The pressure influence on the incommensurate–commensurate ferroelectric phase transition in [4-NH2C5H4NH][SbCl4]

The dielectric properties of the [4-NH2C5H4NH] SbCl4 (abbreviated as 4-APCA) crystal were investigated under hydrostatic pressure up to 300 Mpa. The pressure–temperature phase diagram was given. The paraelectric–ferroelectric phase transition (II→III) temperature (Tc) increases linearly with increasing pressure with a slope dTc/dp=21×10−2 K/MPa. The pressure dependence of Curie–Weiss constants has been evaluated also. In the paraelectric phase (II) the Curie constant (C+) was pressure dependent whereas the C− constant over the ferroelectric phase (III) was almost constant. The results are interpreted in terms of improper and displacive type phase transition model with a soft phonon at a zone boundary.