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

Two new noncentrosymmetric isomeric silver polyiodates, namely, α-AgI3O8 (Pnc2) and β-AgI3O8 (I4̅), 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 c 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.