Abstract; Single crystals of tetramethylammonium tetrachlorozincate [N(CH 3 ) 4 ] 2 ZnCl 4 abbreviated hereafter as (TMA) 2 ZnCl 4 were grown using the slow evaporation technique at 315 K. The X-ray powder diffraction patterns indicated that [N(CH 3 ) 4 ] 2 ZnCl 4 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 (T i =296 K) from normal (parent) to incommensurate phase and (T c =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 (E g ) has the value of 5.903 eV at room temperature. The value of optical energy gap (E g ) decreases with increasing temperature. The changes in the values of the cut off wavelength and optical energy gap (E g ) with changing the temperature were found to take different rates at the two phases under study, besides anomalous takes place at T i and T c . 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.
ZnCr 2%x Fe x O 4 nanoparticles with different compositions (0 : x : 0.8) were successfully synthesized using the microwave combustion method. The effects of Fe ions substitution with Cr ions on the spinel ZnCr 2 O 4 structure, optical, and magnetic properties are investigated. The nanoparticles size was found to increase as a result of Fe substitution, as confirmed by the high resolution transmission electron microscope (HR-TEM) images. A reduction of the optical energy gap and a transition from paramagnetic to superparamagnetic behavior are observed with increasing Fe ratio indicating a single domain ZnCr 2%x Fe x O 4 nanoparticle. The uncompensated surface magnetic moments results in an enhancement in paramagnetic behavior compared with bulk ZnCr 2 O 4 spinel. The saturation magnetization increases by increasing Fe ions in the spinel structure which is expected to originate from a larger surface area.
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