A2B6O9F2 (A = NH4, K): new members of A2B6O9F2 family with deep-UV cutoff edges and moderate birefringence

The fluorooxoborate A2B6O9F2 family (A = alkali metal) has attracted much attention because of its diverse structures and properties. Herein, two new members of this family, (NH4)2B6O9F2 and K2B6O9F2, have been synthesized which exhibit different two-dimensional [B6O9F2]∞ layered structures. Millimeter-sized crystals of (NH4)2B6O9F2 have been grown with NH4F as the flux. The transmittance spectrum indicates that it has a wide UV transparency window with a cutoff edge lower than 180 nm. First-principles calculations show that both (NH4)2B6O9F2 and K2B6O9F2 have deep-UV cutoff edges and moderate birefringence. The contributions of the constituent structural groups to the optical properties are discussed.