Series of Crystals with Giant Optical Anisotropy: A Targeted Strategic Research.

The main commercially used birefringent oxides α-BaB2 O4 and YVO4 have the birefringences of 0.12 and 0.22. We propose a targeted high-throughput screening system to search birefringence-active functional modules (FMs) and large birefringent materials. A series of π-conjugated C-O units [C2 O4 ]2- , [C2 O6 ]2- , [C4 O4 ]2- , and [C6 O6 ]2- are discovered to be birefringence-active FMs. Theoretical and experimental studies on the crystals with C-O units confirm the feasibility of strategy. Based on this, the C-O containing compounds ranging from deep-ultraviolet to near-infrared region with large birefringence from 0.1 to 1.35 are found, and most of them break through the birefringent limit of oxides. The (NH4 )2 C2 O4 ⋅H2 O crystal is grown and its experimental birefringence is 0.248 at 546 nm, which is identified as a promising UV birefringent crystal. The A-site cations play significant roles in optical properties by influencing the density and arrangement of the C-O units.