Ba<sub>3</sub>(C<sub>3</sub>N<sub>3</sub>O<sub>3</sub>)<sub>2</sub>: A New Phase of Barium Cyanurate Containing Parallel π‑Conjugated Groups as a Birefringent Material Replacement for Calcite

A new phase of Ba<sub>3</sub>(C<sub>3</sub>N<sub>3</sub>O<sub>3</sub>)<sub>2</sub> (<b>I</b>) was grown from RbCNO and BaCl<sub>2</sub> by a solid state cyclo-trimerization reaction. It crystallizes in space group <i>R</i>3̅ with the cell parameters of <i>a</i> = 7.0934(2) Å and <i>c</i> = 20.1130(9) Å, featuring a layered structure stacked from the <sub>∞</sub><sup>2</sup>[Ba­(C<sub>3</sub>N<sub>3</sub>O<sub>3</sub>)<sub>2</sub>]<sup>2–</sup> double layers along the <i>c</i> direction. <b>I</b> exhibits wide ultraviolet transparency and large optical anisotropy of Δ<i>n</i> = 0.32 at 800 nm, two times than that of the benchmark birefringent crystal calcite. The discovery of <b>I</b> shows that replacement of calcite is conceivable and opens the novel pathway for designing novel ultraviolet birefringent crystals in metal cyanurates.