Rational Design of Novel Polar Nonlinear Optical Materials in Alkali Metal Rare Earth Iodates

Four new potassium rare earth iodates, namely, acentric K₂Lu(IO₃)₅ and KM(IO₃)₄(HIO₃)_0.33 (M = Ce/Pr) and centric KLa(IO₃)₄, were successfully grown by mild hydrothermal reactions. Three of them exhibit polar structures; K₂Lu(IO₃)₅, KCe(IO₃)₄(HIO₃)_0.33, and KPr(IO₃)₄(HIO₃)_0.33 show second-harmonic generation (SHG) responses of 3.0, 1.0, and 0.8 × KDP, respectively. These three iodates are phase-matchable for second-harmonic generation. The influence of changes in the radius and coordination mode of rare earth ions on the crystal structure and SHG response has been discussed in detail. Our findings suggest that in the alkali metal rare earth iodate, modulating the arrangement of iodate groups by changing the coordination geometry of rare earth ions is an effective strategy for designing polar NLO materials.