Polar or Non-Polar? Syntheses, Crystal Structures, and Optical Properties of Three New Palladium(II) Iodates

Three new novel palladium(II) iodates with square-planar PdO4 units, namely, Pd(IO3)2, AgPd(IO3)3, and BaPd(IO3)4, have been successfully hydrothermally synthesized. They represent the first ternary and quaternary palladium(II) iodates and display three different structural types. Pd(IO3)2 exhibits a novel two-dimensional (2D) layered structure in which each PdO4 square further connects with four neighboring ones by four bridging IO3 groups. AgPd(IO3)3 exhibits a unique three-dimensional (3D) network based on unique one-dimensional (1D) [Pd(IO3)3]− anionic chains along the c-axis which are further interconnected by Ag+ cations. BaPd(IO3)4 is isostructural with KAu(IO3)4, and its structure features zero-dimensional (0D) [Pd(IO3)4]2− anionic units that are interconnected by Ba2+ cations. These materials can be polar or non-polar depending on the different alignments of the lone pairs of the I(V) atoms. Pd(IO3)2 and AgPd(IO3)3 are non-polar and centrosymmetric, hence not second-harmonic generation (SHG) active. BaPd(IO3)4 is polar and displays a moderate SHG response of about 0.4× KTP. Thermal analyses, optical, luminescent, and ferroelectric properties as well as electronic structure calculations have also been performed.