Macroscopic polarity control with alkali metal cation size and coordination environment in a series of tin iodates

A series of stoichiometrically similar tin(IV) iodates, A₂Sn(IO₃)₆ (A = Li, Na, K, Rb, Cs) and Sn²⁺Sn⁴⁺(IO₃)₆ have been hydrothermally synthesized. X-ray diffraction was used to determine the crystal structures of the reported materials. All six materials reveal zero-dimensional molecular structures that consist of SnO₆ octahedra and IO₃ polyhedra. However, the size and coordination environment of cations significantly influence the macroscopic centricities of the materials. While Li₂Sn(IO₃)₆ and Na₂Sn(IO₃)₆ crystallize in the noncentrosymmetric (NCS) polar space group, P6₃, the K, Rb, Cs, and Sn phases crystallize in the centrosymmetric (CS) nonpolar space group, R. Infrared and UV-vis spectroscopy, thermal analyses, and local dipole moment calculations are reported. With NCS polar materials, powder second-harmonic generation (SHG) properties, polarization, and piezoelectric measurements are also presented. The NCS properties are mainly attributable to the parallel alignment of the lone pairs in I⁵⁺ cations.