Effect of the [Ba₂BO₃F]_∞ Layer on the Band Gap: Synthesis, Characterization, and Theoretical Studies of BaZn₂B₂O₆·nBa₂BO₃F (n = 0, 1, 2)

Two new zincoborate fluorides with the common formula BaZn₂B₂O₆·nBa₂BO₃F (n = 1, 2) have been successfully synthesized for the relationship study between the band gaps and crystal structures in zinc-containing borate fluorides. Ba₃Zn₂B₃O₉F with n = 1 in the common formula belongs to the orthorhombic space group Pnma (No. 20), and Ba₅Zn₂B₄O₁₂F₂ with n = 2 in the common formula crystallizes in the monoclinic space group C2/c (No. 62). They can both be seen as compounds with the n[Ba₂BO₃F]_∞ (n = 1 or 2) layer inserted in the structure of BaZn₂B₂O₆. UV–vis–near-IR diffuse-reflectance spectra show that the band gaps of BaZn₂B₂O₆·nBa₂BO₃F (n = 0, 1, 2) gradually increase with more [Ba₂BO₃F]_∞ layers inserted. The first-principles calculation indicates that the inserted n­[Ba₂BO₃F]_∞ 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.