Exceptional Second Harmonic Generation in Ultraviolet Nonlinear Optical Oxyfluoroniobate Crystals via Structural Fingerprint Optimization of Polar Chains

Partial fluorination of oxides has positioned oxyfluorides as a promising class of nonlinear optical (NLO) materials owing to their balanced optical properties. However, effectively arranging optical chromophores to achieve strong optical nonlinearity remains challenging. In this study, we explore the structural chemistry of ¹[NbOF₄]_∞ chain-based oxyfluoroniobates and establish a molecular geometric framework to quantify key structural fingerprint factors, namely, the ∠(O─Nb─O') bond angle, χ[F,Nb,Nb',F'] torsion angle, chain alignment, and distortion of [NbO₂F₄] nodes. Theoretical calculations confirm that these factors critically influence second harmonic generation (SHG) activity. By integrating π-conjugated biuret (C₂H₅N₃O₂) molecules with optimally aligned ¹[NbOF₄]_∞ chains, we synthesized (H₃O)(Biu)₂(NbOF₄) (Biu = biuret), a crystal exhibiting a record-breaking SHG response, reaching 10.8 times that of KH₂PO₄, among transition metal (TM) oxyfluorides. Its moderate birefringence (Δn = 0.062 @1064 nm) and wide band gap (E_g = 4.50 eV) further support its potential as a high-performance ultraviolet (UV) NLO material. These results highlight the power of structural fingerprint optimization in fully activating polar chains and offer a new strategy for designing next-generation UV NLO crystals with enhanced SHG performance.