Effect of the [Ba<sub>2</sub>BO<sub>3</sub>F]<sub>∞</sub> Layer on the Band Gap: Synthesis, Characterization, and Theoretical Studies of BaZn<sub>2</sub>B<sub>2</sub>O<sub>6</sub>·<i>n</i>Ba<sub>2</sub>BO<sub>3</sub>F (<i>n</i> = 0, 1, 2)

Two new zincoborate fluorides with the common formula BaZn<sub>2</sub>B<sub>2</sub>O<sub>6</sub>·<i>n</i>Ba<sub>2</sub>BO<sub>3</sub>F (<i>n</i> = 1, 2) have been successfully synthesized for the relationship study between the band gaps and crystal structures in zinc-containing borate fluorides. Ba<sub>3</sub>Zn<sub>2</sub>B<sub>3</sub>O<sub>9</sub>F with <i>n</i> = 1 in the common formula belongs to the orthorhombic space group <i>Pnma</i> (No. 20), and Ba<sub>5</sub>Zn<sub>2</sub>B<sub>4</sub>O<sub>12</sub>F<sub>2</sub> with <i>n</i> = 2 in the common formula crystallizes in the monoclinic space group <i>C</i>2/<i>c</i> (No. 62). They can both be seen as compounds with the <i>n</i>[Ba<sub>2</sub>BO<sub>3</sub>F]<sub>∞</sub> (<i>n</i> = 1 or 2) layer inserted in the structure of BaZn<sub>2</sub>B<sub>2</sub>O<sub>6</sub>. UV–vis–near-IR diffuse-reflectance spectra show that the band gaps of BaZn<sub>2</sub>B<sub>2</sub>O<sub>6</sub>·<i>n</i>Ba<sub>2</sub>BO<sub>3</sub>F (<i>n</i> = 0, 1, 2) gradually increase with more [Ba<sub>2</sub>BO<sub>3</sub>F]<sub>∞</sub> layers inserted. The first-principles calculation indicates that the inserted n­[Ba<sub>2</sub>BO<sub>3</sub>F]<sub>∞</sub> layers play a positive effect in increasing the band gaps of zincoborate fluorides. Furthermore, the IR spectra, thermal behaviors, and refractive indices of these compounds are also studied.