(NH <sub>4</sub> ) <sub>2</sub> (I <sub>5</sub> O <sub>12</sub> )(IO <sub>3</sub> ) and K <sub>1.03</sub> (NH <sub>4</sub> ) <sub>0.97</sub> (I <sub>5</sub> O <sub>12</sub> )(IO <sub>3</sub> ): Mixed‐Valent Polyiodates with Unprecedented I <sub>5</sub> O <sub>12</sub> <sup>−</sup> Unit Exhibiting Strong Second‐Harmonic Generation Responses and Giant Birefringence
Nan Ma, Jin Chen, Bing‐Xuan Li, Chun‐Li Hu, Jiang‐Gao Mao
Abstract
Abstract Second‐harmonic generation (SHG) response and birefringence are crucial properties for linear and nonlinear optical (NLO) materials, while it is difficult to further optimize these two key properties by using a single traditional functional building block (FBB) in one compound. Herein, a novel IO 4 5− unit is identified, which possesses a square‐planar configuration and two stereochemically active lone‐pairs (SCALPs). By combining IO 4 5− and IO 3 − units, the first examples of mixed‐valent polyiodates featuring an unprecedented bowl‐shaped I 5 O 12 − polymerized unit, namely (NH 4 ) 2 (I 5 O 12 )(IO 3 ) and K 1.03 (NH 4 ) 0.97 (I 5 O 12 )(IO 3 ), are successfully synthesized. Excitingly, both crystals exhibit strong SHG responses (16 × KDP and 19.5 × KDP @1064 nm) as well as giant birefringence (∆ n exp = 0.431 and 0.405 @546 nm). Detailed structure‐property analyses reveal that the parallel aligned planar IO 4 5− units induce the properly aligned high‐density SCALPs, leading to strong SHG response and giant birefringence for both materials. This work not only provides two new potential NLO and birefringent crystals, but also discovers a novel promising FBB (IO 4 5− ) for developing high‐performance linear and nonlinear optical materials.