Ba 2 M(C 3 N 3 O 3 ) 2 (M = Mg, Ca): potential UV birefringent materials with strengthened optical anisotropy originating from the (C 3 N 3 O 3 ) 3− group

(C 3 N 3 O 3 ) 3− is first proposed to be an excellent birefringent active group and the representative compounds Ba 2 M(C 3 N 3 O 3 ) 2 (M = Mg and Ca) are studied in detail.