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, alpha-AgI3O8 (Pnc2) and beta-AgI3O8 (I (4) over bar), have been synthesized through the hydrothermal reactions of AgNO(3)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 alpha-AgI3O8 and beta-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 alpha-AgI3O8 and 345 nm for beta-AgI3O8. Thermal stability studies demonstrate that both isomers are thermally stable up to about 370 degrees C. Theoretical calculations based on DFT methods for the two AgI3O8 phases as well as the NaI3O8 analogue have been performed.