Ba3(C3N3O3)2: A New Phase of Barium Cyanurate Containing Parallel π‑Conjugated Groups as a Birefringent Material Replacement for Calcite

A new phase of Ba3(C3N3O3)2 (I) was grown from RbCNO and BaCl2 by a solid state cyclo-trimerization reaction. It crystallizes in space group R3̅ with the cell parameters of a = 7.0934(2) Å and c = 20.1130(9) Å, featuring a layered structure stacked from the 2[Ba­(C3N3O3)2]2– double layers along the c direction. I exhibits wide ultraviolet transparency and large optical anisotropy of Δn = 0.32 at 800 nm, two times than that of the benchmark birefringent crystal calcite. The discovery of I shows that replacement of calcite is conceivable and opens the novel pathway for designing novel ultraviolet birefringent crystals in metal cyanurates.