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Borate crystals can be chemically and functionally modified by the fluorination strategy, which encourages the identification of emerging fluorooxoborates with a structure and set of characteristics not seen in any other oxide parents. However, the bulk of fluorooxoborates have been found accidentally, rational methods of synthesis are required, particularly for the infrequently occurring poly-fluorinated components. Herein, we reported the use of bifluoride salts as a potent source of fluorine to prepare fluorooxoborates that contain rarely tri-fluorinated [BF 3 X] (X=O and CH 3 ) tetrahedra and eleven compounds were found. We identified the optical properties of the organofluorinated group [CH 3 BF 3 ] and their potential for nonlinear optics for the first time. Among these, two non-centrosymmetric components hold potential for the production of 266 nm harmonic coherent light for nonlinear optics, and more crucially, have the benefit of growing large size single crystals. Our study establishes experimental conditions for the coexistence of the diverse functional groups, enabling the production of poly-fluorinated optical crystals., (© 2023 Wiley-VCH GmbH.)

Second-harmonic generation (SHG) is a fundamental optical property of nonlinear optical (NLO) crystals. Thus far, it has proved difficult to engineer large SHG responses, particularly in the mid-infrared region, owing to the difficulty in simultaneously controlling the arrangement and density of functional NLO-active units. Herein, a new assembly strategy employing functional modules only, and aimed at maximizing the density and optimizing the spatial arrangement of highly efficient functional modules, has been applied to the preparation of NLO crystals, affording the van der Waals crystal MoO 2 Cl 2 . This exhibits the strongest powder SHG response (2.1×KTiOPO 4 (KTP) @ 2100 nm) for a transition-metal oxyhalide, a wide optical transparency window, and a sufficient birefringence. MoO 2 Cl 2 is the first SHG-active transition-metal oxyhalide effective in the infrared region. Theoretical studies and crystal structure analysis suggest that the densely packed, optimally-aligned [MoO 4 Cl 2 ] modules within the two-dimensional van der Waals layers are responsible for the giant SHG response., (© 2023 Wiley-VCH GmbH.)

Polarization modulation of deep-UV light is of significance to current technologies, and to this end, the birefringent crystal has emerged as an invaluable material as it allows for effective light modulation. Herein, a double-modification strategy driven by F and OH anions that makes double effects towards the critical property enhancement of deep-UV birefringent crystals is proposed. This leads to a new hydroxyborate (NH 4 ) 4 [B 12 O 16 F 4 (OH) 4 ] with giant cluster as a deep-UV birefringent crystal with large birefringence (Δn exp. =0.12@546.1 nm). This birefringence is a record among inorganic hydroxyborates with experimentally measured birefringence. Structural analysis shows that the near-plane arrangement of [B 12 O 16 F 4 (OH) 4 ] cluster is responsible for the large optical anisotropy. Theoretical calculations indicate that its π-conjugated [BO 3 ] and [BO 2 OH] units are the main source of this large optical anisotropy., (© 2022 Wiley-VCH GmbH.)

Discovery of new efficient nonlinear optical (NLO) materials with large second-order nonlinearity for the short-wave ultraviolet spectral region (λ PM ≤266 nm, PM=phase-matching) is still very challenging. Herein, a new beryllium-free borate CaZn 2 (BO 3 ) 2 with Sr 2 Be 2 B 2 O 7 (SBBO) double-layered like configuration was rationally designed, which not only preserves the structural merits but also eliminates the limitations of the SBBO crystal. CaZn 2 (BO 3 ) 2 shows a large PM second harmonic generation (SHG) reponse of 3.8×KDP, which is 38 times higher than that of its barium analogue. This enhancement mainly originates from the 1 [Zn 2 O 6 ] ∞ polar chains with a large net dipole moment and [BO 3 ] units with a high NLO active density. Our findings show the great significance of the [ZnO 4 ] tetrahedra introduced strategy to design beryllium-free SBBO-type NLO crystals and also verify the feasibility of using simple non-isomorphic substitution to induce giant second-order nonlinearity enhancement., (© 2022 Wiley-VCH GmbH.)

Maximizing the optical anisotropy in birefringent materials has emerged as an efficient route for modulating the polarization-dependent light propagation. Currently, the generation of deep-ultraviolet (deep-UV) polarized light below 200 nm is essential but challenging due to the interdisciplinary significance and insufficiency of high-performing birefringent crystals. Herein, by introducing multiple heteroanionic units, the first sodium difluorodihydroxytriborate-boric acid Na[B 3 O 3 F 2 (OH) 2 ]⋅[B(OH) 3 ] has been characterized as a novel deep-UV birefringent crystal. Two rare heteroanionic units, [B 3 O 3 F 2 (OH) 2 ] and [B(OH) 3 ], optimally align to induce large optical anisotropy and also the dangling bonds are eliminated with hydrogens, which results in an extremely large birefringence and band gap. The well-ordered OH/F anions in [B 3 O 3 F 2 (OH) 2 ] and [B(OH) 3 ] were identified and confirmed by various approaches, and also the origin of large birefringence was theoretically discussed. These results confirm the feasibility of utilizing hydrogen involved heteroanionic units to design crystals with large birefringence, and also expand the alternative system of deep-UV birefringent crystals with new hydroxyfluorooxoborates., (© 2021 Wiley-VCH GmbH.)