The influence of hydrostatic pressure on the ferroelectric phase transition in (C3N2H5)5Bi2Cl11

Single crystals of pentakis (imidazolium) undecachlorodibismuthate (III); (C3N2H5)5Bi2Cl11, have been studied by the dielectric method at ambient and hydrostatic pressures. At ambient pressure the crystals undergo a ferroelectric phase transition of order-disorder type at 166 K. The character of the dielectric permittivity anomalies is typical of the ferroelectric phase transition close to second-order type. The magnitude of the Curie constants of paraelectric (C+) and ferroelectric (C-) phases is of the order of 103 and the (C+)/(C-) ratio is near 2. At elevated pressures the maximum dielectric permittivity corresponding to the (Tc) value gradually decreases with increasing pressure. In addition, above about 15 MPa the dielectric permittivity shows a step-like behaviour as well as a temperature hysteresis, which is typical of first-order phase transitions. The Curie-Weiss constant (C+) rapidly increases with increasing pressure, reaching a value of the order of 105 at about 181 MPa. The pressure-induced change in the phase transition type is continuous and reversible. The phase boundary has been found to decrease nonlinearly with increasing pressure with the initial slope of dTc/dp = -3.9 x 10-2 K MPa-1. The results have been discussed within the phenomenological model of phase transitions