Secondary‐Bond‐Driven Construction of a Polar Material Exhibiting Strong Broad‐Spectrum Second‐Harmonic Generation and Large Birefringence

Abstract Considerable effort has been invested in the development of non‐centrosymmetric (NCS) inorganic solids for ferroelectricity‐, piezoelectricity‐ and, particularly, optical nonlinearity‐related applications. While great progress has been made, a persistent problem is the difficulty in constructing NCS materials, which probably stems from non‐directionality and unsaturation of the ionic bonds between metal counter‐cations and covalent anionic modules. We report herein a secondary‐bond‐driven approach that circumvents the cancellation of dipole moments between adjacent anionic modules that has plagued second‐harmonic generation (SHG) material design, and which thereby affords a polar structure with strong SHG properties. The resultant first NCS counter‐cation‐free iodate, VO 2 (H 2 O)(IO 3 ) (VIO), a new class of iodate, crystallizes in a polar lattice with VO 2 (H 2 O)(IO 3 )] zigzag chains connected by weak hydrogen bonds and intermolecular forces. VIO exhibits very large SHG responses (18 × KH 2 PO 4 @ 1200 nm, 1.5 × KTiOPO 4 @ 2100 nm) and sufficient birefringence (0.184 @ 546 nm). Calculations and crystal structure analysis attribute the large SHG responses to consistent polarization orientations of the VO 2 (H 2 O)(IO 3 )] chains controlled by secondary bonds. This study highlights the advantages of manipulating the secondary bonds in inorganic solids to control NCS structure and optical nonlinearity, affording a new perspective in the development of high‐performance NLO materials.