“…This is mainly owing to the following reasons: (i) In (H x C 3 N 3 O 3 ) x −3 groups, C, N, and O have strong covalent interactions and exhibit wide band gaps with UV transparency. (ii) The (C 3 N 3 ) ring in (H x C 3 N 3 O 3 ) x −3 has a stronger π-conjugated interaction than that of the (B 3 O 3 ) ring in (B 3 O 6 ) 3– , and this factor increases birefringence of cyanurates. − To date, many cyanurate birefringent crystals, such as K 2 (C 3 N 3 O 3 H), Ba 2 M(C 3 N 3 O 3 ) 2 (M = Mg, Ca), RE(H 2 C 3 N 3 O 3 ) 2 ·(OH)· x H 2 O (RE = La, Y, and Gd), and Li 2 Ca(H 2 C 3 N 3 O 3 ) 4 ·6H 2 O, have been discovered. All these cyanurate crystals exhibit good birefringent performance and could be used as UV birefringent materials if large single crystals are obtained.…”