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]