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

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