AZn4(OH)4(C3N3O3)2 (A = Mg, Zn): Two Zn-Based Cyanurate Crystals with Various Cation Coordination and Large Birefringence

Cyanurate crystals have recently become a research hot spot in birefringent materials owing to the large structural and optical anisotropy of the planar π-conjugated (H_xC₃N₃O₃)^x-3 (x = 0-3) groups. In this study, two new Zn-based cyanurate crystals, Zn₅(OH)₄(C₃N₃O₃)₂ and MgZn₄(OH)₄(C₃N₃O₃)₂, were synthesized by the hydrothermal method. In Zn₅(OH)₄(C₃N₃O₃)₂, the d¹⁰ Zn²⁺ cations have three different coordinating environments, which has never been found in cyanurates. These compounds have wide band gaps (∼5 eV) and large birefringence (∼0.32 at 400 nm), indicating their potential as ultraviolet birefringence crystals.