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, VO₂ (H₂ O)(IO₃ ) (VIO), a new class of iodate, crystallizes in a polar lattice with ${}_{\infty }{}^1[$ VO₂ (H₂ O)(IO₃ )] zigzag chains connected by weak hydrogen bonds and intermolecular forces. VIO exhibits very large SHG responses (18 × KH₂ PO₄ @ 1200 nm, 1.5 × KTiOPO₄ @ 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 ${}_{\infty }{}^1[$ VO₂ (H₂ O)(IO₃ )] 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.

Keywords: Birefringence; Iodate; Second-Harmonic Generation; Secondary Bond; Structure-Property Relationships.