Pressue Dependence of the Ferroelectric-Paraelectric Transition Temperature in KNBO 3 . Comparision with the Ice VII-VIII Transition

New results on the phase diagram of KNbO3 determined, from 10 to 650 K tip to 30 GPa, with Raman scattering are given. On increasing pressure the Curie temperature T-c was found to shift down regularly from 700 K, at atmospheric pressure, to 20 K at similar to25 GPa. Up to 20 GPa, the pressure dependence of T-c follows a classical regime described by the Ising model where the dipolar interaction follow a delta(2)/v law, where delta is the off-center displacement and v the unit cell volume. This behaviour found also for the ice VII-VIII is very likely encountered in a number of (anti)ferroelectric-paraelectric transformations involving positional disorder.