Ba ₄ CdGa ₂ S ₆ F ₄ : A Chalcohalide Birefringent Crystal with Large Band Gap Designed by the Cation and Anion Cosubstitution Strategy

Precise structural design is vital for advancing high-performance birefringent crystals used in mid-infrared polarizing devices. Mixed anion systems, particularly chalcohalides, present a promising avenue by synergistically combining the broad infrared transparency of chalcogenides with the band gap-widening capability conferred by halide ions. Guided by this approach, a novel chalcohalide birefringent crystal, Ba 4 CdGa 2 S 6 F 4, was successfully designed and synthesized by the cation and anion cosubstitution strategy. This compound was derived from the parent phase Ba 5 Ga 2 S 8 by partially replacing one Ba 2+ cation with one Cd 2+ cation and two S 2– anions with four F – anions. Optical characterization via the UV–vis-NIR diffuse reflectance measurement reveals that Ba 4 CdGa 2 S 6 F 4 exhibits a large optical band gap of 3.78 eV. And the broad infrared transparency of Ba 4 CdGa 2 S 6 F 4 is confirmed for the material via both Raman and Fourier transform IR spectroscopy. First-principles calculations indicate that Ba 4 CdGa 2 S 6 F 4 has moderate birefringence with a value of 0.057 at 1064 nm, representing a 26.6% increase over the parent compound Ba 5 Ga 2 S 8 . This work not only reports a promising infrared birefringent crystal with large band gap but also demonstrates that the cation and anion cosubstitution is an effective strategy for designing novel functional crystalline materials.