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.