The p–T phase diagram for ferroelectric bis-thiourea pyridinium nitrate

The effect of temperature and pressure on physical properties of the ferroelectric bis-thiourea pyridinium nitrate inclusion compound has been studied by dielectric spectroscopy and nuclear magnetic resonance (NMR). At ambient pressure the ferroparaelectric phase transition observed at T2 = 216 K is continuous in contrast to the nonferroelectric phase transition observed at T1 = 273 K. Under small pressures, the temperatures of the phase transitions T1 and T2 increase with increasing pressure. Starting from about 250 MPa, T1 temperature decreases with increasing pressure, while T2 temperature increases with increasing pressure. At 450 MPa and 245 K a triple point is observed. Bis-thiourea pyridinium nitrate undergoes a continuous phase transition from the ferroelectric to paraelectric phase under 450 MPa, while above this pressure the phase transition from the ferroelectric to paraelectric phase is discontinuous. The change in the phase transition character is related to the crystallographic change in the group–subgroup relation between the ferro- and paraelectric phases taking place with increasing pressure.

► Bis-thiourea pyridinium nitrate has been studied at high pressures up to 1 GPa. ► p-T phase diagram has been obtained by NMR and dielectric method. ► The triple point is observed in p-T diagram. ► The continuous phase transition is changed to discontinuous at the pressure 450 Mpa.