Crystal structure and phase transitions in the compound [C6H5(CH2)2NH3]2TlCl5

The title compound <f>β</f>-phenylethylammonium pentachlorothallate III crystallizes in the monoclinic system with space group <f>P21</f>. The unit cell dimensions are: <f>a=9.5470(1)</f> Å, <f>b=7.5660(1)</f> Å, <f>c=15.9180(1)</f> Å, <f>β=101.3420°(9)</f>, with <f>Z=4</f>. Its crystal structure was determined and refined down to <f>R=0.0402</f>. The structure of this compound consists of <f>β</f>-phenylethylammonium cations and polynuclear anions in which slightly distorted [TlCl6]3- octahedral sharing two cis vertices are interconnected into chains. These chains are themselves interconnected by means of the N&z.sbnd;H…Cl hydrogen bonds. Two phase transitions at <f>T1=353</f> K and <f>T2=389</f> K were detected and studied by differential scanning calorimetry, dielectric and impedance spectroscopy measurements. The evolution of the dielectric constant and dissipation factor as a function of frequency and temperature revealed the first transition to be ferroelectric–paraelectric. Crystallographic studies show that this material presents a noncentrosymmetric space group at room temperature which is in agreement with our last deduction. The second phase transition at 389 K is characterized by a strong jump in the conductivity plot. It is assumed that the reorientation of <f>NH3+</f> ions is responsible for this phase transition. [Copyright &y& Elsevier]