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 Δ<i>n</i> and wide bandgap (<i>E</i>_g) 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 Δ<i>n</i> values increasing from 0.03 to 0.07-0.08 (at both 1064 and 2050 nm) while maintaining wide <i>E</i>_g characteristics (3.67-4.33 eV vs 2.98 eV), thus successfully addressing the critical Δ<i>n</i>-<i>E</i>_g 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 Δ<i>n</i> and <i>E</i>_g 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.