Co-crystal AX·(H3C3N3O3) (A = Na, Rb, Cs; X = Br, I): a series of strongly anisotropic alkali halide cyanurates with a planar structural motif and large birefringence.

Birefringent crystals with strong anisotropy are important components in modern optical devices. The newly discovered planar π-conjugated cyanurate group (HxC3N3O3)x−3 (x = 0–3) has been demonstrated as an effective functional motif for improving birefringence in the ultraviolet region. Here, single co-crystals of alkali halide cyanurates, RbBr·(H3C3N3O3) (I), RbI·(H3C3N3O3) (II), and CsBr·(H3C3N3O3) (III) were synthesized by the ethanol solution method, and NaBr·(H3C3N3O3) (IV) was obtained via the solvent-drop grinding method. These four co-crystals feature a planar (H3C3N3O3) arrangement and exhibit wide band gaps (> 4.90 eV), tunable birefringence (Δnexp ∼ 0.124–0.256), and high thermal stability (156 °C–349 °C). In addition, first principles calculations were also carried out to evaluate the relationship between molecule density, spatial arrangement and optical birefringence, and suggested a great tailoring effect of the alkali metal and halogen species on regulating the optical anisotropy of co-crystal cyanurates.