·(SbF₃)₂ and noncentrosymmetric Rb₂SO₄·(SbF₃)₂ resulting from cooperative effects of lone pair and cation size. Inorg. Chem. Front. 2019, 6, 3125–3132. doi: 10.1039/C9QI01036J

  39. [39]

      He, F. F.; Deng, Y. L.; Zhao, X. Y.; Huang, L.; Gao, D. J.; Bi, J.; Wang, X.; Zou, G. H. RbSbSO₄Cl₂: an excellent sulfate nonlinear optical material generated due to the synergistic effect of three asymmetric chromophores. J. Mater. Chem. C 2019, 7, 5748–5754. doi: 10.1039/C9TC01249D

  40. [40]

      He, F. F.; Wang, Q.; Hu, C. F.; He, W.; Luo, X. Y.; Huang, L.; Gao, D. J.; Bi, J.; Wang, X.; Zou, G. H. Centrosymmetric (NH₄)₂SbCl(SO₄)₂ and noncentrosymmetric (NH₄)SbCl₂(SO₄): synergistic effect of hydrogen-bonding interactions and lone-pair cations on the framework structures and macroscopic centricities. Cryst. Growth Des. 2018, 18, 6239–6247. doi: 10.1021/acs.cgd.8b01102

  41. [41]

      Yu, H. W.; Young, J.; Wu, H. P.; Zhang, W. G.; Rondinelli, J. M.; Halasyamani, P. S. M₄Mg₄(P₂O₇)₃ (M = K, Rb): structural engineering of pyrophosphates for nonlinear optical applications. Chem. Mater. 2017, 29, 1845–1855. doi: 10.1021/acs.chemmater.7b00167

  42. [42]

      Chen, C.; Sasaki, T.; Li, R. K.; Wu, Y.; Lin, Z.; Mori, Y.; Hu, Z.; Wang, J.; Uda, S.; Yoshimura, M.; Kaneda, Y. Nonlinear Optical Borate Crystals; Wiley-VCH: Weinheim, Germany 2012.

  43. [43]

      Zou, G. H.; Lin, C. S.; Jo, H.; Nam, G.; You, T. S.; Ok, K. M. Pb₂BO₃Cl: a tailor-made polar lead borate chloride with very strong second harmonic generation. Angew. Chem., Int. Ed. 2016, 128, 12257–12261. doi: 10.1002/ange.201606782

  44. [44]

      Zou, G. H.; Ok, K. M. Novel ultraviolet (UV) nonlinear optical (NLO) materials discovered by chemical substitution-oriented design. Chem. Sci. 2020, 11, 5404–5409. doi: 10.1039/D0SC01936D

  45. [45]

      Yang, F.; Wang, L.; Huang, L.; Zou, G. H. The study of structure evolvement of KTiOPO₄ family and their nonlinear optical properties. Coord. Chem. Rev. 2020, 423, 213491. doi: 10.1016/j.ccr.2020.213491

  46. [46]

      Yue, Z.; Lu, Z.; Xue, H.; Guo, S. KBiCl₂SO₄: the first bismuth chloride sulfate being second-order nonlinear optical active. Cryst. Growth Des. 2019, 7, 3843–3850.

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  Figure 1  The reported representative UV NLO-active functional groups

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  Figure 2  Representative borates UV NLO materials discovered by Professor Chuangtian Chen and his collaborators

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  Figure 3  Structural comparison between prototype phase KTiOPO₄ (KTP) and KTP-type UV NLO material CsSbF₂SO₄ synthesized by chemical substitution-oriented design

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