Effect of Rigid Units on the Symmetry of the Framework: Design and Synthesis of Centrosymmetric NaBa₄(B₅O₉)₂F₂Cl and Noncentrosymmetric NaBa₄(AlB₄O₉)₂Br₃

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Two similarly stoichiometric borate halides, NaBa 4 (AlB 4 O 9 ) 2 Br 3 and NaBa 4 (B 5 O 9 ) 2 F 2 Cl, have been successfully designed and synthesized, and their structures were determined by single-crystal X-ray diffraction. Their crystal structures feature the [AlB 4 O 9 ] ∞ and [B 5 O 9 ] ∞ networks, respectively. NaBa 4 (AlB 4 O 9 ) 2 Br 3 is noncentrosymmetric and crystallizes in polar space group P 4 2 nm, while NaBa 4 (B 5 O 9 ) 2 F 2 Cl is centrosymmetric and crystallizes in monoclinic space group P 2 1 / n . Powder second-harmonic generation (SHG) measurements reveal that NaBa 4 (AlB 4 O 9 ) 2 Br 3 has an optical nonlinearity comparable to that of KH 2 PO 4 (KDP) and is type I phase-matchable. In addition, infrared and UV–Vis–NIR diffuse reflectance spectroscopy, as well as electronic band structure calculations, were performed on the reported materials.