Na 3 B 7 O 11 F 2 : a new sodium-rich fluorooxoborate with a unique [B 14 O 24 F 4 ] ring and a short ultraviolet absorption edge.

A new sodium-rich fluorooxoborate Na3B7O11F2 features a novel large [B14O24F4] ring formed by two corner-shared [B7O13F2] groups, each containing two [B3O7] anionic units linked by a [BO2F2] group. The band gap of the title compound calculated by using first-principles calculations is 7.69 eV (similar to 161 nm) and the birefringence refractive index is 0.083 at 193 nm. The results show that it is potentially an excellent deep-ultraviolet (DUV) birefringence crystal.