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

The main commercially used birefringent oxides α‐BaB2O4 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 [C2O4]2−, [C2O6]2−, [C4O4]2−, and [C6O6]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)2C2O4⋅H2O 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. Through a targeted high‐throughput screening strategy, birefringence‐active functional modules and a series of crystals containing the C−O units with giant optical anisotropy spanning from 0.1 up to 1.35 were found. The compound with [C6O6]2− has a birefringence about 11 and 6 times that of α‐BaB2O4 and YVO4. The experimental measurement for the (NH4)2C2O4⋅H2O crystal confirms the feasibility of the screening framework.