The Open Materials Science Journal

2015, 9 : 162-172
Published online 2015 October 6. DOI: 10.2174/1874088X01509010162
Publisher ID: TOMSJ-9-162

Temperature Dependence of the Optical Band Gap and Optical Parameters of Tetramethyl Ammonium Tetrachlorozincate (TMA)ZnCl single Crystals Around the Normal and Incommensurate Phase Transitions

A. Abu El-Fadl and A.M. Nashaat
Physics Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt.

ABSTRACT

Single crystals of tetramethylammonium tetrachlorozincate [N(CH3)4]2ZnCl4 abbreviated hereafter as (TMA)2ZnCl4 were grown using the slow evaporation technique at 315 K. The X-ray powder diffraction patterns indicated that [N(CH3)4]2ZnCl4 belongs to the orthorhombic system with Pmcn symmetry at room temperature. The lattice constants are found to be a= 12.360 Å, b= 15.687 Å and c= 8.985 Å. The values were in good agreement with the values in previous studies. Ultraviolet–visible–near-infrared (UV–Vis–NIR) spectral studies were carried out in the temperature range 276–307 K. This range of temperature involves two phase transition temperatures (Ti=296 K) from normal (parent) to incommensurate phase and (Tc=279 K) from incommensurate to commensurate-ferroelectric phase. The cut off wavelength was found to be 195.016 nm at room temperature. The optical transmittance increases with increasing temperature, and the cut off shifts to higher wavelengths. Analysis reveals that the type of optical transition is the indirect allowed one. The optical energy gap (Eg) has the value of 5.903 eV at room temperature. The value of optical energy gap (Eg) decreases with increasing temperature. The changes in the values of the cut off wavelength and optical energy gap (Eg) with changing the temperature were found to take different rates at the two phases under study, besides anomalous takes place at Ti and Tc. The absorption coefficient (α) as a function of the incident photon energy shows an exponential behavior near the absorption edge which suggests that the Urbach rule is obeyed and indicated the formation of a band tail. Urbach parameters were calculated at different temperatures and the frequencies of effective phonons and electron–phonon interaction constants were determined for various phases.

Keywords:

(TMA)ZnCl single crystals, Crystallization, Optical characterization, Temperature dependence of band gap, Urbach rule.