Two Mixed Alkali Metal Isocyanurates with Wide Band Gap and Giant Birefringence

Cyanurate anions are promising functional bulding blocks for birefringent materials. In this work, two new isocyanurates with mixed alkali cations were successfully synthesized by a hydrothermal method, namely, RbK(H2C3N3O3)2·2H2O (I) and Cs3Li(H2C3N3O3)4·3H2O (II). Compounds I and II are both featured with two-dimensional layers constructed by an isocyanurate six-membered ring extending along the bc plane. The characterization results show that both of these two new compounds exhibit wide band gaps (Eg ∼ 5.2 eV) and giant birefringence (Δn > 0.3) simultaneously. Moreover, theoretical calculations are performed to evaluate the relationship between microscopic structures and their anisotropic optical properties, indicating that the π-conjugated groups make a dominant contribution to the optical anisotropy. These findings demonstrate that the combination of alkali cations and merits of π-conjugated (iso)cyanurates would promote the study of optical integrated devices for emerging photonic technologies.