Linear unit BN₂: a novel birefringence-enhanced fundamental module with sp orbital hybridization

The linear unit BN 2 is discovered as a novel birefringence-enhanced fundamental module. Particularly, Ca 3 (BN 2 )N exhibits a large birefringence (0.411 at 1064 nm), which is about 2.0–3.5 times of the commercial used birefringent crystals, such as α-BaB 2 O 4 , CaCO 3 and YVO 4 .