Field induced ferroelectric phase transition and dielectric spectrum behavior in PMN ceramics

The problem of field induced phase transition into the ferroelectric phase in relaxor ceramics and in particular in PMN ceramics was studied for many years, but the results are still controversial. The temperature and field dependences of PMN polycrystalline ceramics dielectric spectra have been studied. The transitions from the glass-like into the ferroelectric phase were found both in field cooling and zero field heating after field cooling regimes under the fields above 3 kV/cm. Electric field scans near the diffuse maximum of the dielectric permittivity demonstrated the shift of the loss maximum to the high frequency at increasing of applied electric field. The experimentally observed frequency-dependent sign of the C-V effect (the decrease (negative effect) or the increase (positive effect) of dielectric permittivity at increasing field) is due to spectra hardening. In contrast to single crystal, where the application of the electric field higher than 1.6 kV/cm in <111> direction induces phase transition into ferroelectric state, in polycrystalline ceramics no clear evidence of the phase transition from the paraelectric into the ferroelectric state up to 6 kV/cm neither at 242 K nor at 246 K was found. This difference between the single crystal and polycrystalline ceramics can be explained by very strong dependence of the threshold field from the field orientation at this transition. The influence of external field on low frequency relaxation process parameters for PMN single crystal and ceramics is compared.