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

Two similarly stoichiometric borate halides, NaBa4(AlB4O9)2Br3 and NaBa4(B5O9)2F2Cl, have been successfully designed and synthesized, and their structures were determined by single-crystal X-ray diffraction. Their crystal structures feature the [AlB4O9]∞ and [B5O9]∞ networks, respectively. NaBa4(AlB4O9)2Br3 is noncentrosymmetric and crystallizes in polar space group P42nm, while NaBa4(B5O9)2F2Cl is centrosymmetric and crystallizes in monoclinic space group P21/n. Powder second-harmonic generation (SHG) measurements reveal that NaBa4(AlB4O9)2Br3 has an optical nonlinearity comparable to that of KH2PO4 (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.