Synergistic Enhancement of Band Gap and Optical Anisotropy via Functional-Site Cosubstitution in Antiperovskite-Type Chalcohalides [Ba₂K][X][MS₄] (M = Ge, Si; X = Cl, Br)

The advancement of infrared (IR) birefringent crystals has been persistently constrained by the mutually exclusive relationship between high Δn and wide bandgap (Eg) in traditional chalcogenides. Herein, we report the rational design and successful synthesis of antiperovskite-type chalcohalides [Ba2K][X][MS4] (M = Ge, Si; X = Cl, Br) employing a functional-site cosubstitution strategy, where [Ba3][S][GeS4] was utilized as the prototype compound. Notably, the title compounds achieve a substantial improvement over the parent material [Ba3][S][GeS4], with Δn values increasing from 0.03 to 0.07-0.08 (at both 1064 and 2050 nm) while maintaining wide Eg characteristics (3.67-4.33 eV vs 2.98 eV), thus successfully addressing the critical Δn-Eg compromise in IR optical materials. Through a synergistic approach combining structural analysis and theoretical calculations, this study elucidates the pivotal role of isomorphous substitution at distinct lattice sites in optimizing both Δn and Eg in [Ba2K][X][MS4]. These results not only establish antiperovskite-type chalcohalides as highly promising IR birefringent crystals but also demonstrate the effectiveness of functional-site cosubstitution engineering as a powerful strategy for designing novel materials with enhanced birefringent properties.