Two Hydroxyfluorooxoborates Achieving Deep‐Ultraviolet Cutoff Edges and Moderate Birefringence by Assembling Multi‐Anionic Groups

Assembling multi‐anionic groups is conducive to utilizing respective advantage to achieve the enhancement of optical performance. Two new hydroxyfluorooxoborates, Ama 2‐Rb 2 B 3 O 3 F 4 (OH) and K 8 Cs 2 B 15 O 14 (OH) 7 F 20 ⋅ H 2 O with [B 3 O 3 F 4 (OH)] six‐membered rings were synthesized for the first time. The title compounds exhibit short ultraviolet cutoff edges (<200 nm) and K 8 Cs 2 B 15 O 14 (OH) 7 F 20 ⋅ H 2 O possesses a moderate experimental refractive index difference of 0.051@546 nm.