CsICl2: A Record-Birefringent Inorganic Crystal with Ultrawide Infrared Transparency Enabled by Linear Interhalogen ICl2- Units.

Birefringent crystals are essential optical materials for polarized light modulation, yet few exhibit both large birefringence and broad transparency in the mid- to far-infrared (MIR-FIR) regions. Here, we report the successful synthesis of an inorganic birefringent crystal, CsICl2 (CIC), by incorporating the highly anisotropic linear ICl2- birefringence-active group (BAG) with a heavy Cs+ cation under hydrothermal conditions. The large polarization anisotropy, perfect alignment, and dense packing of ICl2- endow CIC with record-high birefringence values in both the visible (Δnexp = 0.994 at 546 nm) and infrared (Δncal = 0.719 at 1064 nm) regions among all known inorganic crystals with Eg > 2.0 eV. CIC also exhibits an ultrawide optical transparency window from 0.367 to at least 25 μm and can be grown into centimeter-scale single crystals. This study introduces CIC as a promising IR birefringent material and highlights the potential of linear interhalogen BAGs for the design of next-generation IR optical crystals.