The effect of pressure on the ferroelectric–paraelectric phase transition in the guanidinium compound [C(NH 2) 3] 4Cl 2SO 4

Abstract

The effect of pressure on the ferroelectric–paraelectric phase transition in the guanidinium compound [C(NH 2) 3] 4Cl 2SO 4 (G 4Cl 2SO 4) was investigated in the pressure p range up to 500 MPa. Its phase transition temperature T c decreases with initial pressure coefficient of d T c/d p = −1.2 × 10 −1 °C/MPa, in contrast to the positive pressure coefficient of the ferroelastic phase transition temperature in G 2PbI 4. The relative permittivity ɛ r in the paraelectric phase above T c obeys the Curie–Weiss law under pressures p. The peak value of ɛ r at T c, ɛ p decreases remarkably with increasing p. Such decrease of the peak value of ɛ p with p results from both the increase in the temperature difference ∆ T between T c and the Curie–Weiss temperature T o and the decrease in Curie constant C. The thermal hysteresis in T c becomes larger (11 and 23 °C at 0 and 300 MPa, respectively) under pressures. The phase transition from ferroelectric II to paraelectric I at T c becomes sluggish and broad with increasing p. The T c deviates from the linear relationship to the non-linear one in T c with p and tends to the change in shape of its curve towards the concave curve, in contrast to the convex one in the KH 2PO 4 type hydrogen-bonded ferroelectric.