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

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 d⁰-metal oxyfluorides A₅(NbOF₄)(TaF₇)₂ (ANTOF: A = K, Rb, Cs, NH₄), synthesized through a synergistic dual-site strategy. Our synthetic strategy enables the targeted incorporation of highly distorted d⁰-metal octahedra into metal fluorides, creating two distinct d⁰-metal polyhedra with complementary microstructural features that can reconcile trade-offs in key optical properties. By introducing 4d⁰-Nb-based [NbO₂F₄] oxyfluoride octahedra into the centrosymmetric parent 5d⁰-Ta-based fluorides A₂TaF₇, 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 d⁰-metal oxyfluorides (3.3−3.8 × KH₂PO₄ @ 1064 nm (visible region) and 0.33−0.38 × β-BaB₂O₄ @ 532 nm (UV region)), wide bandgaps (> 6.36 eV), and suitable birefringence (Δn = 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 [NbO₂F₄] octahedra and the [TaF₇] pentagonal bipyramids, in which ligand-to-metal charge transfer transitions are suppressed.