[ICl 4 ] − : A large anisotropic planar anion group produces giant birefringence

We designed the highly anisotropic planar [ICl₄]^- group. It enabled exceptional birefringence (Δn = 1.162 at 546 nm) and broad mid-infrared transparency (0.36-6 µm) in C₅H₇N₂ICl₄. This study provides a novel strategy for obtaining high-performance mid-IR birefringent crystals.