Calorimetric and optical studies on ferroelastoelectric phase transitions in [(CH3)2CHNH3]4Cd3Cl10 crystal

Temperature dependence of the specific heat and linear birefringence of the tetra(isopropylammonium) decachlorotricadmate (II), abbreviated as IPACdCl, crystal have been measured in the temperature range 250–380 K. In this range three phase transitions were detected previously at about 264, 294 and 353 K. The AC calorimetry enabled a proper analysis of the second order transition at 294 K. We could evaluate the entropy change ΔS = 12.1 J/mol K connected with this transition. On the basis of the structural data one can ascribe the transition to the ordering of IPA cations. The temperature dependence of excess heat capacity and excess entropy in the ferroelectric phase was described in the frame of the classical Landau theory of phase transitions. Precise linear birefringence measurements were performed for three crystallographic (and optical) cuts of the crystal. Critical indices α and β were also determined. Values of indices indicated the transition belonged to the universality class of the Ising model (n = 1, d = 3).