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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-Hg 3 S 2 ZnCl 3 Cl, which features the anion-centered SHg 3 unit. Remarkably, β-Hg 3 S 2 ZnCl 3 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.
Xie et al. (Mon,) studied this question.