Double‐Modification Oriented Design of a Deep‐UV Birefringent Crystal Functionalized by [B12O16F4 (OH) 4] Cluster

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 (NH4)(4)[B12O16F4(OH)(4)] with giant cluster as a deep-UV birefringent crystal with large birefringence (Delta n(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 [B12O16F4(OH)(4)] cluster is responsible for the large optical anisotropy. Theoretical calculations indicate that its pi-conjugated [BO3] and [BO2OH] units are the main source of this large optical anisotropy.