A one-pot method in water to synthesize a new cyanurate K(H₂C₃N₃O₃)·H₂O with large birefringence

A new birefringent crystal, K(H₂C₃N₃O₃)·H₂O, was synthesized via a simple one-pot aqueous method under mild conditions. The crystal structure of K(H₂C₃N₃O₃)·H₂O was determined by single-crystal X-ray diffraction. K(H₂C₃N₃O₃)·H₂O crystallizes in the monoclinic space group C2/m (no. 12) with unit cell parameters of a = 11.0268(5) Å, b = 16.3686(5) Å, a = 7.1958(4) Å, and β = 103.765(5)°. The crystal structure of K(H₂C₃N₃O₃)·H₂O consists of planar isocyanurate anions arranged in hydrogen-bonded ribbons, coordinated by potassium cations and water molecules. UV-Vis measurement showed that the compound exhibits a wide band gap of 4.86 eV, which was verified by density functional theory (DFT) calculations, indicating an indirect band gap of 4.42 eV. Notably, K(H₂C₃N₃O₃)·H₂O displays the largest birefringence reported among cyanurate-based materials to date, reaching Δn ≈ 0.5 at 546 nm, which was confirmed by both DFT and experimental measurements. DFT reveals that the high birefringence arises from anisotropic polarizability associated with the π-conjugated isocyanurate layers, with minimal electronic contribution from the potassium cations. These findings highlight the isocyanurate group as a promising candidate for high-performance birefringence applications.