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For non-π-conjugated [SO 4 ] units, it is challenging to generate sufficient birefringence, owing to the high symmetry of the regular tetrahedron. Unlike the traditional trial-and-error approach, we propose a new paradigm for birefringence engineering to tune the optical properties based on [SO 4 ] units. Through the strategy of ligand substitution, we can predict its effect on the band gap and anisotropy. Theoretical evaluations reveal generalized results that the anisotropic electron distribution of new functional groups induced by the suitable ligand substitution contributes to the band gap and birefringence. To further validate the correctness of the paradigm, we experimentally synthesized and characterized nine novel compounds with selected functional modules. By the new paradigm of ligand substitution, they can reach up to 4-6 times the birefringence of the corresponding sulfate and maintain the wide bandgap. Through rational design, (CN 4 H 7 )SO 3 NH 2 exhibits about 35 times the birefringence of Li 2 SO 4 , which is a significant order of magnitude improvement and verifies the superiority of our proposed paradigm. This work provides a new paradigm for the modification to the non-π-conjugated group and will guide and accelerate the exploration of novel birefringent crystals in the short-wavelength region., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Exploration of nonlinear optical (NLO) crystals that are competent in generating short-wavelength ultraviolet (UV, λ ≤ 266 nm, and even deep-UV, λ ≤ 200 nm) coherent light output by direct second harmonic generation (SHG) remains a formidable challenge. Herein, four UV/deep-UV NLO crystals, M 2 B 4 SO 10 (M = K, Rb, and Cs) and Rb 3 B 11 PO 19 F 3 , were successfully synthesized by evolving the KBe 2 BO 3 F 2 (KBBF) structure into mixed-anionic borosulfate and fluoroborophosphate systems. They display functional [B 4 SO 10 ] ∞ or [B 11 PO 19 F 3 ] ∞ KBBF-type layers that are composed of [BO 3 ], [BO 4 ], and [SO 4 ] groups or [BO 3 ], [BO 4 ], [BO 3 F], and [PO 4 ] groups, respectively. Experimental characterization and numerical computation results indicate that these crystals possess exceptional NLO performance, including large SHG responses (0.9-1.7 × KDP at 1064 nm and 0.1-0.3 × β-BBO at 532 nm) and adequate birefringence to fulfill the SHG phase-matching (PM) condition at 266 nm. In particular, the shortest type-I PM wavelength (λ PM ) of Rb 3 B 11 PO 19 F 3 reaches 180 nm, which implies that Rb 3 B 11 PO 19 F 3 can become a prospective deep-UV NLO crystal. In addition, single crystals of K 2 B 4 SO 10 , Rb 2 B 4 SO 10 , and Cs 2 B 4 SO 10 are easily obtained by the high-temperature solution approach. This work will facilitate the discovery of short-wavelength PM NLO crystals by using the KBBF structure as the template., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)