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.