Linear [ICl ₂ ] Units Induced Giant Optical Anisotropy in AICl ₂ (A = NH ₄ , Rb): A Pathway to Advanced Birefringent Materials

ABSTRACT Designing high‐performance birefringent materials that simultaneously achieve large birefringence, a wide transparency range, and favorable crystal growth habits remains a critical yet challenging objective in polarized optical devices. Herein, two new interhalogens AICl 2 (A = NH 4 , Rb), were rationally designed by coupling [NH 4 ] + and alkali metal ions with linear dihalide units and fabricated by solution methods. Both compounds crystallize in the orthorhombic Pnma space group and comprise linear [ICl 2 ] units. The well‐aligned [ICl 2 ] units in the structures induce large birefringence: 0.75 for RbICl 2 and 0.70 for NH 4 ICl 2 at 546 nm, respectively, which are 3.7/3.4 times larger than that of commercial birefringent crystal YVO 4 (0.204@546 nm). A high‐quality RbICl 2 single crystal (1.1 × 0.5 × 0.2 cm 3 ), exhibiting a wide transparency window extending from 0.50 to 25 µm, was grown via room‐temperature solution crystallization. Theoretical calculations reveal that the optical anisotropy is governed by the anisotropic electron density distribution within the [ICl 2 ] unit and the ordered arrangement of these units, confirming it as an advantageous birefringence‐active motif. The results highlight the inorganic interhalogen as an emerging system for the design of high‐performance photoelectric functional materials, with RbICl 2 as a promising birefringent material.