Recent advances on second-order NLO materials based on metal iodates

Metal iodates with a lone-pair containing I(V) in an asymmetric coordination geometry can form a diversity of unusual structures, including non-centrosymmetric (NCS) structures with promising second-order nonlinear optical (NLO) properties. They have wide transparent wavelength regions (0.4–12 μm), large second harmonic generation (SHG) coefficients (>10 × KDP for many iodates) and high optical-damage thresholds (4–50 GW cm −2 ) as well as good thermal stability (usually >400 °C). In this review, the structures and second-order NLO properties of metal iodates will be discussed. Under reaction media with a high concentration of iodic acid, the iodate groups can be condensed into binuclear or polynuclear iodate anions, these compounds are able to display large SHG responses. The introduction of other lone pair containing cations into the iodate system is also an effective strategy to design new NCS materials. The combination of d 0 transition-metal cations with iodate groups afforded a large number of NCS metal iodates with anionic structures ranging from 0D clusters, 1D chains, 2D layers to 3D networks. These NCS materials can display excellent second-order NLO properties when the polarizations from both types of the asymmetric units are aligned properly. As for the iodates of d 8 -transition metal ion with a square planar TMO 4 geometry, the cis TM(IO 3 ) 4 unit in which the four iodate groups are located at the same side of the TMO 4 plane favors the formation of NCS structures whereas the trans - one in which the four iodate groups being located at both sides of the TMO 4 square plane prefers to a centrosymmetric structure. NCS structures with good SHG properties can also be found in other mixed metal iodate systems.