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• In the realm of optics, birefringent crystal materials play a pivotal role in controlling light polarization, finding extensive applications in optical communication, polarization microscopy, laser phase matching, and various other domains [1-8]. Over the years, significant strides have been made in discovering and commercially utilizing numerous birefringent materials, including MgF₂ [9], CaCO₃ [10], YVO₄ [11], α-BaB₂O₄ (α-BBO) [12], among others. Nonetheless, certain inherent shortcomings, such as limited birefringence, challenges in crystallization, and restricted utility in the ultraviolet (UV) spectrum, constrain their practical efficacy. Consequently, there exists a pressing need to explore UV optical crystals endowed with substantial birefringence.

  The birefringence of a crystal is essentially caused by the anisotropic response of the electron distribution to an applied electric field, and the following groups have been shown to enhance the birefringence of the compound due to their strong anisotropic response: (1) Planar π-conjugated anion groups, including NO₃⁻, CO₃²⁻ and BO₃³⁻. For example, the birefringence of LiZn(OH)CO₃ [13] is 0.147@1064 nm, NH₄B₄O₆F [14] is 0.1171@1064 nm, and C₆H₁₄N₂(NO₃)₃·H₃O [15] is 0.110@546 nm. (2) Cations with stereochemically active lone pair electrons (SCALP) (Sb³⁺, Sn²⁺, Pb²⁺, Bi³⁺), such as Sn₂B₅O₉Cl (0.168@546 nm) [16], RbSbSO₄F₂ (0.10@1064 nm) [17]. (3) Distorted tetrahedral moieties, such as BaB₈O₁₂F₂ (0.116@1064 nm) [18].

  It is well-established that the presence of lone pair electrons significantly influences the birefringence of compounds like Cs₂Pb(NO₃)₂Br₂ (0.147@546 nm) [19], Pb₂BO₃Cl (0.120@1064 nm) [20], K₂Sn₂(C₂O₄)₂·H₂O (0.103@546 nm) [21]. This influence arises from the ability of cations bearing lone pairs to form highly distorted polyhedral, where coordination atoms cluster on one side, resulting in pronounced anisotropic polarization. Consequently, Sb³⁺, with lone pair electrons, and versatile coordination patterns to induce compounds with diverse structures, emerges as a promising candidate for constructing excellent birefringent materials. In our previous researches, we have placed our emphasis on the contribution of Sb³⁺ to the optical properties of compounds [22], and obtained a series of antimony sulfates and phosphates [23,24]. Sulfates and phosphates not only have relatively straightforward crystal growth but also provide a wide transmission band in the UV region. However, sulfate or phosphate anions possess nearly symmetric tetrahedral configurations, exhibiting minimal polarization anisotropy and contributing little to birefringence [25,26]. To achieve a larger birefringence, it is necessary to enhance the contribution of the anions. By seeking suitable inorganic oxygenate anions, we conducted theoretical calculations to assess the polarizability anisotropy (δ) of various inorganic anionic groups NO₃⁻, CO₃²⁻, BO₃³⁻, SeO₄²⁻, PO₄³⁻ and SO₄²⁻ using the Gaussian09 package at the theoretical 6–31 G (d, p) level [27]. Our results, depicted in Fig. 1, demonstrate that compared to SeO₄²⁻, PO₄³⁻ and SO₄²⁻ ions with tetrahedral configurations, as well as other planar CO₃²⁻, BO₃³⁻ groups, the NO₃⁻ anionic group stands out with the highest polarizability anisotropy, rendering it an optimal choice for our study.

  Figure 1

  

  Figure 1.  Comparison of polarizability anisotropy (δ) of NO₃⁻, CO₃²⁻, BO₃³⁻, SeO₄²⁻, PO₄³⁻ and SO₄²⁻ anionic groups.

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  Following the principles outlined before, we conducted detailed experimental exploration of the Sb³⁺-NO₃⁻ system, while searching the ICSD database for compounds with unexplored physical properties that could potentially exhibit excellent birefringent characteristics [28-30]. Five examples of antimony potassium fluoronitrate compounds: SbF₃·KNO₃ (1), SbF₃·3KNO₃ (2), SbF₃·3KSbF₄·KNO₃ (3), KSb₂F₇·3KNO₃ (4), and KSb₂F₇·2KNO₃ (5) were successfully synthesized. The five title compounds are essentially combinations of KNO₃ with various antimony complexes, including [SbF₃], [SbF₄], and their derivatives. Variations in the fundamental structural units and bonding patterns lead to significant structural differences among these compounds, which further lead to significant variations in performance. Notably, compound 5 exhibits exceptionally large birefringence (0.30@546 nm), representing the highest reported birefringence in all the inorganic antimony oxysalts.

  Compounds 1 and 2 each incorporate discrete [SbF₃] units arranged in a trigonal pyramidal geometry. Within these two compounds, the Sb atom coordinates with three F atoms to form [SbF₃] polyhedra, and the N atom is coordinated by three O atoms, constituting the NO₃⁻ anion. Both compounds, as illustrated in Figs. 2a and b, independent [SbF₃] polyhedral coexist with NO₃⁻ anions, and K⁺ cations, with the latter functioning as charge balancers within the crystal lattice. Compound 3 consists of a mixture of [SbF₃] and [SbF₄] units. The [SbF₃], [SbF₄] polyhedra, NO₃⁻ anionic groups, and K⁺ atoms are independently arranged, with K⁺ serving as a charge balancer (Fig. 2c). Compounds 4 and 5 consist of individual [SbF₄] moieties that assemble into [Sb₂F₇] clusters. In these two compounds, the NO₃⁻ groups are orderly positioned within the [Sb₂F₇] cluster's cavity, with K⁺ cations providing charge balance (Figs. 2d and e). In all the compounds, Sb-F bond lengths are in the range of 1.925–2.382 Å, and N—O bond lengths varying from 1.230 Å to 1.277 Å. All the crystallographic data are presented in Tables S1-S11 (Supporting information).

  Figure 2

  

  Figure 2.  The structural representations of compounds (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5.

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  X-ray powder diffraction of compounds 1 to 5 was conducted (Fig. S2 in Supporting information), confirming the phase purity by validating that the experimental results aligned with the fitting results obtained from single crystal X-ray diffraction. To assess the thermal stability, thermogravimetric analysis (TGA) was conducted on compounds 1 to 5 (Fig. S3 in Supporting information), and the heating products at 800 ℃ were identified using X-ray powder diffraction (Fig. S4 in Supporting information).

  Infrared spectroscopy was employed to verify the presence of specific functional groups within the target compounds (Fig. S5 in Supporting information), and the characteristic peaks are aligning with previous studies [31,32]. The UV–vis diffuse reflection spectra were analyzed (Fig. S6 in Supporting information), and their respective band gaps are 3.81, 3.83, 3.87, 3.73, and 3.75 eV [33,34], respectively. These findings suggest that the investigated compounds hold promise as UV crystalline materials. Birefringence measurements were conducted for compounds 1 to 5 at a wavelength of 546 nm using a Zeiss Axio A5 polarizing microscope. The optical range differences (ΔR), with corresponding thicknesses (d) were measured, and subsequently, yielding birefringence (Δn) values of 0.09, 0.14, 0.16, 0.18, and 0.30 at the wavelength of 546 nm [35], respectively (Figs. 3a–e). After reviewing relevant literatures (Fig. 3g), it was noted that compound 5, among all inorganic antimony (Ⅲ) oxysalts, demonstrates the highest level of birefringence [36-49].

  Figure 3

  

  Figure 3.  Representations the experiment birefringence of (a) 1, (b) 2, (c) 3, (d) 4 and (e) 5. (f) The calculated birefringence for five compounds. (g) Birefringence comparison of the title compounds with other inorganic antimony (Ⅲ) oxysalts.

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  The selection and strategic integration of appropriate functional groups are critical for enhancing birefringence performance, as previously discussed. In this study, we explored the synergistic effect between Sb³⁺ cations, recognized for their versatile coordination behaviors and lone pair electrons, and the planar π-conjugated NO₃⁻ groups. However, for compounds 1–5, despite having identical functional groups, notable differences in birefringence were observed. These disparities may be primarily due to variations in the arrangement and concentration of the functional groups. Fig. 4 illustrates the spatial arrangement of [SbF_x] polyhedra and planar NO₃⁻ groups within a unit cell of compounds 1–5. The arrows of different colors in Figs. 4a–e indicate the various orientations of lone pair electrons on Sb³⁺. Completely opposite orientations are represented by the same color, reflecting their identical contributions to the birefringence. Meanwhile, the planes depicted in different colors (Figs. 4f–j) represent NO₃⁻ planes oriented in various directions. A detailed discussion below addresses the factors contributing to the observed discrepancies in birefringence among the five compounds.

  Figure 4

  

  Figure 4.  Orientations of the lone pair electrons on Sb³⁺ in a unit cell for (a) 1, (b) 2, (c) 3, (d) 4 and (e) 5. Orientations of NO₃⁻ planes in a unit cell for (f) 1, (g) 2, (h) 3, (i) 4 and (j) 5.

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  As demonstrated in Figs. 4a–e and Table S12 (Supporting information), the [SbF₃] polyhedra in compound 1, 2 and 4 all manifest two distinct lone pair electron orientations, Sb-Ⅰ and Sb-Ⅱ, each with the density (ρ) of 0.003533/0.002579/0.002600 Å⁻³. The Sb³⁺ cations in compound 3 adopt two types of coordination modes, resulting in three distinct lone pair electron orientations with densities of 0.0005496 Å⁻³, 0.002198 Å⁻³, and 0.004396 Å⁻³, respectively. In contrast, the [SbF₄] polyhedra in compound 5 are uniformly oriented in a single direction and possess a higher density of 0.006513 Å⁻³. The consistent orientation and high density of lone pair electrons in compound 5 significantly contribute to enhancing the birefringence. Figs. 4f–j illustrate the orientations of the NO₃⁻ planes within the unit cells of various compounds. As detailed in Figs. 4f–j and Table S12, compound 1 features four distinct NO₃⁻ orientations (NO₃⁻-Ⅰ, NO₃⁻-Ⅱ, NO₃⁻-Ⅲ, and NO₃⁻-Ⅳ), each with a density of 0.001767 Å⁻³. The NO₃⁻ planes in a unit cell of compound 2 exhibit six orientations with respective densities of 0.002579 Å⁻³ (two planes), and 0.001719 Å⁻³ (four planes). Both 1 and 2 exhibit mussy NO₃⁻ orientations. Compound 3 and 5′s NO₃⁻ planes are uniformly aligned in a single orientation, with a density of 0.002198 Å⁻³ and 0.006513 Å⁻³, respectively. In compound 4, the NO₃⁻ planes are present in two orientations, with densities of 0.003901 Å⁻³ and 0.002600 Å⁻³. Upon comparing these compounds, it is evident that the NO₃⁻ planes in compounds 3 and 5 are well-aligned, contributing to their substantial birefringence values. Specifically, compound 5, which has a higher NO₃⁻ planes density, exhibits the largest birefringence of 0.30 at 546 nm, while compound 3 shows enhanced birefringence of 0.16. Despite having two orientations of NO₃⁻ planes, compound 4, with a higher density in its unit cell than in compound 3, demonstrates a birefringence of 0.18 nm at 546 nm. Conversely, compounds 1 and 2, although possessing higher NO₃⁻ planes densities compared to compounds 3, 4, and 5, display smaller birefringence values of 0.09 and 0.14 at 546 nm, respectively, due to the more chaotic orientations of their NO₃⁻.

  The analysis above affirms that consistency in the orientation of functional groups and higher densities are key factors in achieving greater birefringence.

  Given its classification within the NCS Cmc2₁ space group, the compound 2 was subjected to second harmonic generation (SHG) testing, and it reveals a pronounced frequency doubling signal, approximately 1.7 times greater than that of KDP (Fig. S7 in Supporting information).

  To elucidate the mechanism underlying the optical properties of the five title compounds, first-principles calculations were performed using the CASTEP software suite, based on density functional theory (DFT). The refractive index dispersion curves for compounds 1 to 5 were calculated, demonstrating appropriate anisotropy (Fig. 3f). Among these compounds, 1, 2, 4, and 5 are biaxial crystals, while compound 3 is a uniaxial crystal. Taking compound 1 as an example of biaxial crystal, the refractive indices follow the order n > n > n, where n, and n represent the refractive indices in the x, y, and z directions, respectively. Using the formula Δn = n − n, the calculated birefringence of compound 1 is 0.10 at 546 nm. Similarly, the birefringence values for compounds 2, 4, and 5 are calculated to be 0.13, 0.19, and 0.29 at 546 nm, respectively. For the uniaxial crystal (compound 3), the birefringence is calculated using the formula Δn = |n − n|, resulting in a value of 0.17 at 546 nm. Notably, all calculated values align well with the experimental measurements. In addition, the nonlinear coefficient of compound 2 was also calculated. It has a nonzero independent SHG coefficient (d₂₄) of 1.6 × 10⁻⁹ esu at 1.167 eV (Fig. S7c in Supporting information), which is consistent with the experimental result. The band gaps for compounds 1–5 were calculated to be 3.33, 3.27, 3.85, 3.20 and 3.44 eV (Fig. S8 in Supporting information), respectively. These values are somewhat lower than their respective experimental values attributable to the typical underestimation of the band gap by GGA-PBE calculations [50,51].

  The total and partial state densities (TDOS and PDOS) are presented in Fig. S9 (Supporting information). The TDOS and PDOS profiles of the compounds are similar due to their structural similarities, with compound 5 detailed as a representative example. The valence bands of compound 5, primarily derived from O-2s, O-2p, F-2p, N-2p, and Sb-5p orbitals, with minor contributions from N-2s and Sb-5s orbitals in the range of −10 eV to 0 eV, are depicted. From 0 eV to 8 eV, the primary contributions are from N-2p, O-2p, and Sb-5p orbitals. PDOS analysis indicates a strong overlap between N-2p and O-2p orbitals, suggesting robust covalent interactions within the delocalized π-conjugated NO₃⁻ groups. Furthermore, overlaps between the 5s and 5p orbitals of Sb with F-2p and O-2p states from −5 eV to 5 eV indicate the presence of Sb-O/F bond interactions in compound 5. Optical properties in these compounds are predominantly influenced by electronic transitions near the Fermi level, with significant contributions from the [SbF₄] polyhedra and NO₃⁻ groups. This is confirmed by the electron density difference maps shown in Fig. S10e (Supporting information), where an increase in the electron cloud density around the O atoms suggests electron transfer within the π-conjugated NO₃⁻ units. Concurrently, the highly asymmetric lobes on Sb³⁺ indicate that the lone electron pairs are stereochemically active. Therefore, the synergistic effect between the π-conjugated NO₃⁻ ions and the distorted Sb³⁺ polyhedra generates significant optical anisotropy, resulting in substantial birefringence in compound 5, analogous to that observed in compounds 1–4.

  In summary, five antimony-based nitrates have been successfully synthesized by strategically combining planar π-conjugated NO₃⁻ anions with Sb³⁺ cations possessing lone-pair electrons. Notably, KSb₂F₇·2KNO₃ (5) demonstrates the highest reported birefringence value of 0.30@546 nm among all known inorganic antimony oxysalts. This enhancement is attributed to the consistent orientation of functional groups and their high densities, underscoring its substantial potential in the domain of birefringent materials. This research significantly contributes to the exploration and rational design of novel, high-performance, structure-driven functional materials.

  Declaration of competing interests

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Qin Wang: Writing – review & editing, Writing – original draft, Data curation. Han Luo: Software. Luli Wang: Data curation. Ling Huang: Writing – review & editing. Liling Cao: Formal analysis. Xuehua Dong: Formal analysis. Guohong Zou: Investigation.

  Acknowledgments

  The authors thank Dr. Daichuan Ma at Analytical and Testing Center, Sichuan University for technical help in the Material Studio calculations. This work was supported by the National Natural Science Foundation of China (Nos. 22122106, 22071158, 22375139, 22305166).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110173.

  
  
• 1. [1]

     M.F. Weber, C.A. Stover, L.R. Gilbert, et al., Science 287 (2000) 2451–2456.

  2. [2]

     P.S. Halasyamani, K.R. Poeppelmeier, Chem. Mater. 10 (1998) 2753–2769.

  3. [3]

     S. Liu, X.M. Liu, S.G. Zhao, et al., Angew. Chem. Int. Ed. 59 (2020) 9414–9417. doi: 10.1002/anie.202002828

  4. [4]

     C. Wu, X.X. Jiang, Z.J. Wang, et al., Angew. Chem. Int. Ed. 60 (2021) 14806–14810. doi: 10.1002/anie.202102992

  5. [5]

     W.Q. Huang, X. Zhang, Y.Q. Li, et al., Angew. Chem. Int. Ed. 61 (2022) e202202746.

  6. [6]

     P. Zhang, X. Mao, X.H. Dong, et al., Chin. Chem. Lett. 35 (2024) 109235.

  7. [7]

     D. Zhang, Q. Wang, T. Zheng, et al., Sci. China Mater. 65 (2022) 3115–3124. doi: 10.1007/s40843-022-2088-0

  8. [8]

     G.J. Yi, G.H. Zou, Chin. J. Struct. Chem. 42 (2023) 100020.

  9. [9]

     F. Sedlmeir, R. Zeltner, G. Leuchs, et al., Opt. Express 22 (2014) 30934–30942.

  10. [10]

      G. Ghosh, Opt. Commun. 163 (1999) 95–102.

  11. [11]

      H.T. Luo, T. Tkaczyk, E.L. Dereniak, et al., Opt. Lett. 31 (2006) 616–618.

  12. [12]

      G.Q. Zhou, J. Xu, X.D. Chen, et al., J. Cryst. Growth 191 (1998) 517–519. doi: 10.2307/1592678

  13. [13]

      X.M. Liu, L. Kang, P., . F. . Gong, et al., Angew. Chem. Int. Ed. 60 (2021) 13574–13578. doi: 10.1002/anie.202101308

  14. [14]

      G.Q. Shi, Y. Wang, F.F. Zhang, et al., J. Am. Chem. Soc. 139 (2017) 10645–10648. doi: 10.1021/jacs.7b05943

  15. [15]

      J. Li, X. Lin, Q. Cao, et al., J. Alloys Compd. 908 (2022) 164632.

  16. [16]

      J.Y. Guo, A. Tudi, S.J. Han, et al., Angew. Chem. Int. Ed. 58 (2019) 17675–17678. doi: 10.1002/anie.201911187

  17. [17]

      F. Yang, L.J. Huang, X.Y. Zhao, et al., J. Mater. Chem. C 7 (2019) 8131–8138. doi: 10.1039/c9tc02180a

  18. [18]

      Z.Z. Zhang, Y. Wang, H. Li, et al., Inorg. Chem. Front. 6 (2019) 546–549. doi: 10.1039/c8qi01263f

  19. [19]

      Y. Long, X.H. Dong, H.M. Zeng, et al., Inorg. Chem. 61 (2022) 4184–4192. doi: 10.1021/acs.inorgchem.2c00047

  20. [20]

      G.H. Zou, C.S. Lin, H. Jo, et al., Angew. Chem. Int. Ed. 55 (2016) 12078–12082. doi: 10.1002/anie.201606782

  21. [21]

      L.Y. Ren, L.H. Cheng, X.Y. Zhou, et al., Inorg. Chem. Front. 10 (2023) 5602–5610. doi: 10.1039/d3qi01213a

  22. [22]

      D. Zhang, Q. Wang, L.Y. Ren, et al., Inorg. Chem. 61 (2022) 12481–12488. doi: 10.1021/acs.inorgchem.2c02262

  23. [23]

      X.H. Dong, L. Huang, C.H. Hu, et al., Angew. Chem. Int. Ed. 58 (2019) 6528–6534. doi: 10.1002/anie.201900637

  24. [24]

      Y.L. Deng, L. Huang, X.H. Dong, et al., Angew. Chem. Int. Ed. 59 (2020) 21151–21156. doi: 10.1002/anie.202009441

  25. [25]

      P. Yu, L.M. Wu, L.J. Zhou, et al., J. Am. Chem. Soc. 136 (2014) 480–487. doi: 10.1021/ja411272y

  26. [26]

      Y.Q. Li, F. Liang, S.G. Zhao, et al., J. Am. Chem. Soc. 141 (2019) 3833–3837. doi: 10.1021/jacs.9b00138

  27. [27]

      M.J. Frisch, G.W. Trucks, H.B. Schlegel, et al., Gaussian 09, Revision B. 01, Gaussian Inc., Wallingford, 2009.

  28. [28]

      M. Bourgault, M.B. Ducourant, D. Mascherpa-Corral, et al., J. Fluorine Chem. 17 (1981) 215–224.

  29. [29]

      M. Bourgault, M.B. Ducourant, R. Fourcade, J. Solid State Chem. 50 (1983) 79–85.

  30. [30]

      A.A. Udovenko, L.M. Volkova, R.L. Davidovich, et al., Koord. Khim. 5 (1979) 133–137.

  31. [31]

      X.H. Dong, Y. Long, L. Huang, et al., J. Mater. Chem. C 10 (2022) 17577–17582. doi: 10.1039/d2tc04112j

  32. [32]

      Q. Wang, J.X. Ren, D. Wang, et al., Inorg. Chem. Front. 10 (2023) 2107–2114. doi: 10.1039/d2qi02637f

  33. [33]

      P. Kubelka, F. Munk, Z. Tech. Physik 12 (1931) 593–601.

  34. [34]

      J. Tauc, Mater. Res. Bull. 5 (1970) 721–729.

  35. [35]

      L.L. Cao, G. Peng, W.B. Liao, et al., CrystEngComm 22 (2022) 1956–1961.

  36. [36]

      X.H. Dong, Y. Long, X.Y. Zhao, et al., Inorg. Chem. 59 (2020) 8345–8352. doi: 10.1021/acs.inorgchem.0c00756

  37. [37]

      Q. Wei, C. He, K. Wang, et al., Chem. Eur. J. 27 (2021) 5880–5884. doi: 10.1002/chem.202004859

  38. [38]

      L. Wang, H.M. Wang, D. Zhang, et al., Inorg. Chem. Front. 8 (2021) 3317–3324. doi: 10.1039/d1qi00395j

  39. [39]

      F. Yang, L. Wang, Y.W. Ge, et al., J. Alloys Compd. 834 (2020) 155154.

  40. [40]

      F.F. He, L. Wang, C.F. Hu, et al., Dalton Trans. 47 (2018) 17486–17492. doi: 10.1039/c8dt04400g

  41. [41]

      F.F. He, Y.W. Ge, X.Y. Zhao, et al., Dalton Trans. 49 (2020) 5276–5282. doi: 10.1039/d0dt00846j

  42. [42]

      F.F. He, Q. Wang, C.F. Hu, et al., Cryst. Growth Des. 18 (2018) 6239–6247. doi: 10.1021/acs.cgd.8b01102

  43. [43]

      Q. Wang, L. Wang, X.Y. Zhao, et al., Inorg. Chem. Front. 6 (2019) 3125–3132. doi: 10.1039/c9qi01036j

  44. [44]

      F.F. He, Y.L. Deng, X.Y. Zhao, et al., J. Mater. Chem. C 7 (2019) 5748–5754. doi: 10.1039/c9tc01249d

  45. [45]

      Y. Lan, H. Luo, L.L. Wang, et al., Inorg. Chem. 63 (2024) 2814–2820. doi: 10.1021/acs.inorgchem.3c04404

  46. [46]

      Q. Wei, K. Wang, C. He, et al., Inorg. Chem. 60 (2021) 11648–11654. doi: 10.1021/acs.inorgchem.1c01653

  47. [47]

      X.H. Dong, H.B. Huang, L. Huang, et al., Angew. Chem. Int. Ed. 63 (2024) e202318976.

  48. [48]

      Y. Lan, J.X. Ren, P. Zhang, et al., Chin. Chem. Lett. 35 (2024) 108652.

  49. [49]

      Y.C. Liu, X.M. Liu, S. Liu, et al., Angew. Chem. Int. Ed. 59 (2020) 7793–7796. doi: 10.1002/anie.202001042

  50. [50]

      J. Chen, C.L. Hu, F.F. Mao, Angew. Chem. Int. Ed. 58 (2019) 2098–2102. doi: 10.1002/anie.201813968

  51. [51]

      M. Yan, R.L. Tang, W.D. Yao, et al., Chem. Sci. 15 (2024) 2883–2888. doi: 10.1039/d3sc06683e

• 

  Figure 1  Comparison of polarizability anisotropy (δ) of NO₃⁻, CO₃²⁻, BO₃³⁻, SeO₄²⁻, PO₄³⁻ and SO₄²⁻ anionic groups.

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  Figure 2  The structural representations of compounds (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5.

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  Figure 3  Representations the experiment birefringence of (a) 1, (b) 2, (c) 3, (d) 4 and (e) 5. (f) The calculated birefringence for five compounds. (g) Birefringence comparison of the title compounds with other inorganic antimony (Ⅲ) oxysalts.

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  Figure 4  Orientations of the lone pair electrons on Sb³⁺ in a unit cell for (a) 1, (b) 2, (c) 3, (d) 4 and (e) 5. Orientations of NO₃⁻ planes in a unit cell for (f) 1, (g) 2, (h) 3, (i) 4 and (j) 5.

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