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

Abstract The main commercially used birefringent oxides α‐BaB 2 O 4 and YVO 4 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 [C 2 O 4 ] 2− , [C 2 O 6 ] 2− , [C 4 O 4 ] 2− , and [C 6 O 6 ] 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 (NH 4 ) 2 C 2 O 4 ⋅H 2 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.