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 the 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 F element. 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 . Remarkably, KSb 2 C 2 O 4 F 5 exhibits a large birefringence (Δ n = 0.170 @546 nm) that is even better than the well-known commercial birefringent material α -BaB 2 O 4 , whereas the latter features an optimal arrangement of π -conjugated [B 3 O 6 ] 3- groups. Based on the 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 stereochemical activity lone pair. Combination of lone pair electrons and π -conjugated systems enables to boost the birefringence to a large extent and will help to develop high-performance birefringent materials.