[TeSeS ₂ ] ²⁻ : The First Heterotriple‐Chalcogenide Motifs Decode Giant Mid‐Far Infrared Birefringence

Abstract Optical anisotropy, a cornerstone of polarization control, presents fundamental challenge in functional optical material design due to the intricate interplay between its regulation mechanisms and performance optimization. The development of mid‐far infrared (MFIR) birefringent crystals has long been shackled by the incompatibility between giant optical anisotropy and ultrabroad transparency in conventional chalcogenides. Here, we break this shackle via heterotriple‐chalcogenide engineering in the first thiotellurite birefringent family, BaTeQ 3 (Q = S, Se), featuring isolated [TeQ 3 ] 2 ⁻ motifs. BaTeS 3 achieves a large experimental birefringence (Δ n = 0.193@550 nm, 15× commercial MgF 2 ). More strikingly, the assembly of three distinct Group VIA chalcogens (S, Se, Te) creates the first heterotriple‐chalcogenide motif—[TeSeS 2 ] 2 ⁻ in BaTeSeS 2 , delivering giant Δ n = 0.55@550 nm—a 284% increase from BaTeS 3 and surpassing all commercial benchmarks. BaTeSeS 2 concurrently exhibit ultrabroad transparency (0.5–25 µm), overcoming the classical trade‐off between anisotropy and transparency. First‐principles calculations reveal that their extraordinary optical anisotropy stems from synergistic stereochemically active lone pair (SCALP)‐driven electron localization in [TeQ 3 ] 2− units (electron localization function > 0.8) and Se‐induced pyramidal distortion. The pioneering [TeSeS₂] 2 ⁻ heterotriple‐chalcogenide motif establishes a transformative design paradigm, which not only redefines the materials motifs for giant MFIR birefringence but also unlocks unprecedented opportunities in next‐generation infrared lasers, polarizers, and quantum communication devices.