(CN4H7)2SO4·H2O: high-performance metal-free ultraviolet birefringent crystal with KBBF-like configuration

By modifying the KBBF template structure at the molecular level, the first metal-free sulfate (CN 4 H 7 ) 2 SO 4 ·H 2 O was synthesized, which achieved an effective balance between the short UV cut-off edge (212 nm) and large birefringence (0.132@546.1 nm).