Calorimetric studies of phase transitions in imidazolium perchlorate crystal

The first precise measurements of specific heat changes have been performed for a C3N2H5ClO4 crystal using an ac calorimeter. Phase transitions at about 373 and 219 K were revealed. The continuous second-order phase transition at 373 K to the ferroelectric phase is described by the Landau model using specific heat and linear birefringence data. The Landau expansion coefficients B and C are of the same order of magnitude as those obtained for hard ferroelectric materials. Thermal parameters ( such as the excess enthalpy, Delta H, and the excess entropy, Delta S) of the continuous transition were estimated and discussed. During the first cooling, a distinct (similar to 2 K) supercooling effect was observed at the discontinuous transition at about 219 K.