K₅(W₃O₉F₄)(IO₃): An Efficient Mid-Infrared Nonlinear Optical Compound with High Laser Damage Threshold

The discovery of nonlinear optical (NLO) materials for generating coherent mid-infrared (mid-IR) light is critically important in developing laser technologies. However, all of the commercialized mid-IR crystals thus far possess problems that have hindered their application. We report herein a function-directing structural design strategy to afford a new type of mid-IR NLO material, using fluorinated 5d⁰-transition metal octahedra to construct a polar noncentrosymmetric material K₅(W₃O₉F₄)(IO₃). The Λ-shaped [W₃O₁₂F₄]^10- polyanion, consisting of distorted octahedra, not only guides the alignment of the [IO₃]^- units but also serves as a new kind of NLO-active chromophore in the mid-IR, with K₅(W₃O₉F₄)(IO₃) possessing an optimal combination of large second-harmonic generation response (11 × KH₂PO₄@1064 nm and 0.5 × AgGaS₂@2100 nm), wide visible and mid-IR transparency (0.32-10.5 μm), and a giant laser damage threshold (95 × AgGaS₂). The important role of the [W₃O₁₂F₄]^10- polyanion in improving the NLO properties and extending the IR transparency region has been clarified through first-principles calculations. Our results suggest that the incorporation of fluorinated 5d⁰-transition metal chromophores into iodate materials may afford a viable route to novel promising mid-IR NLO materials with exceptional NLO functions.