BaPb2(CO3)2F2: Enhancing Birefringence by a Multiunit Synergy Effect

Carbonates are ideal birefringent materials because their π-conjugated [CO3] 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 Ba2+ with large ion radius, PbO6F distorted hexagonal pyramid, and paralleled π-conjugated CO3 triangles together, we synthesized a new metal carbonate crystal, BaPb2(CO3)2F2. BaPb2(CO3)2F2 features a typical two-dimensional layered structure, which consists of a PbCO3F monolayer parallel to the ab plane and Ba2+ cations filled between the monolayers. BaPb2(CO3)2F2 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 BaPb2(CO3)2F2. The theoretical calculation shows that the contribution percentages for total birefringence are 57 and 43% for the CO3 group and PbO3F group, respectively. Our work offers a new route for designing new birefringent materials with excellent comprehensive performance.