MM'B 3 O 4 F 3 (M = K; M' = Na, K, Cs): Alkali-Metal Fluorooxoborates with ∞ 1 [B 3 O 4 F 3 ] Chains and Deep-Ultraviolet Cutoff Edges.

Three mixed-alkali-metal fluorooxoborates, KNaB 3 O 4 F 3 ( I ), K 2 B 3 O 4 F 3 ( II ), and KCsB 3 O 4 F 3 ( III ), were acquired in a closed system. I - III are isomorphic and adopt orthorhombic structures [ Pbcn (No. 60)] with wavy parallelly arranged pseudolayers composed of ∞ 1 [B 3 O 4 F 3 ] chains, which exhibit slight differences in the arrangement modes of the fundamental building blocks. First-principles calculations illustrate that they all have moderate birefringence and large band gaps on the order of 7.0 eV, suggesting deep-ultraviolet (DUV) cutoff edges. In order to investigate the main source of the optical properties, the electronic structure and anisotropy of the response electron distribution were analyzed. Experimental characterizations for I confirm the structure and DUV transparence ability.