The bis (tetra-ethylammonium) tetraiodo-zincate (II) was prepared by slowly evaporating an aqueous stoichiometric mixture of tetra-ethylammonium iodide and zinc iodide at room temperature. X-ray diffraction (XRD), thermal analysis, UV-vis spectroscopy and complex impedance spectroscopy were used to fully characterize this compound. Single-crystal X-ray diffraction analysis indicates that the compound crystallizes at room temperature in the tetragonal system, space group P \(\stackrel{-}{4}\)21m(No.113), with lattice parameters a = 13.794 (2) Å and c = 14.900 (3) Å. Differential thermal analysis (DTA) and temperature-controlled X-ray diffraction study revealed a reversible first order phase transition at about 444 K between low and high temperature forms of [(C2H5)4N]2ZnI4. Below 444 K, the low temperature form exhibits two linear regimes of thermal expansion with a change at around 373Kascribed to a second-order phase transition. The diffuse reflectance indicates the existence of optical direct allowed transition mechanisms with the band gap energy equal to 3.9 eV.Complex impedance spectra recorded in the temperature range of 403 to 468 K were satisfactorily modelled by acombination of R//CPE elements assigned tothe electrical response of grains at the first order phase transition at around 444 K, a change of conduction regime is noted in the temperature dependence of DC-conductivity. In both regimes, the conductivity obeys the Arrhenius law with activation energies of 1.37 eV in regime I and 2.37 eVin regime II.The frequency dependent AC-conductivity followed the universal Jonscher’s power law.
