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.