A 2 B 6 O 9 F 2 (A = NH 4 , K): new members of A 2 B 6 O 9 F 2 family with deep-UV cutoff edges and moderate birefringence.

The fluorooxoborate A 2 B 6 O 9 F 2 family (A = alkali metal) has attracted much attention because of its diverse structures and properties. Herein, two new members of this family, (NH 4 ) 2 B 6 O 9 F 2 and K 2 B 6 O 9 F 2 , have been synthesized which exhibit different two-dimensional [B 6 O 9 F 2 ] ∞ layered structures. Millimeter-sized crystals of (NH 4 ) 2 B 6 O 9 F 2 have been grown with NH 4 F 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 (NH 4 ) 2 B 6 O 9 F 2 and K 2 B 6 O 9 F 2 have deep-UV cutoff edges and moderate birefringence. The contributions of the constituent structural groups to the optical properties are discussed.