From Ba₃Nb₂O₂F₁₂(H₂O)₂ to Ba_0.5NbO₂F₂(H₂O): Achieving Balanced Nonlinear Optical Performance by O/F Ratio Regulation

Metal oxyhalides have attracted extensive attention as fascinating nonlinear optical (NLO) materials. Here, two barium niobium oxyfluorides, Ba₃Nb₂O₂F₁₂(H₂O)₂ (1) and Ba_0.5NbO₂F₂(H₂O) (2), have been obtained by regulating the O/F ratio in Nb-O-F units (NbOF₆ pyramids to NbO₄F₂ octahedra). The overall NLO effect is improved from 1 to 2 with larger second-harmonic generation (SHG) intensities (0.9 to 1.7 × KH₂PO₄ (KDP)) and maintainable large band gaps (>3.0 eV) by regulating the O/F ratio in Nb-O-F units (1:6 to 4:2).