THERMAL HYSTERESIS EFFECTS IN THE FERROELECTRIC-FERROELASTIC PHASE-TRANSITION IN [N(CH3)(4)]2ZNI4

The phase-transition sequence undergone by [N(CH3)(4)]2ZnI(4) has been studied by adiabatic calorimetry. The specific-heat results show the P112(1)/a-P2(1)ca phase transformation around 215 K to take place in t wo independent steps. The corresponding specific-heat anomalies differ by only approximate to 0.4 degrees C on heating, and approximate to s 3 degrees C on cooling. An unusual thermal hysteresis effect has been found to be present, which leads to the reversal in the order of appea rance 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 t his anomalous phase transformation.