(NH 4 ) 2 (I 5 O 12 )(IO 3 ) and K 1.03 (NH 4 ) 0.97 (I 5 O 12 )(IO 3 ): Mixed-Valent Polyiodates with Unprecedented I 5 O 12 - Unit Exhibiting Strong Second-Harmonic Generation Responses and Giant Birefringence.

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.