Ferroelectric and relaxor properties of Pb (Sc0.5 Nb0.5) 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 (Sc0.5 Nb0.5) O3, (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, Tc, substantially below the peak temperature, Tm, 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 Tc (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 Tc are found to be comparable to those of simple displacive perovskite oxides such as BaTi O3 and PbTi O3.