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, KSb 2 C 2 O 4 F 5 , by a combination of delocalized π-conjugated [C 2 O 4 ] 2− groups, stereochemical active Sb 3+ cations, and the most electronegative element, fluorine. The [C 2 O 4 ] 2− groups are not in an optimal arrangement in the crystal structure of KSb 2 C 2 O 4 F 5 ; nonetheless, KSb 2 C 2 O 4 F 5 exhibits a large birefringence (Δ n =0.170 at 546 nm) that is even better than that of the well-known commercial birefringent material α-BaB 2 O 4 , even though the latter features an optimal arrangement of π-conjugated [B 3 O 6 ] 3− groups. Based on first-principles calculations, this prominent birefringence should be attributed to the alliance of planar π-conjugated [C 2 O 4 ] 2− anions, highly distorted SbO 2 F 2 and SbOF 3 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. Graphical A synergistic combination of different functional groups has led to the first antimony(III) fluoride oxalate material, KSb 2 C 2 O 4 F 5 , to exhibit a birefringent response (Δ n =0.170 at 546 nm); this value is larger than that of the commercial α-BaB 2 O 4 , despite its suboptimal anisotropic structure.