Thermal hysteresis effects in the ferroelectric-ferroelastic phase transition in

Abstract

The phase-transition sequence undergone by [N(${\mathrm{CH}}_{3}$${)}_{4}$${]}_{2}$${\mathrm{ZnI}}_{4}$ has been studied by adiabatic calorimetry. The specific-heat results show the P${112}_{1}$/a-P${2}_{1}$ca phase transformation around 215 K to take place in two independent steps. The corresponding specific-heat anomalies differ by only \ensuremath{\approxeq}0.4 \ifmmode^\circ\else\textdegree\fi{}C on heating, and \ensuremath{\approxeq}3 \ifmmode^\circ\else\textdegree\fi{}C on cooling. An unusual thermal hysteresis effect has been found to be present, which leads to the reversal in the order of appearance of the two specific-heat peaks. This fact has been unambiguously confirmed by means of an exhaustive experimental specific-heat study and by the use of the thermodynamic-functions values associated with this anomalous phase transformation.