K 5 [B 3 O 3 F 4 (OH)] 2 (NO 3 ): the first hydroxyfluorooxoborate-nitrate with a short ultraviolet cutoff edge and large birefringence.

The first hydroxyfluorooxoborate-nitrate mixed anion compound, K 5 [B 3 O 3 F 4 (OH)] 2 (NO 3 ), was synthesized by the solution evaporation method. It displays a unique structure built by K + cations, the hydroxylated and fluorinated six-membered ring [B 3 O 3 F 4 (OH)] and [NO 3 ] groups. It possesses a band gap of 5.68 eV derived from the diffuse reflectance spectrum, which corresponds to an ultraviolet cutoff edge of 218 nm. First-principles calculations show that it has a large birefringence of 0.095 at 532 nm and the result of the response electron distribution anisotropy method indicates that all three anion groups contribute positively to the birefringence, verifying the synergic contributions from the multiple anion groups.