Breaking the tetrahedral constraint of tetracoordination: Square-planar [ICl 4 ] enables unprecedented birefringence in [C(NH 2 ) 3 ][ICl 4 ]

Birefringent crystals are indispensable for optoelectronic applications, yet commercially available crystals still suffer from the drawback of insufficient birefringence. Meanwhile, tetracoordination units—traditionally confined to a tetrahedral geometry—have long been sidelined from mainstream high-performance birefringent materials due to their intrinsic nearly negligible polarizability anisotropy. Herein, we break this tetrahedral constraint—demonstrating tetracoordinate square-planar [ICl 4 ] anions as a revolutionary platform for unprecedented birefringence. To achieve this, we report an improved synthesis method that avoids the use of toxic Cl 2 gas, enabling the preparation of [C(NH 2 ) 3 ][ICl 4 ] featuring square-planar [ICl 4 ] units. This compound displays a birefringence of 0.975@550 nm, setting a new record that surpasses all existing compounds containing tetracoordinate structural units. The giant birefringence of this material is attributed to the strong polarization anisotropy induced by the unique square-planar geometry of the [ICl 4 ] tetracoordinate units. This research provides theoretical guidance for designing high-performance polar materials based on tetracoordinate units in giant birefringent materials and promotes applications in related optoelectronic device.