CsICl<sub>2</sub>: A Record-Birefringent Inorganic Crystal with Ultrawide Infrared Transparency Enabled by Linear Interhalogen ICl<sub>2</sub><sup>–</sup> Units
Chong‐An Chen, Yang Li, Congcong Jin, Kang Min Ok
Abstract
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, CsICl 2 ( CIC ), by incorporating the highly anisotropic linear ICl 2 – birefringence-active group (BAG) with a heavy Cs + cation under hydrothermal conditions. The large polarization anisotropy, perfect alignment, and dense packing of ICl 2 – endow CIC with record-high birefringence values in both the visible (Δ n exp = 0.994 at 546 nm) and infrared (Δ n cal = 0.719 at 1064 nm) regions among all known inorganic crystals with E g > 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.