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 <i>P</i>4₂<i>nm</i>, while NaBa₄(B₅O₉)₂F₂Cl is centrosymmetric and crystallizes in monoclinic space group <i>P</i>2₁/<i>n</i>. 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