K9[B4O5(OH)4]3(CO3)X·7H2O (X = Cl, Br): Syntheses, Characterizations, and Theoretical Studies of Noncentrosymmetric Halogen Borate–Carbonates with Short UV Cutoff Edges

Two mixed halogen borate–carbonates K9[B4O5(OH)4]3(CO3)X·7H2O (X = Cl, Br) have been successfully synthesized with the hydrothermal method. They crystallize in the space group P62c, and their structures feature 3D networks consisting of the K9O30Cl polyhedra. The isolated [B4O9]6– groups and [BO3]3– triangles are filled in the space of 3D networks. K9[B4O5(OH)4]3(CO3)X·7H2O (X = Cl, Br) show the largest band gaps (6.34 and 5.65 eV, respectively) among the borate–carbonate systems. In addition, we investigated the main-group-elementborate halides based on the Inorganic Crystal Structure Database, compared the structures, and summarized which structures are beneficial to the formation of isomorphic compounds. Herein, the syntheses, structures, first-principle calculations, and optical properties were reported in the work.