Domino‐Effect Amplification via Covalent‐Chain Engineering for Giant Birefringence in NaAsQ 2 (Q = S, Se) Crystals
Bo Zhang, Wei‐Qi Huang, Yang Chi, Xin‐Tao Wu, Hua Lin, Qi‐Long ZhuABSTRACT
Birefringent crystals are vital for photonics technologies, yet conventional materials face trade‐offs between birefringence (Δn), mid‐to‐far‐infrared (MFIR) transparency, and stability. Thioarsenates, with high anisotropy and wide MFIR transparency, are promising alternatives but have been confined to 0D clusters with low Δn. Here, we break this barrier via covalent‐chain engineering, developing NaAsQ 2 (Q═S, Se). Transforming isolated [AsQ 3 ] 3– clusters into continuous helical 1D [AsQ 2 ] – chains, a “0D points to 1D lines” transition enables polarization coupling and domino‐type amplification of anisotropy. This yields Δn of 0.65 (NaAsS 2 ), 0.51 (α‐NaAsSe 2 ), and 0.92 (β‐NaAsSe 2 ) at 550 nm—260%, 183%, and 410% higher than cluster‐based thioarsenates—while maintaining broad MFIR transmission and high stability, overcoming traditional trade‐offs. Mechanistic analysis reveals that isolated [AsS 3 ] 3– units accumulate polarizability anisotropy linearly, whereas 1D [AsS 2 ] – chains exhibit exponential enhancement, triggering a “domino effect” that exceeds 30‐fold amplification at n = 50 and grows further as n → ∞. This exponential amplification, driven by As–S σ‐bonding and delocalized electron channels in the 1D [AsS 2 ] – chains, breaks the intrinsic limitations of isolated clusters. Our work establishes NaAsQ 2 as a promising MFIR birefringent crystal and demonstrates covalent‐chain engineering as a powerful paradigm for designing giant birefringent materials.