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 ultraviolet (UV) cutoff edge, however, the effect of this replacement on π-conjugation units 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 based on first-principles calculations, 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 can act 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.