Linear unit BN(2): a novel birefringence-enhanced fundamental module with sp orbital hybridization

Inorganic planar π-conjugated groups are advantageous to generate large birefringence in optical functional materials, and many excellent materials contain CO(3), BO(3) or B(3)O(6), such as CaCO(3), α/β-BaB(2)O(4) (α/β-BBO), and KBe(2)BO(3)F(2) (KBBF). In view of their microscopic structures, the common characteristics are the planar structures, which are regarded as birefringence-enhanced fundamental modules (FMs). Nowadays, exploring novel birefringence-enhanced FMs is becoming a burning issue. Herein, we investigated the birefringence-enhanced FMs in B–N systems and found that the BN(2) linear unit could produce great birefringence. Through the investigation based on the Inorganic Crystal Structure Database, some compounds with the BN(2) linear group were screened out with the formulas A(3)BN(2) (A = Li, Na), A(3)BN(3) (A = Mg, Ca), and Ba(3)(BN(2))(2). Particularly, Ca(3)(BN(2))N exhibits a great birefringence of about 0.411 at 1064 nm, which is 3.5, 2.5 and 2.0 times those of the most commercially used birefringent crystals α-BaB(2)O(4) (Δn = 0.116 at 1064 nm), CaCO(3) (Δn = 0.164 at 1064 nm) and YVO(4) (Δn = 0.208 at 1064 nm), respectively. To find the origins of the optical properties of compounds with the BN(2) linear group, the first-principles, REDA and polarizability anisotropy analysis methods were used. Owing to the structural arrangement and the polarization anisotropy of the BN(2) linear group, it can influence the birefringence significantly. This work will provide a general way for exploring birefringence-enhanced FMs in B–N compounds.