Double-Modification Oriented Design of a Deep-UV Birefringent Crystal Functionalized by [B 12 O 16 F 4 (OH) 4 ] Clusters.

Polarization modulation of deep-UV light is of significance to current technologies, and to this end, the birefringent crystal has emerged as an invaluable material as it allows for effective light modulation. Herein, a double-modification strategy driven by F and OH anions that makes double effects towards the critical property enhancement of deep-UV birefringent crystals is proposed. This leads to a new hydroxyborate (NH 4 ) 4 [B 12 O 16 F 4 (OH) 4 ] with giant cluster as a deep-UV birefringent crystal with large birefringence (Δn exp. =0.12@546.1 nm). This birefringence is a record among inorganic hydroxyborates with experimentally measured birefringence. Structural analysis shows that the near-plane arrangement of [B 12 O 16 F 4 (OH) 4 ] cluster is responsible for the large optical anisotropy. Theoretical calculations indicate that its π-conjugated [BO 3 ] and [BO 2 OH] units are the main source of this large optical anisotropy.