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.
Yan et al. (Mon,) studied this question.