ZnGa(SeO3)2F: A UV Transparent Birefringent Crystal Explored in M(II)-IIIA-Selenite-F System

In this work, we present a targeted design strategy that combines Group IIIA metal cation and fluorine anion to simultaneously enhance the band gap and birefringence in selenite-based system. Three new ultraviolet (UV) transparent selenite fluorides, SrAl(SeO3)F3 (1), BaAl(SeO3)F3(H2O)0.25 (2), and ZnGa(SeO3)2F (3), were successfully synthesized under mild hydrothermal conditions. Remarkably, all three compounds exhibit wide band gaps as well as large birefringence. In particular, ZnGa(SeO3)2F (3) demonstrates a pronounced birefringence value of 0.176 at 546 nm, representing the highest birefringence reported among all known selenites with a band gap above 5.0 eV. Theoretical investigations indicate that the strong optical anisotropy is predominantly attributed to the stereochemically active lone-pair electrons of Se(IV) cations. The [Zn2O8] dimers and the infinite [GaO4F]∞ chains in ZnGa(SeO3)2F (3) crucially facilitate the optimal spatial arrangement of [SeO3] groups. This study expands the structural diversity of selenite fluorides and provides an effective pathway for designing high-performance birefringent crystals suitable for UV applications.