Na[B₃IO₇(OH)] and Na₂[B₄IO₉](IO₃): Explorations of Boroiodates with Strong Second Harmonic Generation Response and Large Birefringence

In the exploration of nonlinear optical (NLO) materials, developing new functional groups for high performance is essential. Achieving a balance among band gap, second harmonic generation (SHG), and birefringence within a compound is quite challenging. By directly connecting iodates and borates by B-O-I bridges, two boroiodates with novel [B₃IO₈(OH)]^3- and [B₄IO₁₁]^5- anionic functional groups, namely, Na[B₃IO₇(OH)] (<b>NBIO-I</b>) and Na₂[B₄IO₉](IO₃) (<b>NBIO-II</b>), were successfully synthesized for the first time through the boric acid melting method under different molar ratios of H₃BO₃/NaIO₃. <b>NBIO-I</b> (<i>Pbca</i>) features a unique 1D ^∞₁[B₃IO₇(OH)]^- chain composed of corner-sharing [B₃O₆(OH)]^4- groups with [IO₃]^- groups hanging on both sides of the chain. In contrast, <b>NBIO-II</b> (<i>Cc</i>) displays a novel 2D layer of ^∞₂[B₄IO₉]^-, in which all of the [IO₃]^- groups are hanging on the same side of the ^∞₂[B₄O₇]^2- layer formed by corner-sharing [B₃O₇]^5- and [BO₃]^3- groups. Remarkably, <b>NBIO-II</b> exhibits a wide band gap (4.3 eV), large birefringence (0.298@546 nm), and strongest SHG response (7.8 × KDP) among borate-iodates reported previously. This work not only discovered <b>NBIO-II</b> as promising NLO crystals but also provided an effective synthetic route for new boroiodates with excellent optical properties.