CsZrF₄(IO₃): The First Polar Zirconium Iodate with cis-[ZrO₂F₆] Polyhedra Inducing Optimized Balance of Large Band Gap and Second Harmonic Generation

The development of d⁰ transition-metal (TM) iodates is of critical importance because these iodates are promising candidates for second-order nonlinear optical (NLO) materials with excellent optical properties and rich functional and structural diversity. In this work, the first polar zirconium iodate CsZrF₄(IO₃) has been synthesized via a facile hydrothermal reaction. It features an unusual linear chain structure comprised of edge-sharing cis-[ZrO₂F₆] polyhedra and [IO₃] groups. The newly developed cis-[ZrO₂F₆] polyhedra are structure-directing, inducing alignment of the [IO₃] groups and playing a decisive role in the macroscopic polarity of the crystal structure. Crystalline CsZrF₄(IO₃) displays a large optical band gap (4.26 eV, the largest value among the d⁰-TM iodate family), a strong second-harmonic generation response (4.5 × KH₂PO₄), a broad transparency window extending from the UV to the mid-IR region (0.29–5.8 μm), and a high laser damage threshold. First-principles calculations suggest that the cis-[ZrO₂F₆] polyhedra and [IO₃] groups make dominant contributions to the large band gap and SHG response simultaneously. This study shows that high coordination number d⁰-TM oxide-fluoride polyhedra afford a new paradigm for the development of efficient NLO materials.