Sharp Enhancement of Birefringence in Antimony Oxalates Achieved by the Cation–Anion Synergetic Interaction Strategy

Birefringent materials with large birefringence play an important role in in laser science and technology owing to their ability to modulate polarized light. However, the lack of systematic and effective synthesis strategies severely hinders the development of novel superior birefringent materials. Herein, the cation–anion synergetic interaction strategy was proposed to successfully synthesize two excellent UV birefringent materials, RbSb(C2O4)F2·H2O and [C(NH2)3]Sb(C2O4)F2·H2O. Both compounds feature unprecedented [Sb(C2O4)F2]∞– anionic chains composed of planar π-conjugated [C2O4]2– units and a distorted SbO4F2 complex with stereochemically active lone pairs, which induce a large optical anisotropy. Remarkably, further enhancement of birefringence in [C(NH2)3]Sb(C2O4)F2·H2O was achieved via cation–anion synergetic interactions between the [C(NH2)3]+ cationic groups and [Sb(C2O4)F2]∞– anionic chains. It exhibited a giant birefringence of 0.323@546 nm, twice larger than that of its analogue RbSb(C2O4)F2·H2O (0.162@546 nm). A detailed structural analysis and theoretical calculations revealed that the cation–anion synergetic interaction strategy is an effective strategy for the efficient exploration of superior birefringent materials, which will guide the further exploration of new structure-driven functional materials.