Finding a Deep‐UV Borate BaZnB4O8 with Edge‐sharing [BO4] Tetrahedra and Strong Optical Anisotropy

Abstract The polarization modulation of deep‐UV light is an important process that incorporates functionality to selectively respond to light‐mater interaction. Typically, optical anisotropy is foremost to the use efficiency of deep‐UV birefringent crystals. Herein, a new congruently melting polyborate with extremely large birefringence (Δ n (001) =0.14@589.3 nm) and band gap (6.89 eV) is discovered as a high performance birefringent crystal, which breaks the current deadlock of deep‐UV polyborates that usually show small birefringence. The rigid tetrahedra, including [ZnO 4 ] and edge‐sharing [BO 4 ] tetrahedra, make all the planar [BO 3 ] triangles in the lattice adopt preferential arrangement and thereby lead to an extraordinary large birefringence that is larger than all the deep‐UV borates with experimentally measured values. Structural analyses with the additional theoretical calculations were used to study the origin of strong optical anisotropy in BaZnB 4 O 8 .