Pressure-Temperature Phase Diagram of Ferroelectric Pyridinium Tetrafluoroborate Crystal

Abstract

The dielectric constant of pyridinium tetrafluoroborate C₅NH₆·BF₄ (PyBF₄) was measured in the frequency range of 300 Hz to about 10 MHz under pressures up to about 1.5 GPa. The critical slowing down of the dielectric relaxation was observed at all pressures examined, the relaxation frequency of which becomes lower with increasing pressure. The pressure derivative of the transition temperature T_c1 between the paraelectric phase and the ferroelectric phase, (dT_c1⁄dp)₀, is about 120 K/GPa. This is very large compared with those of other typical order-disorder ferroelectrics. The p–T phase diagram was discussed on the basis of phenomenological theory.