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

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, BaTeQ₃ (Q = S, Se), featuring isolated [TeQ₃]²⁻ motifs. BaTeS₃ achieves a large experimental birefringence (Δn = 0.193@550 nm, 15× commercial MgF₂). More strikingly, the assembly of three distinct Group VIA chalcogens (S, Se, Te) creates the first heterotriple-chalcogenide motif-[TeSeS₂]²⁻ in BaTeSeS₂, delivering giant Δn = 0.55@550 nm-a 284% increase from BaTeS₃ and surpassing all commercial benchmarks. BaTeSeS₂ 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 [TeQ₃]^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.