Structural phase transition, optical and electrical properties of the hybrid material [(C2H5)4N]2ZnI4

Abstract

The bis (tetra-ethylammonium) tetraiodo-zincate (II) was prepared by slow evaporation an aqueous stoichiometric mixture of tetra-ethylammonium iodide and zinc iodide at room temperature. This compound has been characterized by X-ray diffraction (XRD), thermal analysis, UV–vis spectroscopy and complex impedance spectroscopy. Single crystal X-ray diffraction analysis shows that the title compound crystallizes in the tetragonal system with space group P4¯21m.The differential thermal analysis (DTA) and temperature-controlled X-ray diffraction study reveal a reversible first order phase transition at about 444 ​K between low temperature-tetragonal and high temperature-orthorhombic forms of [(C2H5)4N]2ZnI4. Below 444 ​K, the low temperature form exhibits two linear regimes of thermal expansion with a change in the slope around 373 ​K which is ascribed to a second-order phase transition. The diffuse reflectance indicates the existence of an optical direct transition with the band gap energy is equal to 3.9 ​eV. The spectra of complex impedances obtained in the temperature range from 403 to 468 ​K were modelled by a combination of R//CPE elements. . A change of conduction regime around 444 ​K is noted in the variation of the Ln (σDC.T) versus reciprocal temperature. In both regimes, the conductivity obeys the Arrhenius law with activation energies of 1.37 ​eV in regime I and 2.37 ​eV in regime II. Frequency dependence of the AC conductivity followed the universal Jonscher's power law.