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

Abstract

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.