Anion-Centered Engineering for Simultaneously Enhancing Birefringence and Band Gap in Chalcogenides

Traditional studies on the enhancement of birefringence have primarily focused on the stereochemically active lone pair of cations; however, the presence of lone-pair electrons often raises the valence-band maximum, thereby significantly reducing the band gap, which hinders their broad-range spectrum application. Anions derived from more electronegative elements exert stronger electron-binding ability, leading to lower energy levels of their nonbonding orbitals compared to those of cations and probably providing a new pathway to overcome the existing limitation. Therefore, in this work, we first propose an anion-centered strategy via structural unit center transformation, similar to that observed in perovskites and antiperovskites, and successfully obtained an anion-centered SCALP-containing chalcogenide, β-[Hg3S2][ZnCl3]Cl, which features the anion-centered [SHg3] unit. Remarkably, β-[Hg3S2][ZnCl3]Cl demonstrates remarkable optical performance with a significant birefringence (0.232@546 nm) and a band gap (3.28 eV) that breaks the limit for cation-centered SCALP-containing chalcogenides and approaches the UV region (320 nm). For the first time, this work confirms the significance of the SCALP-driven second-order Jahn–Teller effect in anion-centered units in simultaneously generating a large polarizability anisotropy and widening the HOMO–LUMO gap, establishing a new paradigm for designing potential birefringent materials.