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, Ba₄CdGa₂S₆F₄, was successfully designed and synthesized by the cation and anion cosubstitution strategy. This compound was derived from the parent phase Ba₅Ga₂S₈ by partially replacing one Ba²⁺ cation with one Cd²⁺ cation and two S²⁻ anions with four F⁻ anions. Optical characterization via the UV-vis-NIR diffuse reflectance measurement reveals that Ba₄CdGa₂S₆F₄ exhibits a large optical band gap of 3.78 eV. And the broad infrared transparency of Ba₄CdGa₂S₆F₄ is confirmed for the material via both Raman and Fourier transform IR spectroscopy. First-principles calculations indicate that Ba₄CdGa₂S₆F₄ has moderate birefringence with a value of 0.057 at 1064 nm, representing a 26.6% increase over the parent compound Ba₅Ga₂S₈. 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.