Two metal-free cyanurate crystals with a large optical birefringence resulting from the combination of π-conjugated units

Abstract

Benefiting from the planar π-conjugated (H_xC₃N₃O₃)^x−3 (x = 0–3) groups, cyanurate crystals have recently become a research hotspot in birefringent materials. Herein, by combining the (H_xC₃N₃O₃)^x−3 (x = 0–3) group with the (CN₃H₆)⁺ cationic group, two metal-free cyanurates, GU(H₂C₃N₃O₃) (I) and GU₃(H₂C₃N₃O₃)₃(H₃C₃N₃O₃) (II), were obtained by the hydrothermal method. These compounds have wide band gaps (∼5 eV) and a large birefringence (∼0.40@400 nm), demonstrating their potential to be ultraviolet birefringent crystals. Moreover, first-principles calculations indicate that their large birefringence values originated from the synergistic effect of the (CN₃H₆)⁺ cations and (H_xC₃N₃O₃)^x−3 (x = 0–3) groups. These findings provide a new design strategy for exploring low-cost UV birefringent crystals with a large birefringence.