A new (C2H5NH3)2ZnCl4 crystal with a pure Pnma-P212121 ferroelastic phase transition

Abstract

Optical, calorimetric, and X-ray measurements performed on (C2H5NH3)2ZnCl4 (C2ZnCl) together with a group theoretical analysis have revealed that this crystal shows a pure first-order ferroelastoelectric phase transition at 243.3 K. In the low-temperature phase the crystal shows two explicit forms of ferrocity: gyrotropy and piezoelectricity, which is a common fact in this class of secondary ferroics. The experimental data show that the corresponding phase transition is similar to that observed in (C5H11NH3)2ZnCl4 (C5ZnCl) at 249 K. The thermodynamic function values for this phase transition in both compounds and the X-ray results confirm that the conformational changes must be excluded from the physical mechanisms involved in the onset of the ferroelastoelectric properties. The microscopic origin of this phase transition should be related to the freezing of the dynamical disorder of the N-H...Cl hydrogen bonding and with the corresponding tilting of the hydrocarbon chains. The experimental data seem to be in agreement with the Landau phenomenological theory except near Tc, where the experimental specific heat is well described by an empirical power law. Finally, as found in some other crystals of this family, C2ZnCl crystals also exhibit an isotropic point around 25 K. The isotropic point temperature shows a very strong dependence on n (chain length) for n<or=5 and an asymptotic behaviour for n>or=12.