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 [B 3 IO 8 (OH)] 3– and [B 4 IO 11 ] 5– anionic functional groups, namely, Na[B 3 IO 7 (OH)] ( NBIO-I ) and Na 2 [B 4 IO 9 ](IO 3 ) ( NBIO-II ), were successfully synthesized for the first time through the boric acid melting method under different molar ratios of H 3 BO 3 /NaIO 3 . NBIO-I ( Pbca ) features a unique 1D ∞ 1 [B 3 IO 7 (OH)] − chain composed of corner-sharing [B 3 O 6 (OH)] 4– groups with [IO 3 ] − groups hanging on both sides of the chain. In contrast, NBIO-II ( Cc ) displays a novel 2D layer of ∞ 2 [B 4 IO 9 ] −, in which all of the [IO 3 ] − groups are hanging on the same side of the ∞ 2 [B 4 O 7 ] 2– layer formed by corner-sharing [B 3 O 7 ] 5– and [BO 3 ] 3– groups. Remarkably, NBIO-II 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 NBIO-II as promising NLO crystals but also provided an effective synthetic route for new boroiodates with excellent optical properties.