From B3O3(OH)3 to B3O3F3: Uncovering series of new functional units based on [BO2F] beneficial for deep‐ultraviolet birefringence

Birefringent materials play indispensable roles in modulating the polarization of light and are vital in laser science and technology. Units with π‐conjugation are regarded as potential functional modules for large optical anisotropy, such as planar [BO3]3‐, [B3O6]3‐, and [B3O3(OH)3]. However, the discovery of deep‐ultraviolet (DUV, λ ≤ 200 nm) birefringent materials still faces a serious challenge due to the limited band gap. Replacing hydroxyl anions [OH]‐ with fluorine anions F‐ in borates can cause the blueshift of the UV cutoff edge, however, the effect of this replacement on optical properties has not been systematically studied. Herein, based on template materials metaboric acid α‐B3O3(OH)3 with planar [B3O3(OH)3] units, we proposed an OH/F substitution strategy designing potential DUV birefringent materials. The designed B3O3(OH)2F, B3O3(OH)F2, and B3O3F3 exhibit large birefringence from 0.123 to 0.146 at 546 nm with DUV transparency, exceeding the performance of traditional α‐BaB2O4. Further electronic structure analysis reveals the band edge and bonding difference between [OH]‐ and F‐, indicating the corresponding designed novel [BO2F], [B3O3(OH)2F], [B3O3(OH)F2] and [B3O3F3] units as potential DUV birefringent‐active functional modules with large structure anisotropy. This work offers the chemical difference of F‐ and OH‐ anions, and design‐strategy of new DUV birefringent materials.