Sulfate Derivatives with Heteroleptic Tetrahedra: New Deep-Ultraviolet Birefringent Materials in which Weak Interactions Modulate Functional Module Ordering

Deep-ultraviolet (UV) birefringent materials are urgently needed to facilitate light polarization in deep-UV lithography. Maximizing anisotropy by regulating the alignment of functional modules is essential for improving the linear optical performance of birefringent materials. In this work, we proposed a strategy to design deep-UV birefringent materials that achieve functional module ordering via weak interactions. Following this strategy, four compounds CN4H7SO3CF3, CN4H7SO3CH3, C(NH2)3SO3CH3, and C(NH2)3SO3CF3 were identified as high-performance candidates for deep-UV birefringent materials. The millimeter-sized crystals of CN4H7SO3CF3, CN4H7SO3CH3, and C(NH2)3SO3CH3 were grown, and the transmittance spectra show that their cutoff edges are below 200 nm. CN4H7SO3CF3 exhibits the largest birefringence (0.149 @ 546 nm, 0.395 @ 200 nm) in the deep-UV region among reported sulfates and sulfate derivatives. It reveals that the hydrogen bond can modulate the module ordering of the heteroleptic tetrahedra and planar π-conjugated cations, thus greatly enhancing the birefringence. Our study not only discovers new deep-UV birefringent materials but also provides an upgraded strategy for optimizing optical anisotropy to achieve efficient birefringence.

Gradual Increase in Birefringence of Antimony Oxalates through Precise Tuning of the Sb3+/[C2O4]2- Ratio

Birefringent crystals play a crucial role in modulating and controlling the polarization of light in the optical communication and laser industries. Recently, adopting appropriate strategies to enhance the birefringence of crystals has become a prominent area of research focus. Herein, four UV antimony oxalate birefringent crystals, namely, K5Sb2(C2O4)5.5·3H2O, BaSb(C2O4)2.5·3H2O, Na4Sb2O(C2O4)4·6H2O, and Na3Sb(C2O4)2F2·2H2O, have been successfully synthesized. These compounds feature a similar zero-dimensional (0D) cluster structure and share the same functional groups, including π-conjugated [C2O4]2- groups and Sb3+-based distorted polyhedra with stereochemically active lone pairs (SCALPs). Interestingly, we achieved a stepwise increase in birefringence by precisely controlling the Sb3+/[C2O4]2- ratio, ultimately resulting in the compound Na3Sb(C2O4)2F2·2H2O exhibiting a large birefringence (0.21@546 nm). Additionally, we confirmed that the synergistic effects between the π-conjugated [C2O4]2- groups and the distorted polyhedra based on Sb3+ are responsible for the excellent optical properties observed in the reported compounds.

(C4H6N3O)(HSO4): A Cytosinium Bisulfate with Large Birefringence and Moderate Second Harmonic Generation Effect Produced via Combining a Promising Planar Nonlinear Optical-Active Motif with a Tetrahedral Group

Investigating novel nonlinear optical (NLO) active units serves as a valuable method for broadening the research landscape of NLO materials. This study showcases the potential of the cytosinium cation (C4H6N3O)+ as a novel NLO-active motif through theoretical calculations. The title compound exhibited a wide band gap of 3.85 eV, along with a moderate second harmonic generation (SHG) response of 1.65 times that of KH2PO4 (KDP) and significant birefringence of 0.47. Its exceptional optical properties are primarily attributed to the synergy interaction between cations and anionic groups in the asymmetric unit.

Two Short-Wave UV Beryllium Selenites Exhibiting Diverse Optical Properties Stemming from Functional Group Arrangements

The arrangement of functional groups exerts a crucial role in determining the characteristics of compounds. In this study, we synthesized two novel short-wave ultraviolet (UV) nonlinear optical (NLO) crystals: KBe2(SeO3)2(OH)·H2O and K2Be(SeO3)2. Interestingly, the two compounds show the same SeO3 triangular pyramids and K-O polyhedra. However, the two compounds exhibit distinct beryllium-oxygen anion groups: BeO3(OH) for KBe2(SeO3)2(OH)·H2O and BeO4 for K2Be(SeO3)2. This results in the SeO3 groups within the structure having different orientations, ultimately leading to the two compounds exhibiting completely different optical properties. KBe2(SeO3)2(OH)·H2O displays a large second harmonic generation (SHG) effect equivalent to 2× KH2PO4 (KDP), coupled with a large birefringence of 0.078 at 546 nm. In contrast, the SHG effect and birefringence of K2Be(SeO3)2 are only 0.33× that of KDP and 0.024 at 546 nm, respectively. Structural analyses and theoretical calculations indicate that these pronounced differences in optical properties stem from variations in the arrangement of the SeO3 functional groups. This study not only sheds light on the correlation between crystal structure and optical behavior but also presents a hopeful avenue for the advancement of materials in the short-wave UV spectrum.

Exceptional Optical Anisotropy Enhancement Achieved Through Dual-Ions Cosubstitution Strategy in Novel Hybrid Bismuth Halides

Guided by a superb dual-ions cosubstitution strategy, two novel, highly optically anisotropic hybrid bismuth halides are designed and synthesized. The first compound, Gu3Bi2NO3Cl8 (Gu = C(NH2)3), is developed using the 2D perovskite halide Cs3Bi2Cl9 as the maternal structure. This involved substituting all Cs+ cations with organic Gu+ and replacing some Cl- anions with [NO3]-. Further substitution of Cl- with additional [NO3]- resulted in the formation of nitrate-rich Gu2Bi(NO3)3Cl2 crystal, exhibiting a 3.4-fold increase in [NO3]- per unit volume. Both compounds have a structurally 0D nature, comprising bismuth-centered polyhedra formed by coordinated chlorides and monodentate/bidentate nitrate moieties, with Gu+ serving as a separator and linker. Notably, the presence of superb optically anisotropic dual-ions, i.e., planar Gu+ and [NO3]-, enables these crystals to possess sharply enhanced optical anisotropy, with birefringence values more than 1 order of magnitude higher than that of the initial crystal Cs3Bi2Cl9 (0.162/0.186vs 0.011 at 546 nm). The discovery and characterization of Gu3Bi2NO3Cl8 and Gu2Bi(NO3)3Cl2 crystals provide new insights into achieving expected modifications in optical properties through the utilization of a dual-ions cosubstitution strategy.

(C8H7N2O2)2[Bi2Br8]·2H2O and (C8H7N2O2)6[Bi2Cl10]Cl2·2H2O: Exploring Birefringent Crystals in Hybrid Halide Systems

In this study, new hybrid birefringent crystals of (C8H7N2O2)2[Bi2Br8]·2H2O and (C8H7N2O2)6[Bi2Cl10]Cl2·2H2O were successfully synthesized by introducing a new birefringent group [C8H7N2O2]+ by a simple aqueous solution evaporation method. They crystallize in the P21/n space group, and their structure consists mainly of the π-conjugated group [C8H7N2O2]+ and the octahedron centered on Bi3+. By first-principles calculations, the birefringence response comes from the [C8H7N2O2]+ group with a planar π-conjugated structure. Meanwhile, the synthesis, structure, first-principles calculations, and optical properties are reported in this paper.

Hg4(Te2O5)(SO4): A Giant Birefringent Sulfate Crystal Triggered by a Highly Selective Cation

Sulfate crystals are often criticized for their low birefringence. The small anisotropic SO4 group is becoming the biggest bottleneck hindering the application of sulfates in optical functional materials. In this study, we report a new method to significantly enhance the birefringence of sulfates. The title compound increases the birefringence recording of sulfates to 0.542@546 nm, which is significantly larger than that of the commercial birefringent crystal of TiO2 (0.306@546.1 nm). At the infrared wavelength, the birefringence of Hg4(Te2O5)(SO4) can be up to 0.400@1064 nm, which is also much larger than the infrared birefringent crystal of YVO4 (0.209@1064 nm). In addition, it also has a wide transparency range, high thermal stability, and excellent environmental stability, making it a potential birefringent material. Hg4(Te2O5)(SO4) features a novel two-dimensional layered structure composed of [Hg4(Te2O5)]2+ layers separated by isolated (SO4)2- tetrahedra. This compound was designed by introducing a highly selective cation in a tellurite sulfate system. The low valence low coordination cations connect with tellurite groups only, making the sulfate isolated in the structure. The steric repulsive action of the isolated SO4 tetrahedra may regulate the linear and lone pair groups arranged in a way that favors large birefringence. This method can be proven by theoretical calculations. PAWED studies showed that the large birefringence originated from the synergistic effect of (Hg2O2)2-, (Te2O5)2-, and (SO4)2- units, with a contribution ratio of 42.17, 37.92, and 19.88%, respectively. Our work breaks the limitation of low birefringence in sulfates and opens up new possibilities for their application as birefringent crystals.

[C(NH2)3]6Mo7O24: A Guanidinium Molybdate as a UV Nonlinear Optical Crystal with Large Birefringence

The key to searching novel nonlinear optical (NLO) crystals was effectively combining the NLO-active units to obtain a noncentrosymmetric structure. Nevertheless, the present predicament lies in the growing challenge of discovering novel crystals within conventional inorganic frameworks that surpass the properties of the current NLO materials. In view of this, researchers expanded their research focus to the organic-inorganic hybridization system; it is foreseeable to concentrate the advantages from several kinds of NLO-active units to acquire novel NLO crystals with superior properties. We herein report an organic-inorganic hybrid molybdate crystal, namely, [C(NH2)3]6Mo7O24 (GMO). It was successfully obtained via combining inorganic NLO-active MoO6 octahedra and organic π-conjugated [C(NH2)3]+ groups. GMO demonstrates a moderate second-harmonic-generation response, specifically measuring about 1.3 times the value of KDP. Additionally, it exhibits a significant birefringence value of 0.203 at the wavelength of 550 nm and possesses a wide band gap of 3.31 eV. Theoretical calculations suggest that the optical properties of the GMO are primarily influenced by the synergy effect of [C(NH2)3]+ groups between MoO6 octahedra.

Guanidinium Vanadate [C(NH2)3]3VO4·2H2O Revealing Enhanced Second-Harmonic Generation and Wide Band Gaps

A new guanidinium-templated vanadate, [C(NH2)3]3VO4·2H2O, has been synthesized in a phase-pure form. It crystallizes in a noncentrosymmetric polar space group, Cc, and the crystal structure is built upon a framework of guanidinium, vanadate tetrahedra, and water molecules linked by hydrogen bonds. Notably, optical measurements reveal that the material exhibits an approximately 9.6-fold enhancement in second-harmonic generation efficiency compared to its phosphate analogue. The enhancement can be attributed to the increased geometrical distortion of the VO4 tetrahedra. Furthermore, we found that the coordination number of the central vanadium atom significantly affects the optical band gaps. Among various coordination numbers, the 4-coordinate VO4 tetrahedra are found to be more favorable for widening the optical band gap of materials compared to the 5- and 6-coordinate vanadium polyhedra, as demonstrated by this work.

Two Short-Wave UV Antimony(III) Sulfates Exhibiting Large Birefringence

In the present work, we have successfully obtained two new UV antimony-based sulfates, NH4Sb(SO4)2 and Ca2Sb2O(SO4)4, by a conventional hydrothermal method. Interestingly, both compounds share similar structural building blocks, such as SbO4 seesaws and SO4 tetrahedra, yet they endow discrepant birefringence values measured at 546 nm with values of 0.150 and 0.114, respectively, owing to the different distortions of the SbO4 groups with SCALP electrons. Moreover, both compounds display large band gaps (4.32 and 4.43 eV, respectively), so they can be used as short-wavelength UV birefringent materials. Moreover, NH4Sb(SO4)2 is a noncentrosymmetric compound, showing a frequency doubling effect of 0.2 × KDP. Detailed structural analyses and calculations confirm the source of superior optical performance and the reasons for the different birefringence of the two compounds. This work provides ideas for the following discovery of antimony-based optical materials with excellent properties.

Two van der Waals Layered Antimony(III) Phosphites as UV Optical Materials

Herein, two new Sb3+-based phosphites, Sb2O2(HPO3) (I) and Sb2O(HPO3)2 (II), were successfully obtained by ingeniously combining Sb3+-based polyhedra containing stereochemically active lone pair (SCALP) and HPO3 polar groups. Both reported compounds exhibit unique 2D van der Waals layered structures, [Sb4O4(HPO3)2]∞ and [Sb2O(HPO3)2]∞, respectively, which favors compounds with large optical anisotropy. Interestingly, the different curvatures of the two layers resulted in the two title compounds showing significantly different birefringences (0.079@546 and 0.046@546 nm, respectively). Both compounds endow wide optical band gaps (4.32 and 4.54 eV, respectively), which indicates their potential as promising ultraviolet (UV) birefringent crystals. The synthesis of the two title compounds enriched Sb3+-based phosphites in the UV region and provided guidance for the subsequent synthesis of superior optical materials.

[C(NH2)2NHNO2][C(NH2)3](NO3)2: A Mixed Organic Cationic Hybrid Nitrate with an Unprecedented Nonlinear-Optical-Active Unit

We herein report a mixed organic cationic hybrid nitrate, namely, [C(NH₂)₂NHNO₂][C(NH₂)₃](NO₃)₂ (1). It was successfully achieved via combining three different planar groups: [(C(NH₂)₂NHNO₂]⁺, C(NH₂)₃⁺, and NO₃^-. First-principles calculations confirm that the [(C(NH₂)₂NHNO₂]⁺ group is an excellent cationic nonlinear-optical (NLO)-active unit. The title compound exhibits a moderate second-harmonic-generation (SHG) response (1.5 × KDP), a wide band gap (3.58 eV), and a suitable birefringence of 0.071 at 550 nm. Theoretical calculations indicate that the synergy effect between the [(C(NH₂)₂NHNO₂]⁺ and C(NH₂)₃⁺ groups dominates the SHG process.

Two Mixed-Alkali-Metal Selenates as Short-Wave Ultraviolet Nonlinear-Optical Materials

Two novel mixed-alkali-metal selenate nonlinear-optical (NLO) crystals, Na₃Li(H₂O)₃(SeO₄)₂·3H₂O (I) and CsLi₃(H₂O)(SeO₄)₂ (II), have been successfully synthesized by an aqueous solution evaporation method. Both compounds feature the unique layers constructed of the same functional moieties including SeO₄ and LiO₄ tetrahedra: [Li(H₂O)₃(SeO₄)₂·3H₂O]_∞^3- layers in I and [Li₃(H₂O)(SeO₄)₂]_∞^- layers in II. The titled compounds display wide optical band gaps of 5.62 and 5.66 eV, respectively, according to the UV-vis spectra. Interestingly, they exhibit significantly different second-order nonlinear coefficients (0.34 × KDP and 0.70 × KDP, respectively). Detailed dipole moment calculations manifest that the large disparity can be attributed to the difference in the dipole moment of the crystallographically independent SeO₄ and LiO₄ groups. This work confirms that alkali-metal selenate system is an excellent candidate for short-wave ultraviolet NLO materials.

SbHPO3F: 2D van der Waals Layered Phosphite Exhibiting Large Birefringence

A novel antimony(III)-based phosphite, SbHPO₃F, featuring a unique two-dimensional (2D) van der Waals layered structure, has been successfully designed and synthesized via the simultaneous employment of optically active moieties including SbO₃F seesaw and tetrahedral HPO₃ groups. Its optimized layered arrangement formed by the alternating connection of 4-membered rings (4-MRs) and 8-MRs endows the title compound with desirable optical properties including a large birefringence and short ultraviolet (UV) cutoff edge, implying that it is a potential UV birefringent material.

Large optical anisotropy differentiation induced by the anion-directed regulation of structures

The modulation of optical anisotropy for two novel UV birefringent materials [C(NH2)3]2Sb3F3(HPO3)4 and [C(NH2)3]SbFPO4·H2O has been successfully achieved via anion-directing regulation structures.