Giant Birefringence Enabled by the Highly Anisotropic Linear IX2− (X = Cl, Br) Building Blocks

Abstract Birefringent crystals are essential for polarized optical devices, yet achieving large birefringence through rational design remains challenging. The key lies in constructing birefringence‐active groups (BAGs) with giant polarization anisotropy and optimal spatial arrangements. Here, we report the successful construction of linear interhalogen BAGs, IX 2 − (X = Cl, Br), enabling giant polarization anisotropy. This was accomplished by simple halogenation of IO 3 − groups in an aqueous solution. Four novel birefringent crystals were synthesized: [H‐4AP][ICl 2 ] ( 1 , 4AP = 4‐aminopyridine), [HDMA] 2 [ICl 2 ]·Cl ( 2 , DMA = dimethylamine), [H‐4AP][IBr 2 ] ( 3 ), and [HDMA] 2 [IBr 2 ]·Br ( 4 ). In all these compounds, the linear IX 2 − BAG adopts parallel arrangements, effectively maximizing synergistic polarization anisotropy. As a result, compounds 1 – 4 exhibit giant birefringence values in both the visible (0.647, 0.585, 0.836, and 0.782 at 546 nm) and near‐infrared (NIR) regions (0.510, 0.356, 0.762, and 0.509 at 1064 nm), surpassing commercial birefringent crystals and many state‐of‐the‐art materials. Furthermore, these compounds achieve an optimal balance between giant birefringence and moderate bandgap among linear BAG‐based materials. Detailed theoretical calculations confirm that the IX 2 − BAGs play a dominant role in this exceptional birefringence. This study demonstrates the remarkable potential of linear interhalogen anions for developing high‐performance birefringent crystals.