Dimensionality–Band Gap–Third-Harmonic Generation Property Relationship in Novel Main-Group Metal Iodates

Three new main-group metal iodates, i.e., AgGa(IO3)4, AgIn(IO3)4, and Ag3In(IO3)6, have been successfully synthesized by hydrothermal reactions at different temperatures. Single crystal X-ray diffraction analysis suggests that AgGa(IO3)4 and AgIn(IO3)4 exhibit unidimensional (1D) structures consisting of IO3 trigonal pyramids, MO6 (M = Ga and In) octahedra, and Ag+ counter cations. Ag3In(IO3)6, however, exhibits a zero-dimensional (0D) structure composed of IO3 trigonal pyramids, InO6 octahedra, and Ag+ counter cations. The metal iodate compounds were further investigated by employing various characterization tools such as spectroscopic analysis, thermogravimetric analysis, density functional theory calculations, and local dipole moment calculations. In addition, the nonlinear optical (NLO) properties such as third-harmonic generation (THG) and two-photon absorption (2PA) of the reported iodates were assessed. The THG measurements on polycrystalline samples of the title compounds reveal that their third-order susceptibility (χ(3)) values are three to five times larger than that of α-SiO2. Based on the experimental 2PA coefficient (β) and laser-induced damage threshold of the metal iodates, it was demonstrated that the optical breakdown arises from NLO light–matter interaction. A comparison of the nonlinear figures of merit (χ(3)/β) for several related main-group iodates indicates that the 0D iodates are better than the 1D iodates and the In iodates are better than the Ga iodates. The results suggest a novel design principle for maximizing the NLO performance of this class of materials in terms of dimensionality, polarizability, and band gap.