Finding a Deep-UV Borate BaZnB₄ O₈ with Edge-sharing [BO₄ ] Tetrahedra and Strong Optical Anisotropy

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 (Δn₍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 [ZnO₄ ] and edge-sharing [BO₄ ] tetrahedra, make all the planar [BO₃ ] 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 BaZnB₄ O₈ .