Two mixed halogen borate-carbonates K-9[B4O5(OH)(4)](3)(CO3)X center dot 7H(2)O (X = Cl, Br) have been successfully synthesized with the hydrothermal method. They crystallize in the space group P (6) over bar 2c, 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. K-9[B4O5(OH)(4)](3)(CO3)X center dot 7H(2)O (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-element borate 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.