Characterization of phase transitions of [N(CH3)4]2ZnCl2Br2 mixed crystals

Abstract

The X-ray powder diffraction patterns show that at room temperature [N(CH3)4]2ZnCl2Br2 crystallizes in the orthorhombic system with Pnma space group. Three endothermic peaks at T1=251K, T2=276K and T3=283K are observed by differential scanning calorimetry technique. In order to characterize these transitions, Raman spectra have been recorded in the wide temperature range (between 170 and 400K) and the frequency range related to the internal and external vibrations of the cations and anions (3200–20cm−1). The evolution of Raman spectra against temperature shows that the compound is homogeneous, and singularities occur in the vicinity of 283K and especially at 251K. The second goal of the present work was to obtain the Tc temperature transition using the dielectric measurements at different temperatures. Measurements of the dielectric constant ε′, ε″ and tanδ at several temperatures showed that this compound becomes ferroelectric below 257K. The impedance spectra and the complex modulus show double relaxation peaks in the paraelectric phase, which suggests the presence of grains and grain boundaries in this sample.