α‑AgI₃O₈ and β‑AgI₃O₈ with Large SHG Responses: Polymerization of IO₃ Groups into the I₃O₈ Polyiodate Anion

Abstract

Two new noncentrosymmetric isomeric silver polyiodates, namely, α-AgI₃O₈ (<i>Pnc</i>2) and β-AgI₃O₈ (<i>I</i>4̅), have been synthesized through the hydrothermal reactions of AgNO₃ with I₂O₅. Both isomers exhibit layered structures that are constructed from I₃O₈^– anions interconnected by Ag⁺ cations. The main structural difference between the two isomers lies in the different stacking fashions of [AgI₃O₈] layers along the <i>c</i> axis in order to meet the requirements of their space groups. Powder second-harmonic generation (SHG) measurements indicate that α-AgI₃O₈ and β-AgI₃O₈ are both phase-matchable materials with large SHG responses of approximately 9.0 and 8.0 times that of KH₂PO₄, respectively. UV–vis–NIR transmission spectra show that the cutoff absorption edges are 328 nm for α-AgI₃O₈ and 345 nm for β-AgI₃O₈. Thermal stability studies demonstrate that both isomers are thermally stable up to about 370 °C. Theoretical calculations based on DFT methods for the two AgI₃O₈ phases as well as the NaI₃O₈ analogue have been performed