K ₂ GdB ₃ O ₆ F ₂ : A Promising Short-Wave UV Birefringent Crystal via Optimized Assembly of π-Conjugated [B ₃ O ₆ ] Groups

Birefringent crystals are indispensable components in optical functional devices that enable the precise manipulation of light polarization. However, achieving a balance between wide ultraviolet (UV) transparency and significant birefringence poses a formidable challenge due to their inherent inverse correlation. Here, we report a novel rare-earth borate fluoride, K 2 GdB 3 O 6 F 2 (KGBOF) that overcomes this intrinsic trade-off via the rational assembly of π-conjugated [B 3 O 6 ] groups. Expectedly, KGBOF exhibits a large birefringence of 0.12@532 nm, comparable to that of the commercially available α-BaB 2 O 4, exceeding that of most rare-earth borate fluorides. Notably, KGBOF demonstrates a wide band gap of 5.71 eV and a short-wavelength cutoff edge (<200 nm). First-principles calculations and structural analysis attribute the exceptional optical anisotropy to the coplanar alignment of π-conjugated [B 3 O 6 ] clusters. These results establish KGBOF as a promising short-wave UV birefringent crystal and provide a basis for the exploitation of new birefringent materials in the short-wavelength region.