K2Au(IO3)5 and β-KAu(IO3)4: Polar Materials with Strong SHG Responses Originating from Synergistic Effect of AuO4 and IO3 Units.

Two new polar potassium gold iodates, namely, K2Au(IO3)5 ( Cmc21) and β-KAu(IO3)4 ( C2), have been synthesized and structurally characterized. Both compounds feature zero-dimensional polar [Au(IO3)4]− units composed of an AuO4 square-planar unit coordinated by four IO3− ions in a monodentate fashion. In β-KAu(IO3)4, isolated [Au(IO3)4]− ions are separated by K+ ions, whereas in K2Au(IO3)5, isolated [Au(IO3)4]− ions and non-coordinated IO3− units are separated by K+ ions. Both compounds are thermally stable up to 400 °C and exhibit high transmittance in the NIR region ( λ=800-2500 nm) with measured optical band gaps of 2.65 eV for K2Au(IO3)5 and 2.75 eV for β-KAu(IO3)4. Powder second-harmonic generation measurements by using λ=2.05 μm laser radiation indicate that K2Au(IO3)5 and β-KAu(IO3)4 are both phase-matchable materials with strong SHG responses of approximately 1.0 and 1.3 times that of KTiOPO4, respectively. Theoretical calculations based on DFT methods confirm that such strong SHG responses originate from a synergistic effect of the AuO4 and IO3 units. [ABSTRACT FROM AUTHOR]