Ferroelectric and relaxor properties of Pb(Sc0.5Nb0.5)O3:: Influence of pressure and biasing electric field

The influences of hydrostatic pressure and biasing electric field on the dielectric properties and phase behavior of a single crystal of the perovskite compound Pb(Sc0.5Nb0.5)O-3, (PSN) have been investigated. On cooling from high temperatures, the crystal first enters a relaxor (R) state and then spontaneously transforms to a ferroelectric (FE) phase at a temperature, T-c, substantially below the peak temperature, T-m, in the dielectric susceptibility. Based on earlier work on ceramic samples, this behavior suggests substantial chemical (Sc and Nb) disorder at the B sites. Pressure enhances the R state with strong indications that the FE phase should vanish at a pressure somewhat higher than the highest pressure reached in the experiments, making the R state the ground state of the crystal at reduced volume. A significant feature of the temperature (T)-pressure (P) phase diagram is the finding that the T-c(P) phase line should terminate at a pressure between 10 and 15 kbar in a manner akin to a critical point; however, in the case of PSN this feature represents a FE-to-R crossover. Such behavior suggests that a path can be defined that takes the crystal from the FE phase to the R state without crossing a phase boundary. A biasing electric field favors the FE phase over the R state, and the results indicate that the R state vanishes at >= 5 kV/cm. The magnitudes of both the high T Curie-Weiss constant, C, and the change in entropy (or latent heat) at T-c are found to be comparable to those of simple displacive perovskite oxides such as BaTiO3 and PbTiO3.