Critical Behavior in the Heat Capacity of Ferroelectric TGS, TGSe, and TGFB

Measurements of the heat capacity were carried out on TGS, TGSe, and TGFB with an AC calorimetry technique to clarify its critical nature near the ferroelectric phase transition point. An anomalous heat capacity with logarithmic temperature dependence was found in all of the three crystals in a temperature range T - T c <10∼20 K above T c . The relation between this anomaly and the quality of sample was studied by examining crystals of different growth conditions. The effect of γ-ray irradiation on this anomaly was also examined. From such studies the observed logarithmic anomaly was ascribed to critical fluctuation. The data are analyzed with a theory for an Ising-dipole system taking into account the effect of fluctuation to the first order. The observed heat capacity anomaly both above and below T c is fitted satisfactorily with reasonable values of parameters.