Breaking the Deep‐UV Transparency/Optical Nonlinearity Trade‐Off: Three‐Parameter Optimization in Oxyfluorides by Tailoring d0‐Metal Incorporation

Abstract Achieving an optimal balance among key optical performance parameters (i.e., bandgap, second‐harmonic generation (SHG), and birefringence) is critically important for addressing application constraints and advancing the development of nonlinear optical (NLO) materials. We report herein the first examples of deep‐ultraviolet (deep‐UV) transparent mixed d 0 ‐metal oxyfluorides A 5 (NbOF 4 )(TaF 7 ) 2 (ANTOF: A = K, Rb, Cs, NH 4 ), synthesized through a synergistic dual‐site strategy. Our synthetic strategy enables the targeted incorporation of highly distorted d 0 ‐metal octahedra into metal fluorides, creating two distinct d 0 ‐metal polyhedra with complementary microstructural features that can reconcile trade‐offs in key optical properties. By introducing 4d 0 ‐Nb‐based [NbO 2 F 4 ] oxyfluoride octahedra into the centrosymmetric parent 5d 0 ‐Ta‐based fluorides A 2 TaF 7 , the resultant ANTOFs not only crystallize with noncentrosymmetric polar structures but, more importantly, demonstrate exceptional performance, including record‐high phase‐matchable SHG responses for deep‐UV transparent d 0 ‐metal oxyfluorides (3.3−3.8 × KH 2 PO 4 @ 1064 nm (visible region) and 0.33−0.38 × β ‐BaB 2 O 4 @ 532 nm (UV region)), wide bandgaps (> 6.36 eV), and suitable birefringence (Δ n = 0.093−0.105 @ 546 nm). Theoretical calculations and crystal structure analyses reveal that the superior linear and nonlinear optical properties of the ANTOFs originate from the dual‐site synergy between the polarizable [NbO 2 F 4 ] octahedra and the [TaF 7 ] pentagonal bipyramids, in which ligand‐to‐metal charge transfer transitions are suppressed.