Cation-anion synergetic interactions achieving tunable birefringence in quasi-one-dimensional antimony(III) fluoride oxalates

Birefringent materials with large optical anisotropy, which can be used to modulate the polarization of light, play a key role in laser techniques and science. However, the exploration studies of new, superior birefringent materials develop extremely slowly due to the lack of effective guidelines for rational design. Herein, three antimony(III) fluoride oxalates, namely, Na2Sb2(C2O4)F6, K2Sb2(C2O4)F6, and Cs2Sb2-(C2O4)2F4·H2O, were successfully synthesized through a rational combination of π-conjugated C2O42− anions and Sb3+ cations with stereochemically active lone pairs. These oxalates feature unique quasi-one-dimensional chain structures that induce large optical anisotropy. Remarkably, Cs2Sb2(C2O4)2-F4·H2O exhibits the largest birefringence (0.325@546 nm) among all reported antimony(III)-based oxysalts. Detailed structural analysis and theoretical calculations confirmed that the optical anisotropy of these oxalates could be tuned through the synergetic interactions of templated cations and anionic functional groups. This work may open the door to efficiently designing excellent birefringent materials and guide the further discovery of other novel structure-driven functional materials.