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

(C₃N₃O₃)³⁻ is first proposed to be an excellent birefringent active group and the representative compounds Ba₂M(C₃N₃O₃)₂ (M = Mg and Ca) are studied in detail.