Kinetics and thermodynamics of the ferroelectric transitions in PbMg1/3Nb2/3O3 and PbMg1/3Nb2/3O3-12% PbTiO3 crystals

The two-step freezing and melting of the field-induced ferroelectric order in PbMg1/3Nb2/3O3 (PMN) and (PbMg1/3Nb2/3O3)0.88(PbTiO3)0.12 (PMN-PT) is investigated. In PMN-PT, direct microscopic images show that both steps occur in the same spatial regions. The higher temperature freezing corresponds to the higher temperature melting, indicating that the stages are not just kinetically but also thermodynamically distinct. The higher-T melting step shows several indications of being a sharp first-order transition near an equilibrium temperature. The lower-T melting step shows more kinetic dependence. Partially poled PMN also spontaneously approaches saturation polarization on zero-field aging, indicating that the true equilibrium state is ferroelectric below ∼200 K. In PMN-PT, a variety of kinetic measurements on the ferroelectric state indicate that the kinetics are governed by a glassy matrix showing aging effects.