Thermodynamics of the phase transitions in ferroelectric pyridinium tetrafluoroborate [C5NH6]+BF-4

The anomaly of specific heat at the phase transitions was studied by differential scanning calorimetry (DSC) and the anisotropy of linear thermal expansion was studied by dilatometry on monocrystals of pyridinium tetrafluoroborate [C5NH6]+BF4-. The effect of hydrostatic pressure on the phase transitions was studied by the methods of pressure dilatometry and pressure DTA. We found that the temperature of ferroelectric phase transition at T-1 = 238 K as well as the temperature of non-ferroelectric phase transition at T-2 = 204 K increase with pressure. The thermodynamic parameters determined on the basis of the calorimetric measurements are consistent with those obtained from the Ehrenfest relationship for second order phase transitions. The pressure experiments allowed us to construct the p-T phase diagram up to 300 MPa. We estimated the uniaxial pressure dependence of the phase transition temperature. The mechanism of both phase transitions is proposed.