Ultrawide Bandgap and Outstanding Second‐Harmonic Generation Response by a Fluorine‐Enrichment Strategy at a Transition‐Metal Oxyfluoride Nonlinear Optical Material

Abstract The development of nonlinear optical (NLO) materials has been hindered by competing microstructure requirements: the need to simultaneously engineer a large hyperpolarizability (a large second‐harmonic generation (SHG)) and a wide HOMO–LUMO gap (a wide band gap). Herein, a non‐centrosymmetric transition‐metal (TM) oxyfluoride K 5 (NbOF 4 )(NbF 7 ) 2 (KNOF) with an extremely high F/O ratio is constructed in high yield. KNOF exhibits an extremely wide band gap (5.88 eV) and a strong powder SHG response (4.0×KH 2 PO 4 )—both being the largest values for TM‐centered oxyfluorides—as well as a birefringence sufficient for applications. The dominant roles of the partially fluorinated [NbO 2 F 4 ] and totally fluorinated [NbF 7 ] groups in achieving the enlarged band gap in KNOF have been clarified by first‐principles calculations. Our results suggest that maximizing the fluorine content of oxyfluorides may unlock the promise of short‐wavelength‐transparent materials with exceptional NLO performance.