A(H3C3N3O3)(NO3) (A = K, Rb): Alkali-Metal Nitrate Isocyanurates with Strong Optical Anisotropy.

The first alkali-metal nitrate isocyanurates, A(H3C3N3O3)(NO3) (A = K, Rb), were synthesized by the tactic of introducing (NO3)- into isocyanurate with a mild hydrothermal technique. They crystallized into the same monoclinic centrosymmetric (CS) space group P21/c, which featured a 2D [(H3C3N3O3)(NO3)]∞ layered structure separated by K+ and Rb+ cations, respectively. Both compounds exhibited short ultraviolet cutoff edges (λcutoff = 228 and 229 nm) and large birefringences (Δn = 0.253 and 0.224 at 546.1 nm). More importantly, in comparison with most of the isocyanurates and nitrates, they have better thermal stability with decomposition temperatures up to 319.8 and 324.4 °C. In addition, our theoretical calculations reveal that the π-conjugated groups play significant roles in improving the optical anisotropy. Remarkably, introducing a π-conjugated inorganic acid radical (NO3)- into isocyanurate is an extremely meaningful strategy to explore new UV birefringent crystals.