MM′B3O4F3 (M = K; M′ = Na, K, Cs): Alkali-Metal Fluorooxoborates with ∞1[B3O4F3] Chains and Deep-Ultraviolet Cutoff Edges

Three mixed-alkali-metal fluorooxoborates, KNaB3O4F3 (I), K2B3O4F3 (II), and KCsB3O4F3 (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[B3O4F3] 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.