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

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.