Na 2 BaB 12 O 18 F 4 : A Mixed Alkali/Alkaline-Earth Metal Fluorooxoborate with Two Unprecedented Interpenetrating Three-Dimensional B-O/F Anionic Networks and a Short Ultraviolet Cutoff Edge.

Fluorooxoborates are promising yet largely untapped crystal materials for linear and nonlinear optical applications. The introduction of a strong electronegative F atom into an oxyboron anionic group offers a virtually unlimited chance for structural engineering and ultimately purposeful tuning of the macroscopic optical properties of the crystal. Herein, a new mixed alkali/alkaline-earth fluorooxoborate, Na 2 BaB 12 O 18 F 4 , was synthesized in a closed system. Na 2 BaB 12 O 18 F 4 features a [B 6 O 11 F 2 ] fundamental building unit (FBB), which polymerizes into two new (first example) independent interpenetrating three-dimensional (3D) B-O/F anionic networks constructed entirely from BO 3 and BO 3 F units. Based on optical characterizations and the first-principles calculations, Na 2 BaB 12 O 18 F 4 exhibits a moderate birefringence (0.054 @ 1064 nm) and a short ultraviolet (UV) cutoff edge (below 190 nm). The successful synthesis and characterization of Na 2 BaB 12 O 18 F 4 may speed up the subsequent discovery of other mixed alkali/alkaline-earth metal fluorooxoborates.