Polar Fluorooxoborate, NaB₄ O₆ F: A Promising Material for Ionic Conduction and Nonlinear Optics

The search of new borates with improved functional properties has attracted considerable attention. Herein, a new polar fluorooxoborate, NaB₄ O₆ F (NBF) was prepared by high-temperature solid-state reaction. NBF belongs to the AB₄ O₆ F family (A=alkali metal or ammonium), a series of compounds that undergoes significant cation-dependent structural changes. NBF is of particular interest owing to the special cation position. Temperature-dependent ionic conductivity measurements show that NBF is a solid ionic conductor, and it has the lowest active energy of 32.5 kJ mol^-1 of fluorooxoborates. NBF also shows a second-harmonic generation (SHG) response of 0.9×KH₂ PO₄ and 0.2×β-BaB₂ O₄ , at 1064 and 532 nm, respectively, and it has a short UV cutoff edge below 180 nm. Based on bond valence (BV) concepts, symmetry analysis, and the first principles calculation, the unique [B₄ O₆ F]_∞ layer can be regarded as the "multifunctional unit", which is responsible for the observed properties of NBF.