Thermodynamics of the phase transitions in ferroelectric pyridinium perchlorate, [C5NH6]+ClO4-

Calorimetry, linear thermal expansion, volumetric dilatometry and dielectric methods were applied to investigate the phase transitions of pyridinium perchlorate, [C5NH6]+ClO4-. Anomalies of the physical properties measured, related to the first-order phase transitions, were observed at T1=248 K and T2=233 K. The values of the entropy change indicate their order-disorder character. An additional thermal anomaly observed at T3=236 K is connected with the next continuous phase transition. The mechanism of the phase transitions is proposed in this paper. The results suggest that the character of the phase transitions do not change up to 800 MPa. On the basis of the high-pressure studies the p-T phase diagram is obtained. The pressure dependences of the phase transition temperatures agree well with the values estimated from the Clausius-Clapeyron relationship. The uniaxial pressure dependences of the phase transition temperatures are estimated. (author)