An Antimony(III) Fluoride Oxalate with Large Birefringence

Birefringent materials play a key role in modulating the polarization of light and thus in optical communication as well as in laser techniques and science. Designing new, excellent birefringent materials remains a challenge. In this work, we designed and synthesized the first antimony(III) fluoride oxalate birefringent material, KSb₂ C₂ O₄ F₅ , by a combination of delocalized π-conjugated [C₂ O₄ ]^2- groups, stereochemical active Sb³⁺ cations, and the most electronegative element, fluorine. The [C₂ O₄ ]^2- groups are not in an optimal arrangement in the crystal structure of KSb₂ C₂ O₄ F₅ ; nonetheless, KSb₂ C₂ O₄ F₅ exhibits a large birefringence (Δn=0.170 at 546 nm) that is even better than that of the well-known commercial birefringent material α-BaB₂ O₄ , even though the latter features an optimal arrangement of π-conjugated [B₃ O₆ ]^3- groups. Based on first-principles calculations, this prominent birefringence should be attributed to the alliance of planar π-conjugated [C₂ O₄ ]^2- anions, highly distorted SbO₂ F₂ and SbOF₃ polyhedra with a stereochemically active lone pair. The combination of lone-pair electrons and π-conjugated systems boosts the birefringence to a large extent and will help the development of high-performance birefringent materials.