BaPb(CO)F: Enhancing Birefringence by a Multiunit Synergy Effect.

Carbonates are ideal birefringent materials because their π-conjugated [CO] groups are beneficial for large anisotropy. How to maximize the birefringence and maintain a wide transmittance simultaneously in carbonates is a great challenge. Adopting a multiple-functional group synergy strategy by combining alkaline-earth metal Ba with large ion radius, PbOF distorted hexagonal pyramid, and paralleled π-conjugated CO triangles together, we synthesized a new metal carbonate crystal, BaPb(CO)F. BaPb(CO)F features a typical two-dimensional layered structure, which consists of a PbCOF monolayer parallel to the plane and Ba cations filled between the monolayers. BaPb(CO)F exhibits a large experimental birefringence of approximately 0.245@546 nm and a short ultraviolet absorption edge down to 264 nm (Eg = 4.70 eV), indicating that it is a promising ultraviolet birefringent material. The reported highest experimental birefringence index among carbonate crystals is surpassed by BaPb(CO)F. The theoretical calculation shows that the contribution percentages for total birefringence are 57 and 43% for the CO group and PbOF group, respectively. Our work offers a new route for designing new birefringent materials with excellent comprehensive performance.