BaPb<sub>2</sub>(CO<sub>3</sub>)<sub>2</sub>F<sub>2</sub>: Enhancing Birefringence\nby a Multiunit Synergy Effect

Carbonates are ideal birefringent materials because their\nπ-conjugated\n[CO<sub>3</sub>] groups are beneficial for large anisotropy. How to\nmaximize the birefringence and maintain a wide transmittance simultaneously\nin carbonates is a great challenge. Adopting a multiple-functional\ngroup synergy strategy by combining alkaline-earth metal Ba<sup>2+</sup> with large ion radius, PbO<sub>6</sub>F distorted hexagonal pyramid,\nand paralleled π-conjugated CO<sub>3</sub> triangles together,\nwe synthesized a new metal carbonate crystal, BaPb<sub>2</sub>(CO<sub>3</sub>)<sub>2</sub>F<sub>2</sub>. BaPb<sub>2</sub>(CO<sub>3</sub>)<sub>2</sub>F<sub>2</sub> features a typical two-dimensional layered\nstructure, which consists of a PbCO<sub>3</sub>F monolayer parallel\nto the <i>ab</i> plane and Ba<sup>2+</sup> cations filled\nbetween the monolayers. BaPb<sub>2</sub>(CO<sub>3</sub>)<sub>2</sub>F<sub>2</sub> exhibits a large experimental birefringence of approximately\n0.245@546 nm and a short ultraviolet absorption edge down to 264 nm\n(Eg = 4.70 eV), indicating that it is a promising ultraviolet birefringent\nmaterial. The reported highest experimental birefringence index among\ncarbonate crystals is surpassed by BaPb<sub>2</sub>(CO<sub>3</sub>)<sub>2</sub>F<sub>2</sub>. The theoretical calculation shows that\nthe contribution percentages for total birefringence are 57 and 43%\nfor the CO<sub>3</sub> group and PbO<sub>3</sub>F group, respectively.\nOur work offers a new route for designing new birefringent materials\nwith excellent comprehensive performance.