CN₄H₇B₃O₃F₂(OH)₂: short-wave UV hydroxyfluorooxyborate crystals with large birefringence

The synthesis of materials possessing substantial birefringence, particularly in the short-wave ultraviolet spectral range, continues to pose a significant challenge in optical materials science. This work reports the discovery of a new compound, CN₄H₇B₃O₃F₂(OH)₂, in the hydroxyfluorooxyborate category. It is synthesized by strategically selecting [CN₄H₇] groups with a wide HOMO-LUMO gap and substantial polarizability anisotropy as cations, while rationally modifying the B-O anionic groups through fluorination and hydroxylation. This compound exhibits a large birefringence value of Δn_exp = 0.185 and a short UV absorption edge at 209 nm, making it a promising candidate for short-wave ultraviolet birefringent crystal materials.