UV Nonlinear Optical Crystal Ba2[B6O9(OH)4] Featuring Unique Chiral Layers with a New B18O42 Circle Based on BO3 and BO4 Units

A new noncentrosymmetric polyborate, Ba-2[B6O9(OH)(4)], has been synthesized under a hydrothermal condition. The polyborate contains chiral layers constructed by two kinds of helical chains and also form new B18O42 circles based on B3O8 units [(3: Delta+2T)]. One kind of Ba atom locates in the cavity surrounded by the B11O11 ring within the anion layer, and the other kind of Ba atom inserts between two adjacent layers. UV-vis diffuse reflectance spectroscopy demonstrates that the compound has a cutoff edge below 190 nm and has a large second-harmonic generation (SHG) effect, which is approximately 3 times that of KH2PO4 (KDP) and is type-I phase matchable.