Ba2M(C3N3O3)2 (M = Mg, Ca): potential UV birefringent materials with strengthened optical anisotropy originating from the (C3N3O3)3- group

Demands for UV birefringent materials are growing dramatically owing to the rapid development of ultraviolet technology. Here, a new family of UV birefringent materials, Ba2Mg(C3N3O3)(2) (BMCY) and Ba2Ca(C3N3O3)(2) (BCCY), have been successfully discovered. It is the first time that the excellent birefringent properties of cyanurates have been studied. These materials exhibit an extremely large birefringence (n = 0.728-0.351 and 0.771-0.346 from 230 nm to 800 nm for BMCY and BCCY, respectively), much larger than that of the commercial UV birefringent crystal -BaB2O4 (-BBO) (n = 0.12@532 nm). Our study indicates that the impressive optical properties of BMCY and BCCY stem from the strengthened optical anisotropy of the planar (C3N3O3)(3-) group compared with the isoelectronic (B3O6)(3-) group. Besides, its congruent-melting properties make it feasible to grow a bulk crystal by the Bridgman-Stockbarger technique. The extraordinary properties of BMCY and BCCY may shed light on a new path to explore birefringent materials for practical application.