A2B6O9F2(A = NH4, K): new members of A2B6O9F2family with deep-UV cutoff edges and moderate birefringence

Two new fluorooxoborates A 2 B 6 O 9 F 2 (A = NH 4 , K) with different two-dimensional [B 6 O 9 F 2 ] ∞ layered structure constructed by the fundamental building block [B 6 O 11 F 2 ] were obtained.