New antimony fluorooxoborates with strong birefringence and unprecedented structural characterisation.

Fluorooxoborates constitute a rich source of optical crystals due to their structural diversity and excellent performance. Antimony fluorooxoborates with stereochemically active lone pairs of electrons still have not been found, although the first antimony borate was discovered several years ago. In this study, we have achieved the successful synthesis of the first antimony(III) fluorooxoborate with an unprecedented [B 2 O 4 F] ∞ chain, namely SbB 2 O 4 F. Remarkably, SbB 2 O 4 F shows strong birefringence (0.171@1064 nm) and short UV cutoff edges (about 220 nm) according to calculations. The birefringence of SbB 2 O 4 F mainly originates from the highly distorted [SbO 4 ] groups.