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.