Abstract
Single crystals of tetramethylammonium tetrachlorozincate [N(CH3)4]ZnCl4 abbreviated hereafter as (TMA)2ZnCl4 were grown using the slow evaporation technique at 315 K. The X-ray powder diffraction patterns indicated that [N(CH)]ZnCl 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 (T=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.
Highlights
The A2BX4 type crystals have been interested because of their incommensurately modulated structures and the successive phase transitions
The X-ray powder diffraction pattern collected at room temperature is presented and the diffraction peaks were indexed in Fig. (2)
The (TMA)2ZnCl4 crystallizes in the orthorhombic system with the space group Pmcn, and the unit cell parameters refined by the least square method and labeled according to international standards ( ! < ! < !)
Summary
The A2BX4 type crystals (with A=K, NH4, Rb; B=Zn, Co, Mn, Cu; X=Cl, Br, I) have been interested because of their incommensurately modulated structures and the successive phase transitions. Most of these materials exhibit many physical properties related to ferroelectric and commensurate or incommensurate phase transitions [1,2]. A2BX4 crystal family shows many physical properties related to structural phase transitions at low temperatures. The highest temperature phase (parent or paraelectric phase, PE) belongs to the Pmcn symmetry In this phase, Theoretical and experimental investigations were carried out on (TMA)2ZnCl4 crystals for their variety of phase transitions. Dielectric relaxation was investigated by Hong et al [9] and
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