The structural diversity of halogen-centered secondary building units: two new mixed-metal borate halides with deep-ultraviolet cut-off edges
Two new mixed-metal borate halides, namely, NaMg2Ba7B14O28F5 (I) and NaBa4B10O18BrF2 (II), were synthesized using the high-temperature flux method. Both structures were characterized by single-crystal X-ray diffraction, and their structures are shown to feature complicated three dimensional (3D) networks with different types of halogen-centered secondary building units (SBUs). Specifically, compound I contains three spatial fluorine-centered SBUs F(1)Ba4Mg2, F(2)Ba(Na)Ba3Mg, and F(3)Ba(Na)Ba2Mg, while compound II exhibits spatial FBa3Na and planar BrBa4 fluorine- or bromine-centered SBUs. Furthermore, four categories of halogen-centered SBUs were investigated among all the 173 available anhydrous halogen-containing borates to elaborate the role of halogen atoms to enrich the structural diversity of borates. What's more, the crystal structures, thermal stabilities, and UV-Vis-NIR diffuse reflectance spectra of synthesised compounds I and II have also been determined.