(NH4)2(I5O12)(IO3) and K1.03(NH4)0.97(I5O12)(IO3): Mixed‐Valent Polyiodates with Unprecedented I5O12− Unit Exhibiting Strong Second‐Harmonic Generation Responses and Giant Birefringence

Abstract

AbstractSecond‐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 IO45− unit is identified, which possesses a square‐planar configuration and two stereochemically active lone‐pairs (SCALPs). By combining IO45− and IO3− units, the first examples of mixed‐valent polyiodates featuring an unprecedented bowl‐shaped I5O12− polymerized unit, namely (NH4)2(I5O12)(IO3) and K1.03(NH4)0.97(I5O12)(IO3), are successfully synthesized. Excitingly, both crystals exhibit strong SHG responses (16 × KDP and 19.5 × KDP @1064 nm) as well as giant birefringence (∆nexp = 0.431 and 0.405 @546 nm). Detailed structure‐property analyses reveal that the parallel aligned planar IO45− 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 (IO45−) for developing high‐performance linear and nonlinear optical materials.